Stability of saturated amplifier and overdrive recovery

Question

I have two questions. First paragraph is background, second is questions.

Background:

I have a photodiode amplifier (low capacitance APD + fast transimpedance amplifier) that has a nice response with small amplitude light pulses. It has good phase margin and there is no overshoot.

When I increase the amplitude of the incoming light pulse, the amplifier output will get close to or hit the rail, and I get overshoot and ringing.

Questions:

1. How do I analyze amplifier phase margin with a large signal response that may saturate the amplifier?

2. How do overdrive recovery circuits work? I can't find any reference to overdrive recovery in my Gray & Meyer book. Where can I learn more about overdrive recovery?

Thanks

Answer 1

1) Predicting overload recovery from small-signal behavior is a lost cause. It depends entirely on things like the input transistor characteristics and the thermal characteristics of the die.

2) Overdrive protection circuits work (typically) by shunting overdrive current away from the input stage. As a result, the input does saturate, but just barely. A typical example would be providing back-to-back diodes across the input (at the expense of increased input capacitance and slightly increased input current).

Answer 2

You have slew-rate limiting at the large signal outputs, which is a time-delay.

Can you add a tiny capacitance across Rfb, value only 10% of what works at small amplitudes but providing the necessary "look ahead" for large inputs?

Or simulate this variable-gain TIA

^{simulate this circuit – Schematic created using CircuitLab}