Utilizing light sensing circuits to reduce photodiode bandwidth and noise effects

Photodiodes convert a basic physical phenomenon (light) into electrical form (current). Design engineers systematically convert the photodetector current into a usable voltage, making the processing of the photodiode signal easily controllable. There are many ways to deal with light sensing circuit problems, but I ran into a particular problem. How to use a circuit that can reduce the photodiode bandwidth and noise effects remotely or through a large parasitic capacitance.

A typical light sensing system circuit has a photodiode, op amp, and feedback resistor/capacitor pair on the front end. This article will start with the circuit introduced in the previous article. In this circuit, the photodiode, amplifier, and feedback capacitive components limit the bandwidth of the circuit.

When implementing light sensing through a photodiode using a large parasitic capacitance or at longer distances, the amplifier input has a large input capacitance. This increase in capacitance increases the noise gain of the circuit unless you increase the feedback capacitance of the amplifier. If the feedback capacitance (CF) increases, the bandwidth of the circuit decreases.

To solve this problem, you can use a bootstrap circuit (see Figure 1). Photodiodes with lower diode capacitance do not benefit from this circuit. Unity-gain buffer A2 removes the cable capacitance and photodiode parasitic capacitance created by the transimpedance amplifier A1 input.

Figure 1: Bootstrap diode capacitance and cable capacitance freed from transimpedance design issues.

When doing this circuit design, the choice of A2 amplifier type can be slightly relaxed. Only four performance specifications are important. These design principles include choosing an amplifier with low input capacitance, low noise, a bandwidth greater than A 1 , and low output impedance.

In this design, the input capacitance of A2 is the only capacitance that plays a role in the AC transfer function of the transimpedance system. The buffer input capacitance replaces the sum of the A 1 input capacitance, cable capacitance, and photodiode parasitic capacitance. A good rule of thumb is C A2 << (CA1 + CCA +CPD), where CA1 and CA2 are the sum of their input differential and common-mode capacitances.

Using this design, you can interchange one noise problem (A 1) with another (A 2). The unity gain buffer eliminates the noise effects of A1. A better principle is to let A 2 noise «= A 1.

In this system the difference between the input signal and the output signal is reduced when it encounters the cable/diode capacitance. You can maintain this low signal difference by choosing A2 with a larger bandwidth than A1 and keeping A2's output impedance low. The A 2 gain rolloff places an upper limit on the bandwidth improvement such that the bandwidth relationship between these amplifiers is equal to A 2-BW >>>> A 1-BW . This circuit requires stable optimization as you balance the CF and A2 input capacitances.