9.2.1 Transimpedance Amplifier

Wide gain bandwidth, low input bias current, low input voltage, and current noise make the OPAx355 series a preferred wideband photodiode transimpedance amplifier family. Low voltage noise is important because photodiode capacitance causes the effective noise gain of the circuit to increase at high frequencies.

The key elements to a transimpedance design, as shown in Figure 31, 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 OPAx355 family (20 MHz). With these three variables set, the feedback capacitor value (C_[FB]) controls 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 31. Dual-Supply Transimpedance Amplifier