7.3.8 Wideband Transimpedance Amplifier

Wide bandwidth, low-input bias current, and low input voltage and current noise make the OPAx354-Q1 family of devices is designed as a 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-Q1], the desired transimpedance gain (R_F), and the gain-bandwidth product (GBW) for the OPAx354-Q1 family of devices (100 MHz). With these three variables set, the feedback capacitor value (C_F) is set to control the frequency response.

Figure 36. Transimpedance Amplifier

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

Equation 1.

Typical surface-mount resistors have a parasitic capacitance of approximately 0.2 pF that required deduction from the calculated feedback capacitance value.

Use Equation 2 to calculate the bandwidth.

Equation 2.

For even higher transimpedance bandwidth, use the high-speed CMOS OPA355-Q1 (200-MHz GBW) or the OPA655-Q1 (400-MHz GBW).