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Photoresistor offers negative feedback to op amp

02 Jul 2015  | Julius Foit, Jan Novak

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Automatic-gain-control (AGC) amplifiers utilise the non-linear 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 non-linear transfer characteristic, which causes a relatively large degree of non-linear 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: A metal tube with an HB LED and a photoresistor forms the optocoupler (left).

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 0.1µA, 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.

Figure 2: The optocoupler's logarithmic response in a feedback loop produces a linear amplifier response.

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