When designing a receiver that is intended to be used for
long-distance, optical through-the-air communications, there
are some things to be considered that are very different
from what might be required in most other situations where
detection (and demodulation) of an optical signal is to be
- Signals are weaker in through-the-air communications -
sometimes by several orders of magnitude - than in more
familiar situations where an optical signal is to be
detected from optical fiber systems such as optical fibers,
TOSLINK, infrared TV remote, etc.: In these
situations, the energy present at the receiver is really
quite high, suffering only modest attenuation through the
- Unlike variable, atmospheric paths, optical cables fibers
tend to have consistent signal levels. Even infrared
remotes, while more-variable, are usually pointed directly
at the device being controlled and have fairly high signal
- Bandwidth requirements can be more modest over
long-distance, through-the-air communications paths.
For weak-signal detection in through-the-air optical
communications and experimentation, voice bandwidth (up to 3
kHz or so) is usually all that is required, while
specialized communications techniques (carrier detection,
WSJT and WOLF modes) work well with frequencies below 300
With the above in mind, the optical receiver was designed to
meet those needs while, at the same time, being constructed
using fairly easily-obtainable components using conventional
A note about using this receiver at higher frequencies:
"Why not use (fill in the blank)
detectors instead of photodiodes?"
- This receiver is intended to detect amplitude-modulated
light from a distant transmitter, "baseband" modulated with
plain speech and what is depicted in Figure 1 has not
been optimized for use with subcarriers or other signaling
techniques above 5 kHz or so.
There are a number of other technologies available that may
allow even better "weak signal" performance than standard
silicon PIN photodiodes - such as Photomultiplier Tubes and
Avalanche Photodiodes. While these technologies exist, the
intent was to come up with a design that was both inexpensive to
easy to replicate. Having said
this, these more-exotic devices (PMTs and APDs) occasionally
appear on the surplus market for reasonable prices and
experimentation using such devices is certainly encouraged!
It is highly recommended that one also
read these web pages:
Light DX Receiver Circuitry - This
article, by Mike Groth (VK7MJ) and Chris Long (VK3AML)
talks about various aspects of optical receiver
design. This article also includes a reproduction
of Application Note
D3011C-3 by EG&G that covers, with somewhat
more rigor, the various properties of photodiode-based
related articles are linked at the bottom of this
version of the "Version 3" optical receiver
Simplified version of the "Version 3" optical receiver -
Click on the image for a larger version.
shows the schematic of an optical receiver from
the link above
. First, a
few comments about the circuit:
- No feedback is used. This avoids the contribution of
additional noise energy at the lowest-level signal portions
of the circuit - namely the "hot" side (anode) of the
- D1, the photodiode is reverse-biased. This is done
to reduce the diode's capacitance and maximize
- Q1 and Q2 form a "cascode"
amplifier circuit. This simple circuit not only
provides a significant amount of AC gain, but it also
minimizes the effects of "Miller
Capacitance" - that is, the effect of the gain
of Q1 seemingly to amplify the effects of its gate-drain
- Q3 is a current source, providing the majority of drain
current for Q1. Being a current source, it has a
fairly high impedance (as opposed to the use of a plain
resistor to supply the same amount of current) and it adds
minimal noise to the circuit, and by running the FET with
more current, it tends to be quieter. A bonus is that
this current source makes the circuit somewhat
less-sensitive to power-supply variations.
- To compensate for the high-frequency rolloff of the
photodiode, U1B is configured as a differentiator to provide
a 6dB/octave boost. This results in a fairly flat
audio response over the speech range. Additionally,
the differentiator circuit itself includes a mechanism to
provide low-pass filtering above 6-7 kHz or so.
Comment: The circuit depicted in Figure 1
differs from the prototype "Version 3" mentioned on the aforementioned web page only in
that is does not include an extra low-pass filter stage, and
or an "active decoupler" in the power supply, the latter not
having found to be necessary when the receiver was operated
from its own, isolated 9 volt battery power source.
- The value of the bypass capacitor in the
cascode amplifier (in this case, C4 - 2.2uF) should be
chosen with care: If very large values are used,
low-frequency oscillation or "ringing" may occur -
particularly at higher supply voltages (e.g. >12
volts) and/or with large, rapid/transient changes in
This circuit is intended to be operated from its own, single
9-volt battery - which is one of the reasons why the
reverse-polarity protection is present in the form of TH1, a
self-resetting thermal fuse, and D4, a reverse-polarity
protection diode: It is extremely easy to momentarily
connect a 9-volt battery backwards while fumbling in the dark -
something that could instantly destroy U1. In order to
maximize the available voltage, a series protection diode was NOT
While the use of an LM833 is shown in the diagram, practically
any dual op-amp with "reasonably" low-noise specifications may
be used, including the venerable TL082/TL072: A 1458-type
op amp was tried, but it was found to be quite noisy. Note
that operating an LM833 (and many other common dual op-amps)
from a single 9-volt battery pushes the low voltage
specification of this device - which is listed as 10
volts: Testing has indicated that the LM833 units that we
have seen seem to operate reasonably down to at least 7 volts,
but this is not
a guaranteed specification!
If you are constructing this circuit, keep in mind that there
are many other op-amps that offer good, low noise performance
but can operate from much lower supply voltages, such as the
LM4562 or the LMC6482, to name but a few!
In Figure 1
, note also the presence of S1, a "gain"
switch across U1B. This switch is optional and is present
to accommodate both situations where signals are very strong
(such as with shots over just a few miles) and also those in
which signals are very weak, close to the noise floor of the
receiver. Of course, this added gain (about 20dB) does nothing
to improve the actual sensitivity of the receiver, but it can
bring a low-level signal up to the range where one's audio
amplifier and monitor/recording system can properly operate!
Note also J1:
jumper is used to short out R4, the resistor in the source lead
of Q1. This 10 ohm resistor (and C3) is present solely for
the purpose of measuring the source current through Q1 and,
after final adjustment, should jumpered .
Finally, note that this circuit is designed such that gate
can actually flow through D1 and into
Q1! This is a rather peculiar way to run a JFET, but this
quirk allows the establishment of a reverse bias across D1.
A few words on the setting of Q1's drain current:
comments about modifications of this circuit and
setting Q1's drain current when adapted for
I have recently seen some variations of the "Version
3" circuit that have been modified for other
uses. In some cases, a capacitor is added
between the JFET's gate and the photodiode to provide
DC blockage to prevent saturation of the JFET should
large amounts of leakage current flow - which may
happen in several cases:
- Operation in high ambient light. If
daylight operation is anticipated, it should be
noted that the photodiode (D1) may conduct
excessively and strongly bias the JFET. If
such operation is anticipated, one of the easiest
things to do is to simply make the reverse bias
voltage variable by placing a potentiometer
(50k-100k will do) across the power supply and
connect the "V+" and of R1 to its wiper.
- It is likely that such a bias adjustment will
be "touchy" at the low end of the bias voltage
range, so it is recommended that a fixed
resistor of 1/3 the potentiometer's value be
connected from the wiper to the "V+" end to
"stretch" out the adjustment.
- Alternatively, a logarithmic-taper
potentiometer could be used: Just wire it
so that the "stretched" portion (e.g. least
amount of resistance change per degree of
rotation) is at the "low" end of the bias
- If you happen to have a 50k-100k multi-turn
potentiometer, you could use that, too!
- If the bias is set too low - especially under
"dark" conditions, the JFET may have too little
bias for the rest of the circuit to work as
evidenced by the receiver seeming to be "too
quiet" and very deaf. If this happens, simply
set the bias to maximum and then reduce it as
- If a device with high leakage is used in lieu of
a PIN photodiode such as an LED. Some
experimenters report good results from
reverse-biasing certain types of LEDs to bring
about a latent "Avalanche Photodiode" mode of
operation and it is likely that the leakage
currents involved in such could saturate the JFET.
In some cases, it may be desirous to simply use
capacitive coupling (as shown in Figure 2
on this page)
in which the gate potential is returned to ground
through a resistor - but if this is done, there are
- Daylight modifications will dramatically reduce
the absolute sensitivity of the receiver.
While this isn't likely to be important for
daytime operation where the sun's thermal noise is
likely to be orders of magnitude higher than the
receiver's noise, it will impact nighttime use
under low-light conditions. In other
words, it's best to build a separate "daytime"
- This will affect the standing
current of Q1, the JFET. Chances are, with
the gate voltage being reduced that it will be
conducting less-strongly! Because of this
you will likely need to reduce the
JFET's current which should be done by increasing
the value of R5! Failure to do this may
yield inconsistent results! Except, perhaps,
in testing, do not use a variable
resistor for R5 as the wiper noise can be
extremely high! A less-desirable alternative
would be to use a resistor between the emitter of
Q3 and ground to sink more current, but here,
again, using a potentiometer is risky! If
this is done, it is recommended that a metal
film resistor be used for the reasons
- The "gate leak" resistor should be as high a
value as is available. Unfortunately, 10
Megohms is about the highest commonly-available
value, although 22 Megohm units may be found.
- The "gate leak" resistor should be metal
film units as these are very low noise in
comparison with the usual carbon-film types.
Unfortunately, this suggestion conflicts with the
previous as high ohmic-value metal film resistors
can be a bit tough to find! They are
available at values of 10 Megohms,
however... if you look hard enough...
- If exclusively "daylight" operation is expected
and additional resistors are used to bias the
photodetector, it is recommended that all of those
resistors be metal film as well! Having said
that, it is more likely that the noise from the
leakage of the photodetector (not to mention the
light itself!) will exceed the noise contribution
of these resistors - even if they are not
With the use of the cascode circuit consisting of Q1 and Q2,
signals from the photodiode, D1, are expressed mainly as
variations in the drain current of Q1 and are reflected as
changes across R6. In order for this circuit to work,
however, it is necessary that the standing current through Q1 be
Because of the circuit topology, there positive
gate-source voltage with a small amount of gate-source current
flowing. As you might expect, this fully "turns on" the JFET
there is still plenty of "room" for the JFET to do its
work. This odd operating condition (that is, with the
JFET conducting gate current)
also requires some care in
the proper selection of the drain current in order for the
circuit to work properly!
Q3 is the primary current source for the JFET, Q1, and as such
it has a fairly high intrinsic impedance (perhaps 10's of
k-ohms) with the standing current set through the selection of
R5, nominally a 120 ohm resistor. If the current is too
high for the particular transistor used for Q1, the gain and
performance is greatly reduced - a condition obviated by the
voltage at the drain of Q1 being within a volt or two of the
positive supply rail. In general, it is better to run the
JFET at a current that is slightly "too low" than slightly "too
high" as the penalties for running slightly "low" current
(slightly degraded S/N ratio) are less severe.
For the typical 2N5457 from the various sources onhand, an R5
value of 120 ohms seems to be about right, resulting in
consistently "good" performance - but this cannot be
guaranteed! If a less-tightly "spec'd" JFET is used (such
as an MPF102) then one may need to do some experimentation to
find the "optimal" value for that specific device. In
experimenting, some MPF102's of various manufacturers and
vintages were tried and while some devices simply did not work
properly with R5 set at 120 ohms, none needed higher values
(which correlated with lower
drain current) than the
180-220 ohm area.
One property of JFETs is that the more drain current you can run
through them, the "quieter" they can become - up to a point - a
phenomenon having to do with "bulk current" effects - and
because of this, best performance (e.g. best signal-noise ratio)
is obtained at higher drain currents. Again, if you go
overboard and run too much drain current, the device is simply
saturated and performance suffers.
Another device with which good success has been obtained is the
Philips BF862. This JFET, available only in a
surface-mount package, is quite remarkable in its combination of
having fairly high transconductance, moderately low gate
capacitance, and high drain current capabilities and in testing
it operated well with drain currents ranging from just a couple
of milliamps to well over 20 milliamps! Of course, the
same advice applies as with other JFETs in terms of optimize
performance with the proper selection of drain current.
It is worth mentioning that Q1's drain voltage can vary widely -
but it is better for it to be low (as low as 1 volt or so - or
even below 0.25 volts for a JFET like the BF862) than high
(close to the positive supply rail) for the simple reason that
the cascode circuit (Q2) requires several volts of drop across
it to function. Since it is the drain current
is being used to convey the signal, one should not be too
surprised by the absolute DC voltage at that point: In
fact, monitoring the drain of Q1 with an oscilloscope is
unlikely to reveal much voltage change at all - again, due to
the fact that it's a change of current that is occurring and
that the signal is likely to be very small, anyway as that's the
whole point of a cascode circuit!
Finally, these components (the JFET, Q1, and Q3, the current
source) are subject to the sorts of changes that semiconductors
undergo as temperature changes. Experience has shown that
when the drain current of Q1 is chosen to be a "safe" value
(that is, at least 25% below the "maximum" current at which the
circuit functions properly) at "room temperature" (around 20C)
then it seems to tolerate temperature changes from about 0C to
35C (the maximum range over which it was tested) while
functioning properly - that is, Q1's drain voltage staying
adequately low to allow Q2's circuit to function - which
also implies that Q1 is not being fed "too much" current by
Q3. If operation over a wide temperature range is
anticipated it is strongly
recommended that it be
thoroughly tested to make sure that Q1 is "happy" (that is, not
fed too much current) over the expected temperature range.
Optimizing weak-signal performance:
As mentioned above, "good" weak-signal performance may be
obtained by setting a nominal Q1 drain current, but if you wish
to eke out the very best possible performance (that is, the best
signal-noise ratio) one can "tweak" to find the optimal drain
current for the specific device being used.
Doing this requires a simple test fixture that will provide a
small but consistent optical signal and a means of looking at
the receiver's output and discerning the signal-noise ratio and
for this, I constructed a "photon range" in a room in my house
that had no windows simply by attaching a normal red LED to the
ceiling and placing the receiver being tested on the floor
directly below. The LED was modulated with a square wave
at the test frequency, with a consistent (but small) amount of
current from a function generator - just enough to be perceived
to be glowing to the dark-adapted eye.
Connected to the receiver, via shielded audio cable, is a laptop
PC running the Spectran program. With this program one can
take measurements of the strength of the audio signal from the
receiver's detection of the modulated LED and compare it with
the noise floor.
This scheme has been used many times and with repeatable
results, but there are a few important factors to consider:
- The room being used for testing must be completely
dark! When running such tests, I even have to
unplug or cover the indicator lights of devices in the
room: Even that dim neon light or LED on a plug strip
can cause a "roar" of AC noise on a sensitive receiver!
- Always run such tests when it is dark outside.
Even though the room has no windows, there is enough light
around doors to other rooms to skew readings.
- Adjacent room lighting is turned off - for the same
reason as above!
- It is best to use a battery-powered, self-contained
tone generator, if possible. Even if you use
shielded cable to feed the LED on the ceiling, it is
possible that enough signal will couple from that wiring
into the receiver: Since the receiver being tested is
very possibly a prototype, it may not be as well-shielded as
it should be and would thus be more susceptible.
- When doing such tests, it is always
advisable to cover the receiver with a box to block the
light source and see that the detected signal
disappears. This is done to make absolutely
certain that the coupling of the signal into
the receiver is, in fact, via photon! (That is, if
the signal goes away when you block the light, you are
probably doing fine! If it doesn't, it's picking up
signal from the wiring! Trust me about this - I
drove myself crazy before I realized what was happening!)
- Always place the receiver under test inside a grounded
metal box. Because my "test range" is in a
utility room, grounded metal pipes are nearby and provide a
convenient earth ground. (My house is old enough
that the water pipes are metal and really are
grounded!) A simple, aluminum project box with
sides a few centimeters high suffices as this is typically
enough to screen the circuit from the stray AC fields that
are prevalent inside a typical house! Again, this step
is particularly recommended when one is testing a
"prototype" circuit that may be built "dead bug" or "ugly"
style and isn't well-shielded!
- The use of battery-operated laptop computer is
recommended. Sometimes one has to have the
computer "floating" in order to minimize pickup of hum and
operate it without any coupling into the
house's AC mains - that is, on battery!
- Make absolutely certain that the noise floor
you are seeing on your computer is that of the optical
receiver being tested and NOT that of the
computer's sound card!
- The easiest way to verify this is to check the noise
floor with the receiver powered up, and again with the
receiver powered down. If you do not
see a 10-20dB change in the noise floor, you may need to
increase the gain of the sound card, or you may need to
add yet more gain to the output of the receiver being
- Normally, I've found that using the "Mic" input with the
"mic boost" box check is sufficient.
- Finally, one should always do the "noise
floor" test described above for every
frequency at which you run tests, as the noise floor will
vary with frequency!
- Because these are measurements based on comparison
against noise, expect there to be significant short-term
variations in readings. These sorts of readings
often require a degree of averaging over a period of time
to minimize the uncertainty to to random noise spikes.
Barry, G8AGN has had excellent (and repeatable!)
results in constructing and using a "Photon Tube" to analyze
receiver performance. This consists of a meter or two of
black ABS sewer pipe of fairly large diameter (7-10cm) with
the receiver under test at one end and an LED at the opposite
Because of internal reflection within the pipe that can
effectively "magnify" the LED's intensity by reflection, it is
absolutely necessary to make the interior of
this pipe anti-reflective. A material suitable for this
purpose is black photographic "flocking" paper specifically
intended for this very sort of purpose. Unfortunately,
this paper can be a bit difficult to locate and expensive, but
it is well worth the effort. Somewhat less effective -
but perhaps usable - is black flocking paint for a similar
purpose or even flat black paint. Yet another method to reduce
spurious reflections is to install a number of baffles within
the pipe made from flat black material such as construction
paper or cardboard.
In the steps below, when setting JFET current
via R5 it is absolutely important that comparative
measurements be made with the photodiode in complete
When optimizing a receiver, one would typically set R5 at a
fairly high value (say, 220 ohms) and the parallel fixed
resistors of decreasing value (typically starting at 1k-2k)
noting the measured signal-noise ratio, the value of the added
resistor, and the voltage across R5 for each test. At some
point, further increases in Q1's drain current (e.g. lower
values at R5) will start causing a decrease in signal-noise
- While a potentiometer may be used for R5, be warned
that these devices can be extremely noisy - especially when
such low signal levels are involved due to the probability
of wiper noise! If a potentiometer is used, one must
always measure the value and substitute it with a
fixed resistor and re-check the signal-noise
ratio! If you don't get as good a signal-noise ratio
with the potentiometer as you did with a fixed resistor, you
can guess what the probably reason for this might be!
Note that "best" performance does not
coincide with maximum gain - which is why it is absolutely
that one is always measuring
signal-to-noise ratio! If, for example, you were to make a
circuit change that caused the signal being received to drop by
3dB - but your noise floor dropped by 6 dB - your receiver is
actually able to "hear" 3dB better, despite the drop in gain!
One important fact is that it is better to supply a bit too
current to the JFET from the current source
than too much! What this means is that with current that
is on the low side, the Q1's drain-source voltage will be quite
low - on the order of a volt or two at the most. If the
current supplied by the current source is too high (e.g. too low
a value of R5) then the drain-source voltage will rise: If
this happens, the cascode has less voltage margin to work
properly and this voltage is likely to be unstable, subject to
the effects of temperature on the FET and the current source
both - something which may make the optical receiver unusable
over certain temperature ranges.
The general recommendation is to determine the maximum current
possible (again, with the photodiode in complete darkness) as
set by lowering the value of R5 and noting the point at which
the drain-source voltage starts to rise rapidly. Taking
this value, R5 is than adjusted upwards in resistance so that
the current is reduced by 20%-25% or so, proving a reasonable
margin of safety with respect to variations of current related
How much difference in performance is there between an
"un-optimized" receiver and one that is fully-optimized?
That's a hard question to answer as it's impossible to know how
good or bad an "un-optimized" receiver really was. In
testing several receivers, the most S/N improvement that was
obtained was on the order of 3dB or so and in that instance the
Q1's drain current was increased substantially from the initial
value, reminding me just how much variation there really is
between individual JFETs of the same part number, from the same
manufacturer, and from the same bag!
A few observations about the receiver's operational
Because no feedback is involved in the operation of the
photodiode portion of the circuit in Figure 1
detector has an intrinsic 6dB/octave amplitude rolloff
characteristic above a few hundred Hz - the precise "knee"
frequency depending on the the capacitance of the photodiode,
the JFET and its cascode circuit, and stray circuit
capacitances. To compensate for this rolloff, U1B, a
differentiator circuit is used, flattening out the frequency
response over the voice bandwidth. It should be noted that
no real attempt was made to achieve maximally-flat frequency
response, but rather a selection of components that yielded
"good" sounding audio.
Below the diode's "knee" frequency, however, the differentiator
has the effect of acting as a highpass filter, reducing the
level of low frequencies (those below 300 Hz or so) but this has
little effect on the reproduction of speech. One advantage
that it offers is that the strong fundamental frequencies
associated with urban lighting (120 Hz and 100 Hz for 60 and 50
Hz power mains frequencies, respectively) are significantly
reduced, offering some degree of prevention of overload of the
audio amplification system used to listen to such
communications: Without this filtering it is possible that
when used near populated areas, the "hum" from urban light
sources could overdrive the speaker amplifier before a
comfortable listening level could be achieved!
The U1B differentiator circuit also has built-in low-pass
filtering, notably R10/C7 and R11/C8 in the feedback path.
These components, along with R6 and C6, work together to provide
high-frequency rolloff above 6-8 kHz or so - well above the
majority of the speech energy. Were the differentiator not
designed to have a reasonably high frequency cutoff frequency it
would, in theory, produce a tremendous amount of high-frequency
hiss owing to its 6dB/octave boost - something that would
actually distract the listener.
If a "flat" frequency response over a much wider range of
frequencies (say, for use with a mechanical television system or
with an FM subcarrier operating in the 10's of kHz) were the
goal then the circuit around U1B would look substantially
different, using a slightly different topology and different
component values, depending on the specific need.
Operation under "high light-level" conditions:
"No, the gate isn't
At first glance of
Figure 1 it may appear that the JFET's gate is
floating: IT IS NOT!
Note that the "cold end" (non-gate side) of the
photodiode may be biased to a rather high voltage and
were the FET of an insulated gate
type the potential would try to rise to roughly match it
- at least until it broke down! Since it is
a junction FET, the "gate-drain diode" junction will
conduct and keep the "hot end" of the photodiode to
within about a "diode's drop" of the drain voltage which
- for most practical purposes - is at drain (ground)
This does several things:
- This allows a bias to be established across the
APD, both reducing its capacitance and allowing its
internal amplification properties to be realized.
- The FET is turned "on." As expected, the
channel resistance of the FET drops with increasing
gate-drain voltage but what is not commonly realized
is that with most JFETs, the channel resistance will
continue to decrease even as the gate-drain voltage
goes positive. Once the gate-drain junction
"diode" begins to conduct, the device's resistance
will continue to decrease as the voltage will still increase
although you now have a diode there with its
expected curve! If you are skeptical of this
observation, the construction of a simple test jig
using almost any common JFET will bear this out as
demonstrated in the graph below:
Gate current versus drain current for a
typical JFET using measured values in a test
Click on image for a larger version.
As can be seen in figure 2 the current increases
exponentially with gate-source voltage in a "diode-like"
manner. Like bipolar transistor, the drain current
(akin to collector current) increases with gate
current (akin to base current), but it's in
linear proportion to the gate voltage rather
than the gate current! This feature is due
to the fact that the "gate-source" junction is
conducting and is doing so in a classic "diode-like"
For our purposes the JFET operates in this mode in a
manner much more "quietly" than a bipolar transistor
would if we were to simply drop one in its place in this
circuit, mainly due to the fact that noise currents are
a small portion of the FET's overall drain current
whereas they would be comparatively large in the case of
a bipolar tranasistor.
Although this graph doesn't extend far enough, this
"semi bipolar-like" property of JFETs is exhibited only
for very low gate currents as the FET itself is "mostly"
saturated at the point that a significant amount of gate
current (e.g. gate current >> gate-source
leakage current) begins to flow and there is
a limit as to how much drain current will flow and still
exhibit any resemblance to the curve above!
Under low-light conditions, the operational and leakage
currents of the photodiode aren't enough to "saturate" the JFET
and it continues to operate "normally" - even with a
high (>100 volt) bias on the APD (Avalanche Photo Diode) used in a test receiver.
If operated under conditions with higher ambient (or
incident) light, the bias voltage should be reduced as
much as necessary and R202 will provide ample protection
to the Photodiode/APD and FET to prevent either from being
damaged. It should be remembered that if there is
plenty of "extra" light, the extremely high sensitivity
of an APD-based isn't going to be required, anyway and
one might be better off using a different (and
Unlike many other optical receivers such as the K3PGP, VK7MJ and
even the "non-daylight" Version 2 receivers which tend to "slam"
to a supply rail and go mute, the "Version 3" circuit will
operate to a degree even with extremely high light levels,
albeit with altered frequency response and distortion
characteristics as noted below. As far as is known, this
circuit is one of the few optical receivers that is useful all
the way from the smallest amount of light to full noonday sun
where it has thusfar been used to span in excess of 20
kilometers (about 12 miles) with fairly good results!
One peculiar quirk of this circuit at higher levels of ambient
light is the fact that the frequency response becomes skewed and
the audio will begin to sound distinctly tinny. The reason
for this is that at higher levels of ambient light, the
photodiode becomes more and more conductive, and as this happens
the capacitance of the photodiode - the primary limitation of
high frequency response - is increasingly shunted by the lower
effective resistance, thereby improving high frequency
response. Additionally, as the photodiode current (along
with the gate current) increases, the gate impedance also drops,
further reducing the effects of the photodiode's
capacitance. Under very low-light conditions, the "knee"
frequency is around 150-250 Hz for the BPW34 (depending on the
photodiode's capacitance and the amount of reverse bias) and is
generally unnoticeable (except as a slight lack of "bass"
response) but with higher levels of light this "knee" moves well
into the middle of the audio range where the post-emphasis
effects of the differentiator become quite obvious in the audio.
With the increase of ambient light comes a dramatic increase in
noise as well - both from the photodiode itself (and possibly
the JFET) and the source of ambient light. While this
effect is, for all practical purposes, negligible in the voice
frequency range at very low light levels, it will eventually
become a roar of noise at much higher light levels - those at
and beyond the point where the audio becomes "tinny." For
this reason, if operating under conditions of high ambient light
or where the transmitting station is backgrounded by some light,
improved performance may result from adding a bit of optical attenuation
to the receiver!
Practically speaking, this effect has been most obvious when
running short-distance paths (under 10 miles or 16 kilometers)
using a 3-watt LED transmitter: One could modify the
circuit to accommodate such shifts in the "knee" frequency (by
modifying the R/C constants associated with U1B), but at such
short distances it is usually more convenient to either reduce
the power of the distant transmitter, put an optical attenuator
(such as a piece of cardboard or "pegboard" with a bunch of
holes in it) in front of the receiver's objective lens to reduce
the amount of energy reaching the photodiode, or a combination
Comment: It would be a fairly easy matter to
provide an extra control to adjust the "knee" frequency of the
differentiator to manually compensate for frequency response
differences as well as an extra resistor and capacitor to
recover the "lost" bass response - but the low-frequency
rolloff may be an advantage because of its tendency to
attenuate 100/120Hz hum from AC-powered light sources.
If properly biased, this receiver should function adequately
under high-light level conditions where the ultimate
sensitivity is determined by the presence of high levels of
As noted in the sidebar above
making the photodiode's reverse bias voltage adjustable can
allow in-field optimization under such conditions!
Comment about the use of a lower-capacitance photodiode at
If a lower-capacitance photodiode than the BPW34 is chosen by
the builder of an optical receiver, it should be noted that the
"knee" frequency will likely be higher than the 200-300 Hz
figure mentioned above. If one wishes to maintain a
relatively "flat" frequency response over the speech range, it
will be necessary to ascertain the new "knee" frequency of
circuit with the chosen photodiode and modify the differentiator
circuit (U1B) accordingly!
Noise and frequency response of the circuit:
As one might expect, the limiting factor of the sensitivity of
this circuit is the noise intrinsic to the components being used
- namely, D1 and Q1. As much as practical, these sources
of noise are minimized, but it is not possible to eliminate them
For the circuit in Figure 1
, the differentiator limits
the frequency response at the low end (below about 300 Hz) - but
if one were to look at the "pre-differentiator" audio (e.g. the
output of U1A) one would see that at very low frequencies (below
a few 10's of Hz) that the noise would increase - largely as a
result of the inevitable "inverse frequency" noise sources
associated with any electronic device: In the
circuit, these noise sources are made somewhat worse by the fact
that the photodiode (D1) is reverse-biased, but since the intent
was to optimize performance in the speech frequency range
(roughly 300-3000 Hz) this is of relatively little importance.
Looking at the "pre-differentiator" audio (the output of U1A)
across the speech range, one notes the expected 6dB/octave
amplitude rolloff of detected signals. If one looks at the
noise floor, however, one sees that it rolls off, too, but not
quite as fast. What this means is that given a constant
input of an optical signal, the signal-noise ratio will decrease
as the frequency increases
. With the
addition of the differentiator (U1B) the frequency response is
constant, but instead of the level of our test signal
decreasing, the noise floor will increase with frequency -
eventually reaching a point at which the noise energy is equal
to or greater than that of the test signal!
Again, a "perfect" differentiator would have a 6dB/octave
increase in gain up to infinity - which, of course is
impossible, not to mention undesirable - but since we are
interested in speech response, the differentiator's response is
intentionally limited to a "reasonable" frequency above the
speech range - specifically, around 6-7 kHz with the selected
values of R9 and C6. To further reduce "noise fatigue" of
the listener - and to better-remove high-frequency components
of, say, a PWM-based transmitter, additional low-pass filtering
is provided with the addition of C7/C8.
Use at very low frequencies
It is important to note that the circuit in Figure 1,
in its entirety,
intended for very-low (<200 Hz)
operation as might be encountered for extremely low-bandwidth,
slow signaling techniques such as those that might be
encountered using QRSS, WSJT, or similar modes. Here are a
few comments on why this might be:
- U1B, the differentiator, effectively rolls off low
frequencies below 200-300 Hz. Strictly speaking, it's
actually the fact that the "front end" (the signal path up
to the output of U1A) rolls off above this 200-300 Hz "knee"
frequency interacting with the differentiator. Because of
this, it is better to couple to the output of U1A (through
a capacitor and series resistor of 47-100 ohms) for such
"low" frequency use. If this is done, keep in
mind that while the frequency response will be "flat" to
200-300 Hz, speech will be muffled or "boomy" as the
intrinsic rolloff will attenuate the higher frequencies.
- If exclusively low-frequency operation is desired (below
200 Hz or so) along with the ultimate in noise performance,
it is recommended that reverse diode bias not be used. In other words,
- Reverse the photodiode. That is, connect the
cathode to the FET's gate.
- Ground the "cold" end of the photodiode - that is, the
- Increase the value of R4 (and the capacitance of C3 -
removing J1, of course!) to effect a slight negative
(relative to ground) gate voltage on Q1 - the precise
resistance being determined on a "photon range" for best
If this is done, note that higher-frequency (speech!) response
will suffer even more. Also note that one might want to
re-check Q1's drain current (doing so with very low test
frequencies) to verify optimal signal/noise performance,
especially since this can interact with adjustments of R4's
value. Finally, be aware that such a configuration is far
sensitive to ambient light and even very
small levels can completely saturate the detector!
Note the "Flat Audio" output in Figure 1
: This is
audio tapped off at the buffer from the JFET/Bipolar cascode
the differentiator consisting of
U1b which acts like a high-pass filter. If the input
impedance of the device following the receiver is fairly high
(as is typical of a sound card input) then the components shown
should be usable down to several 10's of Hz - the low end being
limited largely by the value of the coupling capacitor, C11 and
to a degree, by decoupling capacitor C4. (If the value
of C4 is increased significantly, be aware that the cascode
circuit may become unstable and require a reduction in its
gain which may be accomplished by decreasing the value of R6
along with appropriate adjustments of R5.)
If one's interests are mainly
200 Hz, the use of the K3PGP receiver (and its variants) is
recommended - see this page
for more information and links.
Use at higher frequencies (>3 kHz):
Refer to the drawing in Figure 1 for the following
Because of the lowpass response built into the differentiator
(U1b) it will aggressively roll off frequencies above 10 kHz
even if there is still
high-frequency energy at the
output of U1a. For speech purposes this was done to reduce
"noise fatigue" and to limit the action of the differentiator at
a reasonably high audio frequency (e.g. 5-7 kHz or so.)
While the capacitance of the photodiode and related circuitry is
limitation of the ultimate frequency
response to the circuit, it is still capable of responding to
signals into the 100's of kHz
- albeit with lower
effective sensitivity as the frequency goes up.
If you wish to conduct experiments at higher frequencies than
"audio" (e.g. above 5 kHz) then it will be necessary to obtain a
signal that is not
bandwidth limited - and such a signal
can be found at pin 1 of U1 - the "flat" audio output. In
reality, this signal isn't really "flat" as its ultimate
frequency response is dictated (mostly) by the capacitance of
the photodiode and its amplifier, but this port has nothing it
in that would explicitly limit the high frequency
response. It is at this point that one would connect the
input to a VLF receiver/converter or FM demodulator if one
wanted, say, to experiment with FM/SSB or other subcarrier
schemes in the 10's of kHz range.
The signal at this point is amplified only by virtue of the fact
that the cascode stage (Q1/Q2) has significant gain of its own
and is simply buffered (but not amplified) by U1a. In
testing, there was "usable" output from pin 1 of U1a up to about
1 MHz or so using a BPW34, but the sensitivity was very
reduced at the rate of at least 6dB/octave
above the "knee" frequency of the photodiode/JFET
combination: For example, if the "knee" frequency were 500
Hz, the sensitivity - from the R/C rolloff alone - would be
reduced by nearly 70dB by the time you got to 1 MHz!
Here is a summary of ways to improve the situation at
higher frequencies, but note that these will likely reduce the
ultimate sensitivity obtainable at "baseband" audio frequencies
(e.g. below 3 kHz or so):
- Bypass (or modify) the U1B differentiator circuit.
to roll off both low (<300 Hz) and high (>6 kHz)
frequencies and the "Flat audio Output" does this.
Alternatively, it is possible to modify U1B's operational
characteristics to extend the high frequency emphasis even
higher: You can send an email to me using the link at
the bottom of this page for more details. If you
use the "Flat Audio" output, note that the audio isn't
really "flat" at higher frequencies (e.g. those above the
"knee" of the photodiode circuit) but still roll off - but
they are not low-pass filtered as they would be in the
case of the differentiator.
- Use higher reverse bias voltage. A higher
reverse bias will reduce the diode's capacitance and improve
the frequency response. The amount of added noise due
to diode leakage will be of little consequence as that is
most dominant at lower frequencies (below several hundred
Hz) and the fact that as the frequency goes up, the diode's
capacitance shunts signals anyway. With a higher bias
voltage comes a penalty in that the receiver is less-able to
deal with ambient light conditions as this tends to increase
photodiode conductivity which can more-easily saturate the
amplifier: In extreme cases AC coupling (see an
example of this in Figure 2) may be used at
the expense of the loss of ultimate sensitivity - but since
sensitivity is reduced at these frequencies anyway, the
tradeoff will probably be worth it.
- Use a smaller photodiode. A
smaller-area photodiode has less capacitance. Also,
remember that the smaller the diode, the lower its
capacitance - and the better the high-frequency
response. When reducing the size of the diode one must
make sure that it is still large enough to accommodate the
"blur circle" of the lens being used (see the "Fresnel
Lens Comparison" page for additional info.
about the "blur circle") and that field of view with
the lens/diode combination is appropriate to your needs,
plus it is important to note that if it is also used for
speech purposes, you may want to tweak the values around U1b
(if you do, in fact, plan to use it) to flatten the
frequency response a bit. If the photodiode is smaller
that the blur circle of the lens, some receive efficiency
may be lost as some of the light from the far end is
"missing" the photodiode - but this may be an acceptable
tradeoff for improved high frequency response.
- Use an Avalanche PhotoDiode (APD). These
devices have their own self-amplification, but they can be
rather expensive - on the order of $150USD for a diode with
a 1mm2 active area - and they need a rather high
(100-300) bias voltage to work at maximum gain! The
diode's self-amplification can be used to overcome the R/C
losses and amplifier rolloff and in a test circuit,
reasonable sensitivity was obtained at 2 MHz using an
modified version of the circuit in Figure 7 with an
APD. Additional information about the use of an
APD may be found below. When sizing the
photodiode, note that smallest size that may be used is
limited by the "blur circle" of the lens being used.
- Modify the cascode and buffer amplifiers.
Eventually, the amplifier itself will start to limit
frequency response - but this effect is likely to be less
severe than the R/C rolloff at the photodiode. In
particular speed of the cascode circuit would need to be
increased and this is most-easily accomplished by decreasing
the value R6. Doing so will decrease the gain and
slightly increase the FET's current so some commensurate
adjustment of R5 may be necessary to prevent the FET from
"seeing" too much current. At least some of this gain
may be made up by increasing the power supply voltage to
12-15 volts which also improves the operating margin of the
LM833 - but going too high could cause the circuit to become
unstable. The LM833 is a fairly decent amplifier for
this application as it has good, low noise properties and an
excellent unity-gain bandwidth for a garden-variety
amplifier - on the order of 15 MHz which is higher than the
much noisier TL072! If high gain/high frequency is
your ultimate goal, there are special high-speed op amps out
there that will run circles around the LM833 - but you'll
need to look for them!
- Resonate the photodiode. One "trick" often
employed is to place a resonant circuit across the
photodiode. This would use the capacitance of the
photodiode on conjunction with a parallel inductor to form a
tuned circuit that was resonant at the frequency of
interest. The disadvantage of this is that it tends to
be rather narrowbanded and additional noise can be
introduced with the inductor. Since neither the
photodiode or the external coil have pure reactances (that
is, some loss is involved) they can contribute additional
noise. If broadband response is needed (e.g. video or
high-speed data) the "Q" of the circuit may prevent good
performance, although the "Q" could be spoiled somewhat with
the use of a paralleled resistor - but this adds back losses
and adds the noise generated by the resistor - either of
which can degrade performance! Again, if you do
use a resistor, use only a metal film device
to minimize noise contribution.
- Use a TIA. At some point it may be better to
simply use an TIA - a TransImpedance Amplifier. The
circuit shown in Figure 3 uses feedback to
compensate for the capacitance of the photodiode and can
achieve good frequency response - but it, too, does so at
the sacrifice of ultimate sensitivity, but it may be
better-suited overall, especially if your needs are for a
broadband response. Many of the tricks noted above
will apply to this circuit as well.
Why would you want to operate a "subcarrier" at
Good Through-hole FETs
are getting difficult to find these days...
If you are an avid builder, you have probably noticed
that over the past several years, through-hole JFET
transistors have almost disappeared from the catalogs of
manufacturers current offerings! You used
to be able to buy the venerable MPF102 from any number
of places for a low cost, made by a lot of different
manufacturers, but nowadays, you have to resort to
surplus places or even EvilBay. The "problem" with
Ebay is that unless you buy from a reputable vendor, you
are never quite sure what you are getting - even if the
parts that arrive have a particular number stamped on
Such is the case for the transistor described here, the
2N5457 as this, too, has disappeared from the
catalog in the through-hole version - but it is still
available in the surface mount
version: Look for the MMBF5457 (the
suffix will vary).
Don't let the idea of a surface-mount part scare
you: Just use a good pair of glasses, a
fine-tipped soldering iron and some tweezers - and buy a
few extra transistors.
As has been noted, ultimate sensitivity will be reduced as the
frequency increases due to device/circuit capacitance - so why
would one use a subcarrier?
If the optical path crosses urban areas, the "baseband"
frequency may contain mains harmonics (of 100 or 120 Hz) from
urban lighting that can degrade signals. These harmonics
tend to be strongest at the lower harmonics (300/360 Hz) but
they diminish as frequency increases, so "shifting" the audio up
by several kilohertz may move things away from the frequencies
Again, doing so will likely reduce ultimate sensitivity by
10-20dB - particularly if FM is used - but if one has plenty of
signal available - such as the use of a fairly short path - then
this may be an acceptable tradeoff.
If you do
use a subcarrier scheme, there are
several things to consider:
- You'll probably need to build an "AM" (e.g. "baseband")
transmitter and receiver anyway to connect it to your FM
modulator or SSB transverter/exciter/demodulator, so it
would be worth the minor inconvenience (adding a switch or
two!) to be able to switch between "AM" and "Subcarrier"
- You will need to build "extra" gear in addition to the LED
transmitter and optical receiver to do the FM/SSB
generation/conversion on transmit/receive. If you are
handy with building this, then that's fine, but remember the
"FM or SSB?"
- Expect that ultimate sensitivity will suffer
significantly. Again, if your link needn't be the
highest-performance (e.g. rather limited distances involved)
then all of this may be an acceptable tradeoff.
You can find, on the web, a number of schemes that use FM
subcarriers - most of them being examples of poor design - but
one of the best-conceived of these circuits is this
one by Max Carter
. Typically, an FM carrier is
modulated at a frequency between 20 and 100 kHz - well above the
baseband. As can be seen from the linked article,
modulation is pretty easy, but efficient demodulation is a bit
trickier: If one wants a high-performance system, bandpass
filtering of the carrier is important and every feature you add
contributes a bit to overall system complexity.
It is possible to use a receive converter (such as one intended
for "VLF" - Very Low Frequency - use) along with a
communications receiver (such as a ham rig) that has FM and
demodulate using that: This has the advantage the the
performance of such a receiver, with its built-in filtering, is
likely to work much better than a simple PLL-type FM demodulator
that one might build to operate at the subcarrier frequency
itself. If you go both ways, you can use the ham rig to both
receive and modulate a carrier if you have a "transverter"
available - but all of this assumes that you have a transceiver
that can do this along with you!
Again, with the higher frequency, performance degrades, but if
your link can tolerate 20-30dB reduction in effective
sensitivity, you can achieve excellent fidelity.
As was mentioned above, if you have a transverter (e.g. a
"transmit/receive converter") you can use an existing amateur
rig to modulate and demodulate the signals. In addition to
FM, you can also use other modes available to you - which may
include SSB. The advantage of SSB over FM is that it takes
much less bandwidth (about 1/6th) and this in combination with
the very nature of SSB means that signals 10-15dB weaker than FM
can be detected: As long as the FM signal has a 10dB or so
signal-noise ratio, it will be reasonably well "quieted", but
the trained ear can copy an SSB signal that much weaker and is
very near the noise!
Since SSB requires a lower bandwidth than FM (e.g. 2.5 kHz versus 15
kHz) you can actually set the SSB "carrier" frequency much lower than
with FM - as low as 3-5 kHz if the transverter is properly designed -
which minimizes the amount of sensitivity reduction due to device
capacitance in the optical receiver. SSB has the advantage of
efficiency over FM owing to the fact that it is, in fact, only as wide
as the audio signal itself and could be considered to be the AM voice
"baseband" frequencies simply shifted up. With the use of an SSB
signal and a well-designed mixer (e.g. one that is well balanced and
has minimum "bleedthrough" of the local oscillator) it is possible to
make use of the radio's noise blanker to reduce effects of AC-induced
noise as well as the radio's DSP filtering - if it has the capability,
that is! One advantage of SSB is that DC power to operate the
transceiver is reduced on the optical transmitter since, unlike AM or
FM, no "carrier" (constant light) is required during periods of no
Again, the use of SSB or FM would imply that one has equipment
to use and could be - as suggested - be in the form of a VLF
"transverter" to work with one's portable HF multi-mode
transceiver. Of course, all of this assumes that you have
a transceiver to use and have built the necessary converter gear
to go with it. Again, since higher frequencies are used,
expect 10-20 dB reduction in effective sensitivity, but if your
link isn't all that far (a few 10's of km) then there should be
enough margin under "clear air" conditions.
A few additional comments on modifying the circuit for
higher-frequency operation may be found here.
Constructing the circuit:
of this optical receiver
Ron, K7RJ, has been helpful in the
testing of the "Version 3" receiver design for several
years, having been involved in many of the short and
long-distance communications experiments.
Because of his interest, he has done something that I
have not: Designed and laid out a circuit
board for the receiver.
Using an "online" circuit board manufacturing house,
he has also had several boards made for these
receivers, which have also been tested.
Furthermore, Ron has put together a web page that has
documented his efforts and includes information on how
can get boards made.
page describing his receiver may be
found here: (link)
Note that the schematic on Ron's page differs in some
very minor ways with the one shown in Figure 1
(above) and that parts designations (e.g. "R1", "C5",
etc.) are different, but all of the other comments about
the circuitry made on this page apply to his version -
with the appropriate parts translations, of course!
Also note that Ron is NOT set up
to supply people with boards directly, but this page
provides all the information needed to download the
design, download the (free!) board layout program, and
get you to the point at which you can take your credit
card and get some boards (6 of them) made for you and
your friends! Also included on this page is a
parts list and a few pictures pertaining to some of the
more-important details of the receiver's construction.
Note that Ron's page includes several pictures and
practical advice pertaining to how the photodiode is to
be mounted: These techniques are valid no matter
what means of construction you end up choosing!
As simple as the circuit is, there are a few key points that
must be made pertaining to its construction:
- Use "high frequency" (VHF) wiring techniques.
Even though this circuit operates at "audio" frequencies, treat
it as if it were a VHF/UHF radio circuit, minimizing
lead length and maintaining solid component support,
- Impedances, especially around D1 and Q1, are extremely
high! Excess lead length not only increases the
possibility of the pickup of stray signals (such as hum!)
but it also implies higher capacitance - something that
you want to minimize!
- In a circuit such as this were there is both high
impedance and sensitivity to capacitance, this circuit can
- that is, it will respond, electrically, to mechanical
vibrations - like a microphone. This is an
unavoidable result, but keeping leads fairly short and
using mechanically solid mounting for the most sensitive
components can greatly minimize this!
- This circuit should be shielded. As mentioned
before, the extremely high impedances and sensitivity
involved make it susceptible to being affected by stray
fields - which could be RF, electrostatic or magnetic.
While the aforementioned "high frequency" wiring techniques
go a long way to preventing problems, it is still a good
idea to enclose the circuitry in a metal box and, if
practical, bypass the signal leads going in and out to
prevent conduction of signals on these leads into the
circuitry - especially if you are planning to use
transmitters (e.g. handie talkies, cell/mobile phones, etc.)
Operational voltage range:
- The connection between the anode of D1 and the gate of
Q1 should be done in the air, and be kept as short as
- Due to the extremely high impedances, combined with low
signal levels, there should be nothing else
connected at this junction!
- One should take care that this connection is kept free
of flux, dust, and other contaminants, as these can cause
leakage paths - which can contribute noise! Clean
the junction with denatured alcohol when you are done to
- Do NOT make this connection using a circuit board
trace or pad! Doing so will not only add capacitance
to this portion of the circuit, but it also be a potential
leakage path - a noise source!
- For pictures and descriptions on how to properly mount
and connect the photodiode, see Ron's
web page referenced in the sidebar above.
With the components shown in Figure 1
, the circuit is
designed to operate properly from about 8 volts to 12
volts. At and below 7.5 volts, the receiver performance
begins to degrade, so one should keep spare 9 volt batteries
Note that the front-end circuit (Q1, Q2, Q3) does not
have good power-supply rejection which means that any hum or
audio that is riding on the power supply you use will likely
appear in the audio output. This can become an issue if an
AC-operated supply is used and/or if an audio amplifier or other
device is operated from the same supply as the receiver.
When the supply voltage rises above 12-13 volts, instability
arises in the cascode circuit: If one anticipates
operating exclusively at such voltages, the resistor values in
the cascode stage should be adjusted - or simply add a regulator
to keep it in the 9-10 volt range! Note that only
the "front end" portion of the circuit (Q1, Q2, Q3) would need
to be regulated in such cases as the op amp (U1) will happily
operate at these higher voltage and provide good power supply
- Using a fresh alkaline 9 volt battery, the receiver
typically draws between 10 and 20 milliamps with the
component values shown in Figure 1.
- It is recommended that this circuit be operated on its own,
independent power supply! If you decide to
run this circuit from the same power supply as the speaker
amplifier, transmitter, etc. then you are asking for
trouble as mixing an extremely sensitive
receiver with the possibility of ground loops is a recipe
for disaster! Remember: 9 volt batteries
- Remember to bring spare batteries with you!
On more than one occasion we've forgotten to turn off the
receiver when we were done, finding a dead battery next time
we tried to use it. Fortunately, we have (thusfar)
followed our own advice!
- It is recommended that you NOT put this
receiver in the same box with any other circuitry -
especially an audio amplifier and/or speaker. As
mentioned above, the audio-modulated currents from an audio
amplifier will likely find their way into the
ultra-sensitive front-end - plus the fact that the receiver
itself will inevitably be somewhat microphonic and pick up
the speaker's vibrations, likely resulting in feedback!
Miscellaneous comments on the receiver design:
(Note: An attempted minimally-technical description
Because the goal was simply to allow reception of speech,
optimal performance was desired in the 300-3000 Hz range.
With this in mind, there are several observations that one might
- Less concern with 1/F-type noise. This type of noise
sources increase with decreasing frequency, becoming
most severe at very low (<100 Hz) frequencies.
- With a high end of only 3000 Hz or so, we need not be as
concerned with making sure that our frequency response
extends much above this.
As it turns out, the above two points actually have something to
do with each other: One of the main limiters in the upper
frequency response of an optical receiver is the intrinsic
frequency response of the detector being used. Let us
take, as an example, the BPW34 PIN photodiode. This is an
inexpensive device ($1.00 US or less in small quantities) that
offers reasonably good performance. It is also a
"medium-area" photodiode - that is, it is neither very small or
very large, having an active surface area on the order of 7.5
One unchangeable fact is that the larger the diode, the more
self-capacitance that it exhibits. In the case of the
BPW34 at zero volts, it has a capacitance of around 70pF.
While this amount of capacitance may not sound like much -
especially at audio frequencies - one must appreciate the very
high impedances involved. For example, if the circuit's
impedance were about 10 megohms, one could reasonably expect
that frequencies above about 225 Hz would be attenuated at 6dB
When one is trying to detect extremely weak signals the primary
limitation of the detection system is likely to be noise:
If the level of the signal being sought is below that of the
intrinsic noise of the receiver itself, it will be lost!
It makes sense, then, that in a receiver system, that one try to
preserve as much of the original signal as possible. As
can be seen from the above, by the time we reach 225 Hz, the
signals impinging on the photodiode are already being attenuated
and dropping toward the noise floor!
One way to reduce this attenuation is to decrease the
capacitance of the photodiode. In addition to simply using
photodiode, one can also put reverse bias on the diode to
exploit a property in which the capacitance decreases with
increasing voltage. For example, the same BPW34 diode that
has a capacitance of about 70pF at zero volts will have a
capacitance of about 25pF at just 3 volts, dropping to about
10pF at 25 volts. Taking the example above, this would
raise the "knee" frequency from 225 Hz at zero volts bias to
about 625Hz at 3 volts and about 1600 Hz at 25 volts.
Ideally, going from 0 volts bias to 25 volts would improve the
level of the recovered signal at, say, 1000 Hz by roughly
15dB. (Note that the above example is illustrative
only and doesn't take into account other sources of
capacitance, such as wiring and the JFET itself.)
The caveat to increasing the bias voltage on the photodiode is
that leakage currents also increase - which also results in
increased noise! Fortunately, the majority of this
increase in noise occurs at low frequencies, mostly below 300
Hz, so it has only minimal impact on the higher (speech)
frequencies. For the most part, the increase in the
diode's noise at speech frequencies caused by the higher bias
voltage is much less than the amount of signal gained by the
reduction of the device's capacitance!
Why not use a feedback amplifier?
Typically, a TransImpedance
is used in conjunction with a
photodiode for detection of weak signals. When properly
designed, this offers flat frequency response up to a "knee"
frequency - which is dependent, in large part, to the amount of
feedback being applied and how it relates to the photodiode's
capacitance. In this way, the effects of the diode's
capacitance on frequency response can be greatly mitigated, as
it is the properties of the photodiode and the amplifier's
closed loop gain and the compensation of that loop that
establish the available bandwidth. If you needs require
flat frequency and phase response above ultimate low-bandwidth
sensitivity, a TIA may be the better choice.
While a TIA is an attractive option, it has the downside of
requiring the addition of additional components in the signal
path - each component being a potential source of noise.
For example, a TIA circuit has a feedback resistor through which
current flows to close the loop - but not only does the current
flowing through this resistor produce noise on its own, but
noise produced by the amplifier circuit itself (and its
associated components - including the feedback resistor) is also
fed back into the input, further contributing to signal
Generally, when building a TIA one uses a low-noise operational
amplifier. As it turns out, however, even specialized "low
noise" op amps can be noisier in this situation than a discrete
junction FET device! For this reason, it is not uncommon
to see extremely low-noise TIA circuits that use low-noise op
amps preceded by garden-variety JFETs - or even ones that avoid
the use of op amps entirely.
Light DX Receiver Circuitry" on
Light DX page. These pages contain a
wealth of information on related topics.
Amplifiers - Turning Light into Electricity
- From National Semiconductor, an online seminar
about various aspects of using photodiodes and how to
amplify their output. This page links not only to
some .PDFs of slides and transcripts of the seminar, but
it also has an online video of the original
presentation. Related to this topic is National
Semiconductor's application note AN-1244
which also contains information about this same topic.
All This Transimpedance Amplifier Stuff, anyway?"
- This is an article by Bob Pease that appeared in the
January, 2001 issue of Electronic Design that discusses,
among other things, the sources of noise in
transimpedance amplifiers and techniques to deal with
- Linear Technologies Corp. has some useful
information on the design of photodiode amplifiers,
Photonics has, on its website, a number of
papers about the "what and how" of many types of optical
components. For more info, look at:
- The Technical
Notes page. This page describes the
general theory behind the operation of many types of
optical devices, such as photodiodes, photomultiplier
tubes, and many more devices.
- The Application
Notes page. This page has a number
of articles describing how optical devices are used in
the real world.
- The Hamamatsu Document Libraray
- This links to a search for documents on various topics, but it does
not have a "browseable" list of titles with descriptions that I could
Diminution of receiver
sensitivity at higher frequencies in an inevitable consequence
of the capacitance of any photodiode/amplifier circuit.
Typically, a "TIA" (TransImpedance Amplifier) circuit is used in
wide-bandwidth receivers and a combination of feedback signals
and capacitance is used to flatten the frequency response as it
goes up - but this does not
high frequency sensitivity! What it does
"flatten" it out - but once the you get past the design
frequency response of the TIA, rolloff occurs.
In the "Version 3" non-feedback circuit described here, NO
feedback is used so the frequency starts rolling off immediately
- past several hundred Hz, typically. With proper design,
however, while the absolute signal level may drop with
increasing frequency, it will still be present. Without
additional feedback circuitry, however, it may be that the signal-to-noise
at higher frequencies may be better
with this circuit than with a TIA circuit as there are fewer
components and signal paths that can degrade the overall
signals. In other words, if its not
important that the amplitude response be perfectly flat, this
circuit may suffice. An example of this would be the use
of an FM or SSB subcarrier above the speech frequency range
(e.g. above 3 kHz or so) in which some amplitude rolloff with
frequency isn't going to be a particular problem.
It is worth mentioning that possible to "tweak" the circuit in
several ways if one wanted to forgo the use of "Speech Range"
operation in favor of higher-frequency (>5 kHz)
operation. If you are interested in this sort of
operation, you can contact me at the link below.
to the KA7OEI Optical communications Index page.
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