High transimpedance gain applications require very large R_F to be used, which can give rise to potential stability problems. R_F interacts with the input capacitance (C_IN) of the op amp, the photodiode capacitance (C_PD), and stray PCB capacitance to create a low-frequency zero (f_Z) in the noise gain transfer function (1/β) as illustrated in Figure 7-13. Remember that C_IN includes the differential (C_DF) and common-mode (C_CM) capacitance of the op amp. The typical value of C_DF and C_CM are found in the Electrical Characteristics. The zero in 1/β causes the gain to increase over frequency and is the basis for instability problems. To counteract the zero, add a compensation capacitor (C_F) in the feedback loop to create a pole (f_P). Increasingly, larger R_F requires a decreasingly lower capacitor to remain stable. In some cases, parasitic capacitance from the resistor and PCB layout can alone be sufficient to maintain stability.

Figure 7-13 Transimpedance Amplifier Noise Gain

The optimized selection of C_F depends on several parameters and extensive literature exists on this topic. Equation 2 provides a good starting point for the selection of C_F.

Increasing the value of C_F yields a higher phase margin and limits the peaking response at the expense of signal bandwidth. The bandwidth of the transimpedance amplifier is given by Equation 3. Large R_F significantly limit the achievable bandwidth of the circuit. A compromise between gain, bandwidth, and stability can be made according to the specific requirements.