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Photoresistor offers negative feedback to op amp

02 Jul 2015  | Julius Foit, Jan Novak

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This LED current-control circuit has an important advantage: It permits an almost-independent adjustment of the attack and release time. You can adjust the attack time through variable resistor P1, using a higher value if necessary. You can also adjust the release time using P2. The photoresistors used have a rather good response speed, and the introduced delay at a stepwise illumination variation is acceptable for most practical requirements.

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 70µV 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.


Figure 4: The amplifier system has a constant output from 0.1 mV to 1V-rms input.


The key parameter this design follows is its linearity. Because of the photoresistor's linearity and the separation of the non-linear rectifier load from the output, the gain control introduces negligible non-linearity. Thus, A1 alone, in principle, determines the overall linearity of the system.

Harmonic analysis of the output signal at 1kHz yields higher harmonics with amplitudes lower than A1's noise level for all input voltages to 200µV rms and below 275 dB for input voltages to 1.5V rms. The non-linear 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 100µV and 50 mV rms, the attack and release times are approximately 0.2 and 2 seconds, respectively. The recovery time from a 1kHz-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.


References
1. Foit, Julius, "AGC amplifier features 60-dB dynamic range," EDN, Aug 4, 2005, pg 87.

2. Foit, Julius, "Logarithmic Processing Amplifier," Proceedings of the Fifth WSEAS International Conference on Microelectronics, Nanoelectronics, Optoelectronics, March 2006, pg 6.

3. Opto-isolator Catalogue, Tesla Blatná.


About the authors
Julius Foit and Jan Novak are with Czech Technical University in Prague, Czech Republic.


This article is a Design Idea selected for re-publication by the editors. It was first published on May 27, 2010 in EDN.com.


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