I designed a front-end for the S5972 high-speed photodiode. See the following schematic.
In Multisim an AC sweep analysis gives the following Bode plot:
1. Is the presented front-end stable? If not, what part of the Bode plot tells me about the stability? What value do I need to adjust to stabilize the design?
From my understanding, we need constructive interference at the inverting input of the op-amp as well as a gain larger than unity. In the phase diagram of the Bode plot, the blue curve describes the difference between the voltage at the inverting input of the op-amp and the output of the op-amp. When the blue curve hits -180 degrees at about 230 MHz the constructive interference condition is satisfied, however, the magnitude of the frequency response is below unity, therefore, there is no positive feedback that makes the circuit ring.
2. How much of my understanding is correct?
What I believe I am missing is that usually the stability is defined in terms of noise gain. The reasoning behind using the noise instead of the signal gain (am I using the terms correctly in this context?) is that both gains can be different. For example, the standard inverting amplifier has a signal gain of -Rf/Ri while it has a noise gain of 1+Rf/Ri, thus, although if Rf/Ri falls we still have a unity amplification of the noise and the noise can be caught in a positive feedback loop.
3. Is this correct? How would I simulate the noise gain with Multisim?
As requested a transient analysis of a pulsed current source. The pulsed current source is configured to start at 0 A. After a delay of 100 ns the current source jumps in 1 ns rise time to a value of 440 uA. The pulse width is 50 ns. Fall time is 1 ns. The period between pulses is 300 ns.