Twin Tee Sine Wave Oscillator

output and

output

-180

op-amp

AB =0?

The notch has finite depth, typically 60dB i.e. B = 0.001

The potential divider on the +ve input provides a little positive feedback to make sure the oscillator starts reliably.

Reply to
Andrew Holme
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How does a twin-tee sine wave oscillator work?

The schematic I have shows an op-amp with a twin-tee between the output and the - input, and a voltage divider formed by a 10K resistor from the output to the + input, and a 1K resistor from the + input to ground.

I understand that at the frequency w= 1/ RC the twin-tee produces a -180 degree phase shift. But at w= 1/RC, wouldn't the - input of the op-amp become 0V because of the twin-tee's notch effect? Then the loop gain AB =0?

Reply to
zero

maybe your talking about a wheatstone or wien bridge ? the Wien is used mostly in AC type applications. some times to 2 combined OSC's are used in a bridge to generate the lower side band of the harmonic. this technique used to generate very smooth low audio freqs.

Reply to
Jamie

input of

desired

No.

For one thing: B is not 0.001; that's just a typical "ballpark" figure. It will vary with component tolerances, temperature e.t.c.

Another thing: you can't control gain like that with POSITIVE feedback.

Sustained oscillation requires zero (or 360 degrees) phase shift and unity gain. Due to temperature coefficients and component tolerances, you can't get exactly unity gain, so oscillators are designed with gains greater than unity. The question then is: what is the amplitude limiting mechanism?

The amplitude may be allowed to grow until limited by the power supply rails, the designer may use the non-linear characteristic of an active component to limit the amplitude, or some form of automatic gain control may be employed - whichever way it is done, the gain is effectively unity!

The amplitude limiting mechanism affects the purity of the sine wave.

Reply to
Andrew Holme

If B = 0.001 would I have to change the voltage divider on the + input of the op-amp to produce a gain of A = 1000 to keep AB = -1 at the desired frequency?

Reply to
zero

The positive feedback path wants to make it oscillate; assume it wants to oscillate at all frequencies. The negative feedback through the t-t overcomes the positive feedback and kills the oscillation. But at the tt notch frequency, there is no neg feedback, so that's the only frequency it can oscillate at, so it does.

John

Reply to
John Larkin

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