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Fool'n around with DSO & AWG
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nctnico:

--- Quote from: RoGeorge on September 19, 2022, 09:01:50 pm ---
--- Quote from: mawyatt on September 17, 2022, 10:45:27 pm ---we utilized certain transistors upside down or reverse as analog switches because they had such a low saturation voltage when operated reverse
--- End quote ---

Is the BJT model critical for the reversed Vsat trick to work?

Asking because in simulation it worked great, seeing reversed Vsat of less than 2mV, while in practice the reversed Vsat was about the same as the normal Vsat.  Wanted to use the reversed transistor's low Vsat to switch on a discharging resistor for a 1.2V Ni-MH cell (discharge current in the range of 100mA) only to find out Vsat is about the same for either normal or reversed Vsat (70mV Vsat reversed, instead of 80mV Vsat normal).  :-\ 

Tried with two transistor, both with B=430 normal, and with CE reversed one was showing B=3, the other B=12.  The transistors type is SF128 D, video/RF NPN 600mW, TO-39 metal can.  Will look for other medium power transistors to try, and will use soldered wires instead of alligators next time.

How to choose the type of the transistors better for reversed mode?

--- End quote ---
Likely the older transistors work better. A long time ago I played with an avalanche pulse generator. I found out that the modern transistors don't work well or not at all. The good old 2N3904 worked like a charm though. Seems to have to do with different production methods.
tautech:
Yep as Nico says the old bipolar types particularly 2N2369 was famously used for the Jim Williams avalanche pulser.
Dave did a vid on it years ago and for a while member free_electron was making kits however IMO todays Leo Bodnar pulser is a superior product.
Some reading:
https://www.eevblog.com/forum/blog/eevblog-306-jim-williams-pulse-generator/

IIRC it was based on App note AN47 from Linear
HighVoltage:

--- Quote from: tautech on September 20, 2022, 08:26:02 am ---Yep as Nico says the old bipolar types particularly 2N2369 was famously used for the Jim Williams avalanche pulser.
Dave did a vid on it years ago and for a while member free_electron was making kits however IMO todays Leo Bodnar pulser is a superior product.
Some reading:
https://www.eevblog.com/forum/blog/eevblog-306-jim-williams-pulse-generator/

IIRC it was based on App note AN47 from Linear

--- End quote ---

Yes, the fastest rise time I ever got out of the Jim Williams 2N2369 avalanche pulser
was around 400ps. The Leo Bodnar pulser gives faster than 50 ps rise time.
The concept of the Jim Williams avalanche pulser is just amazing!

RoGeorge:
The Vec saturation voltage in mV range is expected for reversed mode, reversed meaning here that the CE terminals were swapped between them.  It all works at voltages lower than 5V (lower than Vbe reverse breakdown).  I don't see how the reversed mode BJT, and its expected very low Vsat, is related with the controlled avalanche breakdown from that pulse generator.

If the pulse generator mention was only as a side note while recommending older transistors, then these SF128D tested in reversed mode happens to be old indeed.  They are NOS found this weekend while digging in one of the scrap boxes I have from 20+ years ago, found together with some other NOS transistors and ICs from the same era, fabrication date probably during the 80s-90s era.
mawyatt:
The ones we used reversed way back in 60~70 were a fast pulse transistor, but can't remember the number!! Don't think it was the 2N2369 tho.

BTW thanks for the MIT find, that's a great reference :-+

Best,
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