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Fool'n around with DSO & AWG
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RoGeorge:


This is too much fun already!
You sure it's not illegal?  ;D

A switch with zero AC resistance seems a very interesting component, is that exploited in practice?
mawyatt:
Don't know if it's useful as a switch since the zero ohms would only be over a narrow bias current range. Just did a quick test where we injected a signal into the emitter of the 2N3904 at 1KHz 2Vpp thru a 100K resistor and 1uF decoupling capacitor. You can see the signal collapse as you slowly sweep thru the zero ohm region at ~11.9 VDC bias, and then expand as you pass thur. It's like sweeping a notch filter only in the bias domain!!

Best,
RoGeorge:

--- 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.

Attaching the simulation for a generic NPN (not SF128) and the only datasheet I could find for SF128.

How to choose the type of the transistors better for reversed mode?
mawyatt:
A lot will depend on what parasitic resistances the transistor has, and how well the model takes care of these as well if it's even capable of accurate reverse behavior. When we utilized this way back, recall the switching type transistors behaved the best (have a higher doping level in the base/emitter) while reversed but can't remember the exact device type tho.

Best,
RoGeorge:
Will test some switching transistors then, thank you.  :)

Just for the docs, found a brief class notes for the reversed transistors, has formulas for reversed Beta:
http://web.mit.edu/6.012/www/SP07-L18.pdf
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