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From: piglet <erichpwagner@hotmail.com>
Newsgroups: sci.electronics.design
Subject: AF Osc AGC alternatives to lamp/thermistor/FET
Date: Sun, 20 Oct 2024 14:15:19 +0100
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Yesterday in the thread "Random thoughts on sinewave oscillators" I 
posted a LT Spice wirelist showing amplitude levelling without fet, ntc 
thermistor or ptc filament instead using current steering long tail pair 
- kinda poor mans multiplier.

Here is another alternative, using diode conductance bridge "variolosser".

For the full 1960s vibe replace U2 U3 with bjt current sources.

Version 4
SHEET 1 1316 888
WIRE 288 -208 -32 -208
WIRE 368 -208 288 -208
WIRE 464 -208 432 -208
WIRE 608 -208 544 -208
WIRE 672 -208 608 -208
WIRE 880 -208 736 -208
WIRE 960 -208 880 -208
WIRE 1056 -208 960 -208
WIRE 608 -176 608 -208
WIRE 880 -144 880 -208
WIRE 960 -128 960 -208
WIRE -32 -112 -32 -208
WIRE -304 -80 -304 -112
WIRE 608 -64 608 -112
WIRE 1056 -64 1056 -208
WIRE 880 0 880 -64
WIRE 960 0 960 -64
WIRE -32 16 -32 -32
WIRE -304 48 -304 0
WIRE 752 48 752 16
WIRE 1056 64 1056 16
WIRE 1056 64 784 64
WIRE 688 80 448 80
WIRE 720 80 688 80
WIRE 928 96 784 96
WIRE 288 128 288 -208
WIRE 448 128 448 80
WIRE 176 144 176 112
WIRE 928 144 928 96
WIRE 144 160 -128 160
WIRE 752 160 752 112
WIRE 288 176 288 128
WIRE 288 176 208 176
WIRE -32 192 -32 80
WIRE 32 192 -32 192
WIRE 144 192 32 192
WIRE -304 208 -304 144
WIRE 1056 208 1056 64
WIRE 288 224 288 176
WIRE 32 256 32 192
WIRE 176 256 176 208
WIRE 448 256 448 208
WIRE 448 256 384 256
WIRE 512 256 448 256
WIRE 928 256 928 224
WIRE 384 288 384 256
WIRE 512 288 512 256
WIRE 688 288 688 80
WIRE 1056 288 1056 272
WIRE 1056 288 688 288
WIRE -32 304 -32 192
WIRE -304 336 -304 288
WIRE 688 336 688 288
WIRE 288 384 288 304
WIRE 384 384 384 352
WIRE 384 384 288 384
WIRE 512 384 512 352
WIRE 560 384 512 384
WIRE 384 400 384 384
WIRE 512 400 512 384
WIRE -32 432 -32 384
WIRE 32 432 32 320
WIRE 688 448 688 416
WIRE 896 448 688 448
WIRE -128 496 -128 160
WIRE 288 496 288 384
WIRE 288 496 -128 496
WIRE 688 496 688 448
WIRE 384 512 384 464
WIRE 448 512 384 512
WIRE 512 512 512 464
WIRE 512 512 448 512
WIRE 288 544 288 496
WIRE 448 560 448 512
WIRE 800 640 800 608
WIRE 896 656 896 448
WIRE 896 656 832 656
WIRE 448 672 448 640
WIRE 688 672 688 576
WIRE 688 672 448 672
WIRE 768 672 688 672
WIRE 288 688 288 624
WIRE 896 688 832 688
WIRE 896 720 896 688
WIRE 800 752 800 704
FLAG -304 -112 P10
FLAG 176 112 P10
FLAG -304 144 N10
FLAG 176 256 N10
FLAG -304 48 0
FLAG -304 336 0
FLAG 288 128 OUT
FLAG 288 688 0
FLAG 752 16 P10
FLAG 752 160 N10
FLAG 800 608 P10
FLAG 800 752 N10
FLAG 896 720 0
FLAG 608 -64 0
FLAG 880 0 0
FLAG 960 0 0
FLAG 928 256 0
FLAG -32 432 0
FLAG 32 432 0
FLAG 560 384 0
SYMBOL OpAmps\\UniversalOpamp2 176 176 R0
SYMATTR InstName U1
SYMBOL voltage -304 -96 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V1
SYMATTR Value 10
SYMBOL voltage -304 192 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V2
SYMATTR Value -10
SYMBOL res 272 208 R0
SYMATTR InstName R3
SYMATTR Value 3300
SYMBOL res 272 528 R0
SYMATTR InstName R4
SYMATTR Value 2200
SYMBOL diode 368 288 R0
SYMATTR InstName D1
SYMATTR Value 1N914
SYMBOL diode 368 400 R0
SYMATTR InstName D2
SYMATTR Value 1N914
SYMBOL diode 496 288 R0
SYMATTR InstName D3
SYMATTR Value 1N914
SYMBOL diode 496 400 R0
SYMATTR InstName D4
SYMATTR Value 1N914
SYMBOL res 432 112 R0
SYMATTR InstName R5
SYMATTR Value 100k
SYMBOL res 432 544 R0
SYMATTR InstName R6
SYMATTR Value 100k
SYMBOL OpAmps\\UniversalOpamp2 752 80 M0
SYMATTR InstName U2
SYMBOL OpAmps\\UniversalOpamp2 800 672 M0
SYMATTR InstName U3
SYMBOL res 704 432 R180
WINDOW 0 36 76 Left 2
WINDOW 3 36 40 Left 2
SYMATTR InstName R8
SYMATTR Value 10k
SYMBOL res 704 592 R180
WINDOW 0 36 76 Left 2
WINDOW 3 36 40 Left 2
SYMATTR InstName R9
SYMATTR Value 10k
SYMBOL cap 432 -224 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C3
SYMATTR Value 100n
SYMBOL res 560 -224 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R7
SYMATTR Value 2200
SYMBOL res 896 -48 R180
WINDOW 0 36 76 Left 2
WINDOW 3 36 40 Left 2
SYMATTR InstName R10
SYMATTR Value 470k
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