In this web page: http://physicsopenlab.org/2017/09/27/charge-sensitive-preamplifier/

they show the input voltage to a preamplifier and the output voltage. enter image description here

If I do the quick computation (see below), I deduce that the web page made an error in the sign of the output voltage: it should be a voltage that goes more and more negative, and not as they show: more and more positive.

Am I right or is the web page right?

Now, a related question: if electrons would be created in the detector by an ionizing particle, will the current "id" be a positive pulse or a negative pulse ? I would believe that actually, id should be a negative pulse. In this case, we would obtain the positive output voltage.

Any comment?

Here is my computation:

enter image description here

  • $\begingroup$ Kind of up to the particular preamp. Check the specs. $\endgroup$ – Jon Custer Oct 6 '19 at 23:35
  • $\begingroup$ I thought that preamp was a simple integrator with an operational amplificator, thus giving the minus sign ? $\endgroup$ – Mathieu Krisztian Oct 7 '19 at 5:04
  • 1
    $\begingroup$ Classic Ortec preamps had the output with the same sign as the input. While a simple model might give a different result, you need to look inside at the actual design. Having an inverting buffer has some advantages. $\endgroup$ – Jon Custer Oct 7 '19 at 12:43
  • $\begingroup$ Thank you so much. You may copy your comment to the answer. $\endgroup$ – Mathieu Krisztian Oct 9 '19 at 6:52

You are right. The first graph on the web page you referenced should be a negative pulse. Further down the referenced page is the actual circuit with the reversed biased SiPM connected to the negative input of an opamp with a capacitor in the feedback. An ionizing particle causes electrons to flow out of the "n" end of the SiPM which are integrated by the classic inverting opamp integrator … producing a positive output voltage that slowly decays. Further down the referenced page are scope traces showing the correct polarity pulses. This is all how you correctly understood it.


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