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Sine wave generator

Discussion in 'Misc Electronics' started by Noonie, Nov 25, 2003.

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  1. Noonie

    Noonie Guest

    Hi,
    I am doing a project on a sine wave generator circuit. problem: when I
    simulated it, it gave me triangular wave instead of sine!!!
    So, Could you please help me discover what's wrong?! or help me in
    analysing how it works?
    The circuit schematic is on the following site:
    xoomer.virgilio.it/fladelle/Page13.htm

    Thanks in advance...
     
  2. mikem

    mikem Guest

    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
    2N3904, so I used them instead of BC238.

    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:

    __________________________________________________________________________-


    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
     
  3. 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.


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  4. mikem

    mikem Guest

    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.
     
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