Figure 9-5 shows how the OPAx863 devices can be used as a transimpedance amplifier (TIA) for converting a photodiode current into an output voltage. The factors which determine the minimum required gain-bandwidth product (GBW) of the amplifier are the transimpedance gain, small-signal bandwidth, and the photodiode capacitance. Increase in the values of any of these three parameters requires higher GBW. Large-area photodiodes with higher sensitivity have a relatively larger photodiode capacitance, sometimes up to 100 pF, which impacts the small-signal bandwidth, and is given below.
The small input capacitance of OPAx863 helps to achieve a wider small-signal bandwidth, mainly limited by the photodiode capacitance. The feedback resistor RF introduces a zero in the noise gain with the total input capacitance CIN and a 40 dB/dec rate of closure. Figure 9-5 shows the feedback capacitor CF that is needed to cancel the zero due to RF, CIN with a pole in the noise gain, whose value is given below for a 65° phase margin and butterworth response.
Equation 1 estimates a closed-loop bandwidth of 2.6 MHz. Figure 9-6 and Figure 9-7 show the closed-loop gain and phase plots from TINA-TI simulation of the TIA circuit of Figure 9-5. The 1/β gain curve has a zero from RF and CIN at 142 kHz and a pole from RF and CF cancelling the 1/β zero at 1.87 MHz, resulting in a 20-dB per decade rate-of-closure at the loop-gain crossover frequency (the frequency where AOL equals 1/β), ensuring a stable circuit. A phase margin of 65° is obtained with a closed-loop bandwidth of 2.6 MHz and a 100-kΩ transimpedance gain.