I have made a minimal pulse generator based on
Jim Williams application note 47
http://cds.linear.com/docs/en/application-note/an47fa.pdfin the variant using a 2N3904 transistor, as mentioned several time on the Web, for example in
http://www.kerrywong.com/2013/05/18/avalanche-pulse-generator-build-using-2n3904/As I do not intend to use this pulse generator outside of my lab, I did not bother to make the
power supply, and just made the pulse generator part.
with the following setting (AN47, FIg. D1)
Q1 : 2N3904
C1 : 5pf (with some margin, this is a 45 years old component, but was the only ceramic cap I had in this range with high voltage (500V) ).
resistors : 1M, 12k, 51 ohm.
I had thus the pleasure to use my Fluke 415B HV power supplied which I salvaged some time ago from
the dumpster, but had no use of it yet.
I increased the voltage until I got the avalanche behaviour at 100V, but with some flickering, and it
stabilised at 102V.
But as time goes on, I had to increase the voltage to 106 V to keep a stable pulse.
I verified that the change did not come from the power supply.
Question 1 : Does the avalanche behaviour change as the transistor heats ?
In his note, JW mention that all transistors do not present avalanche behaviour, and that in a 50 samples set, only 82%
presented this behaviour.
I tested only one which worked, so my sample is not sufficient to make statistics.
Question 2 : Is the selection just to have the avalanche effect ? Doe the rapidity of the avalanche changes
a lot with the various transistors among the same denomination (2N3904).
Will the avalanche threshold voltage change a lot ?
My design is not as compact as Jim Williams' (Fig. D3 in AN47).
Question 3 : Should I gain a lot in rising time with a more compact design ?
In any case, I do not have the proper gear to test a fast pulse.
Below is what I got with my rigol DS1054z (100 Mhz).