Sine wave generator

Noonie wrote:

In the real circuit, the amplitude is stabilized (and distortion is minimized) by the non-linear resistance of the lamp. As the voltage across the lamp increases, the lamp's resistance increases, thereby reducing the amount of feedback, thereby reducing the loop gain such that the oscillator is just at the verge of dropping out of oscillation, where the output looks truly sinosoidal. If there is too much feedback (or the loop gain >1.000), then the distortion gets so bad that the wave looks triangular or even square.

The nominal resistance of the lamp is 12/0.04 or 300 Ohms. If the filament is not showing any color (cold), then the resistance is much lower, like 100 to 150Ohms. I simulated this circuit using LTspice. In my circuit, the lamp is myR7 (150 Ohms). I only had a model for

By playing with the values of yourR5(myR8) and yourR6(myR6), I was able to get everything from no oscillation at all to a low-distortion sine wave to trapezoids to square waves. The collector of Q1 consitantly has a lower distorion output than the emitter of Q2...

I'll leave it as an exercise for someone to come up with a model of the non-linear resistance of the Lamp to improve the attached simulation.

MikeM

LTspice circuit "oscs.asc" follows:

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Version 4 SHEET 1 880 680 WIRE 16 -48 16 -16 WIRE 16 144 16 208 WIRE 16 336 16 368 WIRE 16 368 160 368 WIRE 416 368 416 352 WIRE 224 384 224 368 WIRE 224 368 416 368 WIRE 496 288 496 368 WIRE 496 368 416 368 WIRE 160 320 160 368 WIRE 160 368 224 368 WIRE 160 240 160 192 WIRE 160 128 160 96 WIRE 160 96 80 96 WIRE 256 96 160 96 WIRE 352 160 368 160 WIRE 368 160 368 96 WIRE 496 96 496 32 WIRE 496 208 496 96 WIRE 416 272 416 256 WIRE 416 160 416 96 WIRE 416 96 496 96 WIRE 16 208 96 208 WIRE 16 208 16 256 WIRE 224 208 224 256 WIRE 224 256 288 256 WIRE 368 256 416 256 WIRE 416 256 416 240 WIRE 496 -192 496 -64 WIRE -160 176 -160 128 WIRE -160 -192 16 -192 WIRE 16 -16 16 48 WIRE 16 -192 496 -192 WIRE 432 -16 176 -16 WIRE 272 48 256 48 WIRE 256 48 256 96 WIRE 368 48 352 48 WIRE 288 160 256 160 WIRE 256 160 256 96 WIRE 368 96 416 96 WIRE 368 96 368 48 WIRE 544 96 496 96 WIRE 176 -16 16 -16 WIRE 96 208 224 208 WIRE 16 -128 16 -192 WIRE -160 48 -160 -192 FLAG 224 384 0 FLAG -160 176 0 FLAG 544 96 out FLAG 160 96 B FLAG 176 -16 C FLAG 96 208 E SYMBOL res 0 -144 R0 SYMATTR InstName R1 SYMATTR Value 5.6K SYMBOL res 0 240 R0 SYMATTR InstName R2 SYMATTR Value 1.8K SYMBOL res 144 224 R0 SYMATTR InstName R3 SYMATTR Value 15k SYMBOL cap 144 128 R0 SYMATTR InstName C1 SYMATTR Value 10n SYMBOL npn 80 48 M0 SYMATTR InstName Q1 SYMATTR Value 2N3904 SYMBOL npn 432 -64 R0 SYMATTR InstName Q2 SYMATTR Value 2N3904 SYMBOL res 368 32 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R4 SYMATTR Value 15k SYMBOL cap 352 144 R90 WINDOW 0 0 32 VBottom 0 WINDOW 3 32 32 VTop 0 SYMATTR InstName C2 SYMATTR Value 10n SYMBOL res 480 192 R0 SYMATTR InstName R5 SYMATTR Value 500 SYMBOL res 400 256 R0 SYMATTR InstName R6 SYMATTR Value 450 SYMBOL res 400 144 R0 SYMATTR InstName R7 SYMATTR Value 150 SYMBOL res 384 240 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R8 SYMATTR Value 0.1 SYMBOL Misc\\battery -160 32 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V1 SYMATTR Value 9 TEXT 182 -110 Left 0 !.tran 0 0.06 50m

In article , snipped-for-privacy@hotmail.com mentioned...

This is _supposed_ to be a simplified form of a Wien Bridge Oscillator. Here's what I see. The C1, C2 and R3, R4 are in the _negative_ feedback loop; they're supposed to be in the positive feedback loop. The lamp is in the _positive_ feedback loop; it's supposed to be in the negative feedback loop. So I don't see why the circuit should work at all. It looks like it's been _too_ simplified! Long ago I had serious doubts, so I cobbled one together, and it didn't work for me at all.

Nope, the loop gain is reduced as the lamp resistance increases, meaning that the lamp limits the amplitude of oscillation, and it keeps the output relatively distorion free. This means that the circuit is a true Wien Bridge Oscillator.

I further simplified the circuit! In my simulation, I opened the feed back loop between the emitter of Q2 and R4. I injected a sine wave and swept it from 10 to 10KHz. The frequency where the phase shift goes to zero is near 1Khz (which is where it normally oscillates). As the lamp resistance is varied from 100 to 200 Ohms, the Gain varies from 0.9 (wont oscillate) to about 1.2 (oscillates with distortion); The critical value for oscillation with no distortion is ~190 Ohms.