# Opamp Wein Bridge Oscillator Simulation help needed ?

#### mishra87

Joined Jan 17, 2016
1,010
Hello All,

I have below query on wein bridge oscillator simulation :

1. What should be the amplitude of a any oscillator. I simulated wein bridge oscillator i found it is getting saturated towards positive and negative supply ? My simulation shows 14.7V peak.
What should optimum amplitude we should achieve for application ?

My perception is still not that much clear in amplitude of oscillator .

2. What is reason for oscillation frequency is not matching with calculated value ?
Theroritical value is 5.2KHz and Simulated value is 4.1KHz.

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#### OBW0549

Joined Mar 2, 2015
3,566
1. What should be the amplitude of a any oscillator. I simulated wein bridge oscillator i found it is getting saturated towards positive and negative supply ? My simulation shows 14.7V peak.
What should optimum amplitude we should achieve for application ?

My perception is still not that much clear in amplitude of oscillator .
There is nothing in your circuit to control or limit the amplitude of oscillations to a level below that at which the op amp's output saturates against the supplies. Lacking this, the oscillator will do exactly what you've observed.

Here is one method for controlling oscillator amplitude. There are many more ways.

2. What is reason for oscillation frequency is not matching with calculated value ?
Theroritical value is 5.2KHz and Simulated value is 4.1KHz.
If the op amp output is allowed to saturate, that will lower the frequency of oscillation by creating a "dead time" at the positive and negative peaks of the waveform. Fix the saturation problem, and the frequency of oscillation will more closely match your calculations.

#### crutschow

Joined Mar 14, 2008
33,367
My perception is still not that much clear in amplitude of oscillator .
If the oscillator loop gain is too low, it will not oscillate.
If the loop gain is too high, the oscillations will increase until the amplifier saturates.
If the loop gain is Goldilocks right, then it will operate with the desired, undistorted, output amplitude.
But it's impossible to get the exact gain required for that (rather like balancing a blade on its edge) without some form of automatic gain control or non-linear amplitude limiting, such as the ones OBW0549 referenced.

Last edited:

#### Bordodynov

Joined May 20, 2015
3,117
See

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#### mishra87

Joined Jan 17, 2016
1,010

#### mishra87

Joined Jan 17, 2016
1,010
This is what i tried to learn.
I understood the circuit well but i am not much familiar in LT Spice accept basic use of LT Spice. May be i have to learn more about LT Spice.
I did not understand the use of below function in red block.

#### mishra87

Joined Jan 17, 2016
1,010
If the oscillator loop gain is too low, it will not oscillate.
If the loop gain is too high, the oscillations will increase until the amplifier saturates.
If the loop gain is Goldilocks right, then it will operate with the desired, undistorted, output amplitude.
But it's impossible to get the exact gain required for that (rather like balancing a blade on its edge) without some form of automatic gain control or non-linear amplitude limiting, such as the ones OBW0549 referenced.
Thanks for support !

#### mishra87

Joined Jan 17, 2016
1,010
There is nothing in your circuit to control or limit the amplitude of oscillations to a level below that at which the op amp's output saturates against the supplies. Lacking this, the oscillator will do exactly what you've observed.

Here is one method for controlling oscillator amplitude. There are many more ways.

If the op amp output is allowed to saturate, that will lower the frequency of oscillation by creating a "dead time" at the positive and negative peaks of the waveform. Fix the saturation problem, and the frequency of oscillation will more closely match your calculations.
Thanks for guidance !

#### Bordodynov

Joined May 20, 2015
3,117
This is what i tried to learn.
I understood the circuit well but i am not much familiar in LT Spice accept basic use of LT Spice. May be i have to learn more about LT Spice.
I did not understand the use of below function in red block.

View attachment 208817
.opt plotwinsize=0 directive removes the compression of the output file, this setting improves accuracy for the directives Measure and Four.
With the help of Meas I've calculated the moments of zero crossing the output voltage. The tenth intersection and the eleventh intersection. Then I calculated the frequency.
From the output file, I put a comment on the circuit. The comments are in blue.
Also I have calculated your expression on the calculator and put as a comment on the scheme.

#### mishra87

Joined Jan 17, 2016
1,010
.opt plotwinsize=0 directive removes the compression of the output file, this setting improves accuracy for the directives Measure and Four.
With the help of Meas I've calculated the moments of zero crossing the output voltage. The tenth intersection and the eleventh intersection. Then I calculated the frequency.
From the output file, I put a comment on the circuit. The comments are in blue.
Also I have calculated your expression on the calculator and put as a comment on the scheme.

#### MisterBill2

Joined Jan 23, 2018
16,630
If the oscillator loop gain is too low, it will not oscillate.
If the loop gain is too high, the oscillations will increase until the amplifier saturates.
If the loop gain is Goldilocks right, then it will operate with the desired, undistorted, output amplitude.
But it's impossible to get the exact gain required for that (rather like balancing a blade on its edge) without some form of automatic gain control or non-linear amplitude limiting, such as the ones OBW0549 referenced.
Certainly the explanation is correct, and the "right gain" for stable non-saturation operation is a very narrow band. Also be aware that the gain control method must not add distortion of it's own in order to not affect the frequency. That is why all clipping types of gain control are less than optimum.