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From: grege@gold.gvg.tek.com (Greg Ebert)
Newsgroups: sci.electronics
Subject: Re: MOSFET Puzzle
Message-ID: <2178@gold.gvg.tek.com>
Date: 11 Apr 91 18:28:29 GMT
References: <1285@tusun2.mcs.utulsa.edu>
Distribution: na
Organization: Grass Valley Group, Grass Valley, CA
Lines: 53



In article <1285@tusun2.mcs.utulsa.edu> lansford@tusun2.UUCP (Wendell Wayne Lansford) writes:
>The following problem was recently posed by my professor in Advanced 
>Networks:
> 
>TRANSISTOR IN A BOX PUZZLE
> 
>               -------------
>        +  ----|           |----  +
>               |   Black   |
>       Vin     |    Box    |    Vout
>               |           |
>        -  ----|           |----  -
>               -------------
> 
> [...]

Awright, how 'bout this :


A depletion-mode MOSFET is actually a 4-terminal device. It looks like this:


			o Drain
			|
               ----||---+---K|---
               |   Cgd  |       |
               | 	 \      *---o  substrate
       gate o--*        |       |
	       |        |       |
	       ----||---*--K|----
	           Cgs  |
			o  Source

 Those -K|- devices are parasitic diodes. The switch inside is controlled
 by Vgs: OFF if Vgs < Vp; on otherwise.

 Connect an AC source across the gate and substrate. Now connect the source
 and drain terminals together. Finally, connect an imagionary load across
 the source-drain terminal and the substrate. Look familiar ? It's a
 capacitive voltage doubler.

 Imagine the gate is negative with respect to the substrate. Cgs and Cdg
 are charged 'negatively', thus the MOSFET is pinched-off. In fact, you
 can now forget that it's even a MOSFET. Now let the gate go positive
 with respect to the substrate. There is no path to discharge Cgs & Cgd
 (thanks to the diodes), so the device remains pinched-off. Looking
 at the drain-source node, the voltage with respect to the substrate is
 the voltage on Cgs/Cgd PLUS the gate-substrate voltage. 

 Thinking about it some more, I think the issue of whether or not
 the MOSFET is pinched-off is irrelevant.