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
x
Notes:
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