Photoresistor provides negative feedback to an op amp, producing a linear response
A photoresistor, as part of an optocoupler, provides negative feedback to an op amp, producing a linear response.
Julius Foit and Jan Novak, Czech Technical University, Prague, Czech Republic; Edited by Martin Rowe and Fran Granville -- EDN, May 27, 2010
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AGC (automatic-gain-control) amplifiers use the nonlinear characteristics of control devices. The magnitude of the real component in some of their differential parameters changes depending on variations in their dc operating points. A typical example is the VA characteristic of a silicon PN junction, which results in the differential conductance directly proportional to the passing dc current (Reference 1). In this form of control, the main problem is the control element's nonlinear transfer characteristic, which causes a relatively large degree of nonlinear signal distortion once the processed voltage amplitude exceeds millivolts (Reference 2).
A photoresistor, which has a VA characteristic that's linear in a large range of voltages, is up to the task. Common photoresistors remain perfectly linear for signal amplitudes of 100V or more. Therefore, the amplification-control device can be an optocoupler whose controlled element is a photoresistor. The circuit in this Design Idea uses a radiation source whose spectral characteristic fits the spectral characteristic of the photoresistor, and its radiated power should, if possible, be a linear function of the drive signal. Such optocouplers are commercially available, but few have properties good enough for this purpose. Common photo-resistors have spectral characteristics close to the spectral characteristics of the human eye, whose peak sensitivity has approximately a 500-nm wavelength. So a white or green LED (light-emitting diode) is a good alternative. To obtain the highest possible sensitivity, this circuit uses a white HB (high-brightness) LED.
Figure 1 shows the individual components of the optocoupler
and the assembled device. The optocoupler comprises a cylindrical holder that
accepts a standard 5-mm HB LED from one end and a photoresistor at the other
end. An opaque nonconductive seal prevents external light from entering the
device. The polished metallic inner wall of the holder results in minimum light
loss between the LED and the photoresistor. Available off-the-shelf
photoresistors include the LDR 05, the LDR 07, and a standard white, 5-mm HB LED
type L-53MWC*E, with output-light flux of 2500 mcd at a 20-mA drive current
(Reference 3).
Figure 2 shows the transfer function of the optocoupler
using the LDR 07-type photoresistor. The output resistance of the device can
vary from 100Ω to 10 MΩ with LED-drive currents from 34 mA to 0.1 μA,
respectively. The photoresistor's linear VA characteristic, even for
large-amplitude signals, lets you use it as the control element even in
situations that require a relatively large signal voltage, such as when the
photoresistor is part of the feedback loop of an operational amplifier. Figure 2 also shows that you can obtain a variation of linear
output resistance over at least five decades with a maximum LED-drive current
within the limits of permitted output current of common monolithic operational
amplifiers.
Such an amplifier can control the overall amplification of the system in the same range without additional current amplification. Due to the photoresistor's linearity, the resulting degree of processed signal nonlinear distortion is almost solely due to the nonlinearity of the operational amplifier. Within the normal operating range, the overall linearity of the system improves with increasing input-signal amplitude because the amount of negative feedback increases with increasing signal amplitude.
Figure 3 shows the amplifier system. The basic
signal-processing device is inverting op amp A1. Its inverting
connection lets you set the absolute value of the overall amplification from
input to output to a value smaller than unity, permitting correct processing of
an input-signal amplitude even larger than the regulated output value.
Optocoupler IC1 is the core component of the system, whose output,
the photoresistor, serves as a variable part of A1's
negative-feedback network. At no-signal conditions, the LED does not illuminate
the photoresistor. Thus, its resistance rises to a high value, which can cause
dc runaway and the loss of the quiescent operating point of A1. Such
a condition is not harmful in principle because the signal path is ac-coupled,
preventing the dc error value from getting any further. When a nonzero signal
suddenly appears at the input, however, A1's open-loop amplification
would amplify it, causing a rapid rise in LED current. This action would drop
the optocoupler's output resistance almost stepwise to a value sufficient to
restore the dc operating point of A1. The ac coupling transfers this
transient to the output, and it may cause problems in signal-processing circuits
following the adaptive amplifier. To prevent this effect, you should limit the
maximum value of the feedback resistance to a reasonable value, such as 47 MΩ,
the value of R6. Because the op amps have JFET inputs, the value of
R6 can be rather high. The value of 47 MΩ is a reasonable compromise,
limiting the maximum absolute value of voltage amplification in A1 to
approximately 82 dB. The limiting factors for selecting a value for
R6 are the noise and the open-loop amplification of
A1.
Buffer A2 separates the nonlinear load through the rectifying diodes from the output signal, thus preventing the nonlinear load from the rectifying diodes from distorting the output signal. Diodes D3 and D4 compensate the threshold voltage, including its temperature coefficient, of rectifying diodes D1 and D2. If you do not need to set the regulated output-voltage amplitude to a value smaller than the threshold value that the bias current in R4 sets, you can replace D3 and D4 with a short circuit and omit R7. You can set a larger-than-unity voltage amplification in A2 to obtain a regulated output amplitude lower than the threshold that the bias in R4 sets. Just insert an additional resistance in series with the D3/D4 pair.
The rectifier uses Schottky diodes, which have a lower threshold voltage than conventional PN diodes. They also have a short recovery time, keeping the same rectification efficiency at high signal frequencies. The rectifier operates as a full-wave voltage doubler, providing peak-to-peak rectification even for signals with nonsymmetrical waveforms. The rectifier output feeds to A3, a voltage-to-current converter, which drives the LED in the optocoupler. A rectification threshold-shifting bias-current source connects to current-sensing resistor R4. In this case R5 simulates a current source, setting the regulated output-voltage amplitude. If the 15V supply voltage isn't perfectly stable, obtain bias current from a separate stable source. An opposite-polarity diode connects across the optocoupler's input to protect the LED from reverse polarization at no-signal conditions.
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Figure 4 shows the overall response of the adaptive
amplifier system. The output signal remains constant at 350 mV rms ±1 dB for
input-signal voltages of less than 70 μV rms to more than 1.2V rms-that is, over
a more-than-85-dB range. The no-signal output noise is less than 6 mV rms,
yielding an SNR (signal-to-noise ratio), or processed-signal dynamic range,
better than 20 dB at the onset of regulation in the worst-case condition and
improving proportionally with increasing input-signal level.
The key parameter this design follows is its linearity. Because of the photoresistor's linearity and the separation of the nonlinear rectifier load from the output, the gain control introduces negligible nonlinearity. Thus, A1 alone, in principle, determines the overall linearity of the system.
Harmonic analysis of the output signal at 1 kHz yields higher harmonics with amplitudes lower than A1's noise level for all input voltages to 200 μV rms and below 275 dB for input voltages to 1.5V rms. The nonlinear distortion becomes noticeable only at large input amplitudes exceeding the regulation range of the system, raising the second harmonic to -45 dB and the third harmonic to -40 dB at 2.5V-rms input.
Within the AGC's range limits, the overall transfer linearity improves with increasing input-signal amplitude due to the increasing degree of negative feedback to A1 at increasing input-signal amplitudes. With a value of 10 kΩ for P1 and 1 MΩ for P2 and a stepwise input-signal variation between 100 μV and 50 mV rms, the attack and release times are approximately 0.2 and 2 seconds, respectively. The recovery time from a 1-kHz-more than 10V-rms input overdrive-to full no-signal sensitivity is less than 2 minutes. You can adjust all of these time intervals in a wide range by varying the values of C4, C5, P1, and P2, with P1 setting the attack time and P2 setting the release time.
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Talkback
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The image in this URL has a 2006 dateline but Dave does not claim the idea as original.
wireless.org.uk/circuits2.htm
It has been on his site since about 2000. I am afraid being a regular reader of design ideas I have seem many from the 1960s and later reheated usually by university students. I guess there is a possibility they didnt see the original but many of these ideas are in standard text books too. Sad.
Alan Melia - 2010-8-6 14:54:53 PDT -
The idea may be new to the gentlemen that submitted it. Every decade, someone utters the word "peace" and claim that is a new idea. Get the point?
Dave Amos - 2010-8-6 14:45:40 PDT -
I have designed compressor- limiter when at Fairchild Recording Equipment Corp. over 50 years ago and we have used 741 opamp with Cadmuim Sulfide cell in FB loop with LED in power feed. Compression ratio was 40/1 over 40 db range. Distortion was less than .05%. By selecting fast cells, one could achieve very fast response, excellent for microphone circuits and other audio signals. Cost of this circuit then was less than a Dollar. Johny Carson studio consoles we have built used many optical controls with Cadmium Sulfide Cells.
George Alexandrovich Sr - 2010-4-6 07:09:56 PDT -
Shame on the editors for accepting decades-old recycled ideas as "unpublished and original". National
Semiconductor's AN-20 "Analog Multiplier" describes a
linearized photoconductive cell in a feedback loop, this has been in print since the 1970's. Google search for "compressor schematics" and find many commercial and hobby circuits using the same technique (Craig Anderton among others). I wish the Ideas for Design were given the same careful consideration that the late
Bill Travis once brought to EDN magazine.
Harry Bissell - 2010-3-6 09:33:51 PDT -
This is an interesting circuit, although the concept is indeed not new by any means. I designed and built a tube-type compressor circuit in 1964.
I offer two concerns, the first is that the apparently flat output amplitude Vs input amplitude curve is not quite believeable, also, there certainly will be a bit of instant overload for any sharp increase in input, since there is definitely a time constant in the gain controller circuit.
What I really wonder about is the frequency response, does that change with gain? And what would be the application intended for this design.
The explanation of how it functions is indeed very good, I would give this project a good grade.
William Ketel - 2010-1-6 17:10:36 PDT
