8.3.8 Wideband Transimpedance Amplifier

Wide bandwidth, low input bias current, low input voltage, and current noise make the OPAx354 a preferred wideband photodiode transimpedance amplifier for low-voltage single-supply applications. 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 36) are the expected diode capacitance [including the parasitic input common-mode and differential-mode input capacitance (2 + 2) pF for the OPAx354], the desired transimpedance gain (R_F), and the gain-bandwidth product (GBW) for the OPAx354 (100 MHz, typical). With these three variables set, the feedback capacitor value (C_F) may be set to control the frequency response.

Figure 36. Transimpedance Amplifier

To achieve a maximally flat, second-order, Butterworth frequency response, the feedback pole must be set as shown in Equation 1:

Equation 1.

Typical surface-mount resistors have a parasitic capacitance of approximately 0.2 pF that must be deducted from the calculated feedback capacitance value. Bandwidth is calculated by Equation 2:

Equation 2.

For even higher transimpedance bandwidth, the high-speed CMOS OPA355 (200-MHz GBW) or the OPA655 (400-MHz GBW) may be used.