I assume you want to run the mosfet in the region where current
is essentially independent of drain source voltage. What about making
a current source out of it with a source resistor. I've done an ASCII
sketch below of an AC coupled amplifier. For DC and lower frequency
applications we use an opamp looking at the current viewing resistor
and driving the gate. This turns the mosfet into a controlled current 
source (sink?). In your case you would need a low power (single 
supply maybe ?) op amp.

By the way, if you're trying to drive a power mosfet from only a 5V 
supply make sure it is a "Logic Level" device (i.e the gate threshold
is low enough to ensure that 5V can drive it fully on.) 

If the picture below comes out funny try to line it up so that every 
line starts with an x.

x                                 _____
x                                   |
x                                   |
x              5V______            RRR
x                   |              RRR (load)
x                   |              RRR
x                   |              RRR           ||
x                  RRR              |            ||
x                  RRR (R1)         @------------||------- 
x                  RRR              |            ||  Output        
x            ||    RRR           |--|            ||   (AC) 
x            ||     |          | |    
x  Signal - -|| ----@----------| |    Mosfet
x            ||     |            |--|  
x            ||     |               | 
x                   |         ------@-------------- 
x            (C1)    \   |    |     |             |
x                     \  |    |    RRR            | 
x                      \ |----     RRR (R2)       |
x                npn   / |         RRR         _______ 
x                     /  |         RRR         _______ 
x                  |/_              |             |    (C2)
x                   |               |             | 
x                  _|_______________|_____________|_____ 
x                0V

	Quiescent drain current approx: 0.6V/R2
	Load must ensure  that there is an appropriate DC voltage
	across the mosfet with this quiescent current flowing.

	Input impedance at signal frequencies R1 (can be large).

	R1*C1 timeconstant must be large w.r.t. to signal frequencies

Date: Sat, 16 Nov 1996 15:42:14 -0800

Original Subject: Re: Mosfet Information

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