# What are stray capacitance and parasitic capacitance

What does a circuit look like that can reduce the bandwidth and the effects of the noise of the photodiode from the outside or with a parasitic capacitance?

Photodiodes transform a basic physical phenomenon (light) into electrical form (current). Electronic components convert the current of the photodetector into a usable voltage, whereby the changes in the signal of the photodiode can be controlled. There are a number of different approaches to the problems with photosensitive circuits. A reader asked about a circuit that could reduce the bandwidth and the effects of noise from the photodiode from outside or with a parasitic capacitance.

Fig. 1: Classic circuit of a light-sensitive system

The classic circuit of a light-sensitive system consists of a photodiode, an operational amplifier and a pair of feedback resistor and feedback capacitor at the front end (Fig. 1). In this circuit, the bandwidth is limited by the photodiode, the amplifier and the feedback capacitance.

When measuring light with a photodiode which has a large parasitic capacitance or which is far away, there is consequently a large capacitance across the input of the amplifier. As a result of this additional capacitance, the noise gain of the circuit increases as long as the feedback capacitor is not increased. When the feedback capacitor (C.F.) is increased, the bandwidth of the circuit decreases.

Figure 2: Removal of the diode capacitance and line capacitance by means of bootstrapping

You can use a bootstrap circuit to solve this problem (pic 2). However, photodiodes with a relatively low diode capacitance do not benefit from this circuit. A unity gain voltage follower, A.2, removes the line capacitance and thus the parasitic capacitance of the photodiode from the input of the transimpedance amplifier, A1.

In designing this circuit you have a relatively free choice as to what type of amplifier for A2 concerns. Only four specifications are important here. These development guidelines include that the selected amplifier has a low input capacitance, low noise, higher bandwidth than A.1 and has a low output impedance.

In this circuit, the input capacitance is A.2 the only capacitance that plays a role in the AC transfer function of the transimpedance system. The input capacitance of voltage follower A.2 replaces the sum of the input capacitance of A.1, the line capacitance and the parasitic capacitance of the photodiode. As a rule of thumb, CA2 << (CA1 + CCA + CPD), where CA1 and CA2 correspond to the sum of their input differential and common mode capacitance.

However, a noise problem (A.1) by another (A.2) replaced. The voltage follower removes the noise effect from A1. As a rule of thumb, the noise of A2 <= A1 is.

In this system, the difference between the input and output signals drops across the line / diode capacitance. You can keep this difference low by using A2 with a higher bandwidth than A1 and select the output impedance of A2 keep it low. By decreasing the gain of A2 there is an upper limit for the bandwidth improvement. The bandwidth ratio between the amplifiers is A.2-BW >> A1-BW. This circuit requires optimization of stability by calling CF. to the input capacitance of A2 align.

Bonnie C. Baker, Texas Instruments

literature

[1] Baker, B .: "The eyes of the electronic world are watching", EDN, Aug. 7 (2008)

[2] Baker, B .: "Transimpedance-amplifier stability is dye", EDN, Sept. 4 (2008)

[3] Graeme, J .: Photodiode Amplifier, McGraw-Hill, ISBN 0-07-024247-X

[4] Kurz, D., Cohen, A .: "Bootstrapping Reduces Amplifier Input Capacitance", EDN, March 20 (1978)