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I am wondering what would be purpose of a setup like this?

schematic

simulate this circuit – Schematic created using CircuitLab

If you notice there is no input in this setup and the voltage is fairly high on the Common leg which exceeds the tolerance level of 2N3904. The output with the CRO shows a spike of roughly 10v high in approx. 300pS rise.

Would appreciate it if somebody point me into the right direction.

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But it looks more like a common collector setup to me. –  nidhin Jun 13 at 2:46
    
@nidhin thanks for it. My bad. Corrected it –  Mehrad Jun 13 at 2:55

2 Answers 2

up vote 4 down vote accepted

The 2N3904 is rated to at most 40 Volts collector-emitter, and we have a 100 Volt source here. It looks like it is an avalanche-breakdown oscillator. There are avalanche BJTs designed to be used this way, but any ordinary BJT will work.

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From the clue I have been given by @motoprogger, I did a bit of a reading and this is what I ended up with. This circuit is possibly an Avalanche Pulse Generator and one of many uses can be to measure the bandwidth for oscilloscopes.

This article, Avalanche Pulse Generator Build Using 2N3904 by Kerry D.Wong gives a pretty good discretion of how this simple circuit operates.

and here is the pulse he generates in his experiment

enter image description here

Apparently this is a typical Jim William design :)

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