Electronic – stretching ns analog pulses for digital conversion

adcoperational-amplifierpulse

Problem statement

in one of my design i need to stretch narrow 10 ns pulses coming from photo diode after TIA stage, if i have to measure the pulse amplitude i have to stretch it at least by a factor of 100, in this search i have found a approach in which the circuit was using a capacitor after its feedback path, but i have not understood how exactly the circuit works, but when i tried to replicate it, no pulse stretch was happening, here below is the circuit and kindly suggest few modifications to the circuit or suggest an approach for the analog pulse stretch

circuit
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Opamp is a high slewrate 3000V/us and high BW of 100M as BW = 0.35/risetime and slewrate = 2* pi * BW * Vpp, so for my 10ns input the opamp would do the job.

pulse output is just zero !

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EDIT1:
primarily i forgot to mention that my 10ns pulses will repeat at a rate of 1Hz-50Khz, so just after some small modifications, i came with below circuit where i increased the gain and obtained pretty decent results, kindly provide your valuable comments

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Results

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PS: i was using TINA from TI as simulator in above case

Best Answer

You are trying to charge a 330 pF capacitor up very quickly. Going by information from your other question this needs to be done in about 1ns. Given your pulse maybe be (say) 1V in amplitude you can work out how much current the op-amp has to supply to charge the capacitor up....

Q=CV or...

dQ/dt = C dV/dt (note that rate of change of charge equals current)

So current = 330pF * 1V per nanosecond = 330 mA

You are not going to find a fast enough op-amp that can supply this amount of current with any degree of accuracy.

You are going to be looking at something around 10pF rather than > 100 pF and you are going to need a much better op-amp as has been mentioned in the other answer.