8.2.1 Transimpedance Amplifier

Wide gain bandwidth, low input bias current, low input voltage, and current noise make the OPAx354-Q1 family of devices a preferred wideband photodiode transimpedance amplifier. Low-voltage noise is important because photodiode capacitance causes the effective noise gain of the circuit to increase at high frequency.

The key elements to a transimpedance design, as shown in Figure 37, are the expected diode capacitance (C_(D)), which must include the parasitic input common-mode and differential-mode input capacitance (4 pF + 5 pF); the desired transimpedance gain (R_(FB)); and the gain-bandwidth (GBW) for the OPAx354-Q1 family of devices (20 MHz). With these three variables set, the feedback capacitor value (C_(FB)) is set to control the frequency response. C_(FB) includes the stray capacitance of R_(FB), which is 0.2 pF for a typical surface-mount resistor.

1.

NOINDENT:

C_(FB) is optional to prevent gain peaking. C_(FB) includes the stray capacitance of R_(FB).

Figure 37. Dual-Supply Transimpedance Amplifier