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From: lansford@tusun2.mcs.utulsa.edu (Wendell Wayne Lansford)
Newsgroups: sci.electronics
Subject: Re: MOSFET Puzzle
Message-ID: <1294@tusun2.mcs.utulsa.edu>
Date: 12 Apr 91 16:09:13 GMT
References: <1285@tusun2.mcs.utulsa.edu> <2178@gold.gvg.tek.com>
Reply-To: lansford@tusun2.UUCP (Wendell Wayne Lansford)
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Organization: University of Tulsa,  Tulsa, Oklahoma
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Your solution is interesting.  My professor, who originally solved the 
problem for an instructor in New York who was witnessing this
phenomenon in his use of MOSFETs, gave me his solution.   
The solution is rather tedious, but the theory is as follows:

The fact that the device is a transistor is only incidental to the observation
that it is a distributed RC structure.  Vinput is from the Gate to the 
Source and  Voutput is from the Drain to the Source (ie a typical connection).
The channel exhibits a total resistance, Rt.  The gate and channel form a 
distributed capacitor, C. The idea is that at some frequency, Vgd (Voltage
from gate to drain) will be 180 degrees out of phase with Vinput.  Noting 
that Voutput =  Vinput - Vgd, it can be seen that at this 180 degrees out
of phase frequency, Voutput will be greater than Vinput.  This "passive"
voltage gain phenomenon should be observed at frequencies around 1 GigaHz
for uncased, leadless devices. 


Wendell Lansford
lansford@tusun2.mcs.utulsa.edu