Wide gain bandwidth, low input bias current, low input voltage, and low current noise make the OPAx328 excellent wideband photodiode transimpedance amplifiers. Low-voltage noise is important because photodiode capacitance causes the effective noise gain of the circuit to increase at high frequency.

Figure 7-4 shows that the key elements to a transimpedance design are the:

• Expected diode capacitance (C_D), including the parasitic input common-mode voltage and differential-mode input capacitance
• Desired transimpedance gain (R_F)
• Gain-bandwidth (GBW) = 40 MHz

With these three variables set, the feedback capacitor (C_F) value can be set to control the frequency response. C_F includes the stray capacitance of R_F, which is 0.2 pF for a typical surface-mount resistor.

NOTE: C_F is optional to prevent gain peaking, and includes the stray capacitance of R_F.

Figure 7-4 Dual-Supply Transimpedance Amplifier

For an optimized frequency response, use Equation 6 to set the feedback pole:

Equation 6.

Equation 7 calculates the bandwidth:

Equation 7.

For single-supply applications, the +IN input can be biased with a positive dc voltage to allow the output to reach true zero when the photodiode is not exposed to any light, and respond without the added delay that results from coming out of the negative rail. Figure 7-5 shows this configuration. This bias voltage also appears across the photodiode, providing a reverse bias for faster operation.

NOTE: C_F is optional to prevent gain peaking, and includes the stray capacitance of R_F.

Figure 7-5 Single-Supply Transimpedance Amplifier

For more information, see the Compensate Transimpedance Amplifiers Intuitively application report.