Another photocircuit with enhanced dynamic behavior is shown in Figure 7.17. The small signal transfer characteristics of this photocircuit with and without the amplifier can be found as
The photocircuit using the amplifier may be unstable if the miller capacitance due to the gate-drain capacitance of the MOS transistor is larger than the input parasitic capacitance.
The DC operating point at Vi, similar to the buffer-like pull up photocircuit, is determined by the DC characteristics of the amplifier. If a differential amplifier is used, with the negative input connected to a reference voltage, the input voltage will be set at that reference voltage. The output voltage still has a logarithmic relationship with the input photocurrent.
Figure 7.17: a) Without an amplifier in the feedback loop. b) Buffering
photocircuit with an amplifier in the feedback loop.