I have a system that detects pulses amplitudes with a peak detector, the signal from which is then digitized with an ADC. Pulses can sometimes overlap, so I've decided to "zero" the peak detector input when ADC conversion is in progress (there can be sometimes multiple ADC conversions which are averaged afterwards). The problem is with the schematic before the peak detector.
simulate this circuit – Schematic created using CircuitLab
My idea was to short the input signal to ground with R1 and Q1. However, the problem is that a small positive impulse and a large negative impulse are present when the transistor base switches (see scope picture: top graph is base voltage, middle graph is the collector voltage, bottom one is the input signal from a buffer). The positive impulse is very short and does not concern me: first of all, C1 just reduces it a bit, and, secondly, the peak detector does not even react to it (it's too short). The negative impulse, however, concerns me very much since the registered amplitude is reduced when the next input pulse comes when negative voltage is present on the transistor collector, and deadtime is therefore significantly increased .
However, I once accidentally inserted the transistor in wrong orientation and swapped collector and emitter. For some reason, the schematic works a bit better in this mode, though the negative impulse is still present. I've also tried other NPN bipolar transistors (2n3904, BC547, 2n5551, KT368A); some behave a bit better, some a bit worse, but the general idea is the same: they all work better when connected in reverse active mode.
My questions are:
- Why does this negative pulse happen? I do understand the cause for the positive pulse (collector-base capacitance most probably), but the negative impulse is way too long for that and timing is a bit off.
- How to combat this negative "bump"?
- Why in reverse-active mode this schematic works better? Is it just due to lower beta?
P.S. Yes, I've tried mosfets: the positive pulse from them is way higher and does influence the peak detector, and the negative pulse is also present (about the same voltage as a bipolar transistor in reverse-active mode)