Electronic Design

Reactance Circuit Stabilizes Photosensor, Eliminates Ambient Light Interference

Operation of a silicon photodiode as an optical receiving element is typically accomplished using the reverse-biased mode at a specified bias voltage (Fig. 1). Incoming light is converted by the photodiode D to a photocurrent, causing a voltage drop at resistor R. Thus, the output voltage of the operational amplifier is proportional to the amplitude of the incoming light. If modulated light is used for transmission, the interference caused by the ambient light can be eliminated by filtering the output voltage, UA.

To achieve high sensitivity, a large value of resistor R is necessary. That’s because large output voltages of the op amp could occur due to ambient light interferences. Either the value of R has to be reduced, with loss of sensitivity, or a high supply voltage must be used for the op amp.

By using modulated light, these disadvantages can be avoided if R is replaced by an inductance L or a parallel resonant circuit. The disadvantage here is the inductor’s size and its high sensitivity to electromagnetic interference.

These disadvantages can be avoided if the inductance is simulated by the reactance circuit shown in Fig. 2. The 90° phase shift of an RC element and a transistor’s 180° phase shift result in an inductance-like phase shift of 90° between voltage U1 and current I1.

The receiver circuit presented in this Idea for Design is used by our company in a laser navigation system for a Mars rover. This system operates with modulated laser light at a frequency of 10 kHz.

Two reactance circuits work as load resistances for a photodiode, BPW34 (Fig. 3). The voltage drop at each reactance circuit is amplified by a differential amplifier, built with two 2N4416 FETs. This symmetrical design makes and a diode bias voltage of about 2 V,which is nearly independent of ambient light conditions. the circuit insensitive to commonmode interference. The circuit operates using a single 5-V power supply.

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