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From: Bill Sloman <bill.sloman@ieee.org>
Newsgroups: sci.electronics.design
Subject: An LTSpice test bed for the gain control FET in "low distortion"
 oscillators
Date: Sun, 22 Dec 2024 01:39:21 +1100
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The Jim Williams low distortion Wein bridge depends on a FET for 
amplitude control, but the FET isn't the perFectly linear resistor we'd 
like it to be.

And minimising the rectifier ripple in the gain control voltage applied 
to the gate of the FET is tricky.The test bed example below is set to 
apply 10uV of 2kHz ripple, which is less than any of my circuits can 
manage - 2mV is closer to what the circuit I've posted could manage.

I've put together a little LTSpice test bed to try to get some idea of 
how non-linear the FET channel is.

Mind the line wraps - the last three lines should be a single line in 
the .asc file.

It looks as if there about 70uV of higher harmonic content in the 10.4mV 
peak to peak voltage across the FET for the 1V peak to peak 1kHz test 
signal.

Version 4
SHEET 1 1252 680
WIRE 304 -592 -496 -592
WIRE 304 -512 304 -592
WIRE 64 -496 -96 -496
WIRE 64 -448 64 -496
WIRE -96 -432 -96 -496
WIRE 64 -336 64 -368
WIRE 304 -336 304 -432
WIRE 304 -336 64 -336
WIRE -496 -304 -496 -592
WIRE 64 -256 64 -336
WIRE 304 -192 304 -336
WIRE 64 -128 64 -176
WIRE 112 -128 64 -128
WIRE 256 -128 112 -128
WIRE 64 -80 64 -128
WIRE -496 48 -496 -224
WIRE -128 48 -496 48
WIRE -96 48 -96 -352
WIRE -96 48 -128 48
WIRE 304 48 304 -96
WIRE 304 48 -96 48
WIRE -128 80 -128 48
WIRE -496 112 -496 48
WIRE -496 240 -496 192
WIRE 64 240 64 0
WIRE 64 240 -496 240
FLAG 112 -128 Fet-gate
FLAG -128 80 0
SYMBOL res 80 16 R180
WINDOW 0 31 76 Left 2
WINDOW 3 31 40 Left 2
SYMATTR InstName R1
SYMATTR Value 10k
SYMATTR SpiceLine tol=0.1
SYMBOL res 48 -272 R0
WINDOW 0 43 37 Left 2
WINDOW 3 47 73 Left 2
SYMATTR InstName R2
SYMATTR Value 10k
SYMATTR SpiceLine tol=0.1
SYMBOL njf 256 -192 R0
SYMATTR InstName J1
SYMATTR Value MMBF4391
SYMBOL res 288 -528 R0
WINDOW 0 43 37 Left 2
WINDOW 3 47 73 Left 2
SYMATTR InstName R3
SYMATTR Value 5k
SYMATTR SpiceLine tol=0.1
SYMBOL voltage -496 -320 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V1
SYMATTR Value SINE(0 1 1kHz 0 0 0 1000)
SYMBOL voltage -496 96 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V2
SYMATTR Value SINE(7.724 0.00001 2k 0 0 90 2000)
SYMBOL voltage -96 -448 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V3
SYMATTR Value 7.724
SYMBOL res 48 -464 R0
WINDOW 0 43 37 Left 2
WINDOW 3 47 73 Left 2
SYMATTR InstName R4
SYMATTR Value 20k
SYMATTR SpiceLine tol=0.1
TEXT 128 144 Left 2 !.tran 0 1s 0s startup
TEXT -648 304 Left 2 !.MODEL MMBF4391 NJF VTO=-4.6 
BETA=0.02779LAMBDA=0.00595 RD=1 RS=1 IS=1e-14 CGD=14p CGS=10.5p PB=1 
B=1KF=1e-18 AF=1 FC=0.5 mfg=Motorola

-- 
Bill Sloman, Sydney