Unwanted negative spike on the output of resetable charge amplifier

[rgawron]

I try to simulate charge amplifier, it will be used as a first stage for photomultiplier tube. It doesn't have resistor in parallel to feedback capacitor (C1), instead, when output voltage is close to supply voltage, feedback capacitor is discharged by a transistor (J4).

I found this idea somewhere on the internet, but it was just briefly described, the idea was to avoid noise created by (high value) resistor in feedback loop.

I came with circuit presented below, it kind of works, but when the capacitor is discharged, a rellativelly huge voltage spike is visible on the output.

How can I improve my design to remove this spike?

Does below circuit contains other errors?


Here is link to the LTSpice simulation file:

https://github.com/RobertGawron/PhotomultiplierRadioactivityDetector/tree/master/Simulation/ChargeAmplifierWithAutomaticalReset

[StillTrying]

"relatively huge voltage spike is visible on the output."

Maybe the junction capacitances of the J4 transistor, you could try highering that 5 miliohm resistor quite a bit to reduce the spike current and add a low-ish resistance in series with the J4's gate.

The LTC6268/LTC6269 supplies are 5V max. total. I couldn't get the github .asc to load.

[rgawron]

@StillTrying: thanks, I will play with it. Why gate resistor would be useful here?


Here is a direct link to .asc file, GitHub adds extra .txt in filename after downloading:

https://raw.githubusercontent.com/RobertGawron/PhotomultiplierRadioactivityDetector/master/Simulation/ChargeAmplifierWithAutomaticalReset/ChargeAmplifierWithAutomaticalReset.asc

It's still GitHub link, just when linking directly to a content of a file, hostname change from github.com to raw.githubusercontent.com