Hello,

I am trying to simulate and then breadboard a simple Wien oscillator.
I want to use this to drive an 8Ohm speaker to generate an audio beacon
signal. 3 circuits were simulated in LTSpice using a UA741 (because I
have these at hand) and diodes/zeners for stabilization. I have set the R
and C values for +/- 1KHz.

When performing the simulations I get the expected output sine wave
(with the distortion of course) but don't understand the amplitudes I get.
To be concrete I have tested:

1. Wien oscillator with no stabilizing elements powered with +5V and -5V.
I get a clipped sine-like wave as expected with a positive amplitude of
3V and negative 3V.

2. Wien oscillator with 2 diode stabilizing elements powered with +5V and
-5V. I get a sine-like wave as expected with a positive amplitude of
about 747mV and same negative value.

3. Wien oscillator with 2 zener stabilizing elements powered with +12V and
-12V. I get a sine-like wave as expected with a amplitude from
-10V to +10V.

I do not understand amplitudes in cases (1) and (2). In case (1) I
expected a sine wave with a p-p amplitude of 10V but get 6V. In
case (2) I expect a p-p amplitude of 12.4 (zener breakdown of 6.2).

Can anyone explain this?
I have added simulation (2) and (3). Simulation (1) is simulation (2)
with the diodes remove.

TIA.

 

1) the 741 does not have rail to rail output capability, it can only swing to within 2-3V of each rail. What you see is expected.
2) the back to back diodes in the feedback path reduces the gain as the diodes turn on (about 700mV). This is why the output is limited to +/-700 mV.

 

1) the 741 does not have rail to rail output capability, it can only swing to within 2-3V of each rail. What you see is expected.

Ok. I will have to learn to read the data-sheet. I'll try and see where that information is given.

2) the back to back diodes in the feedback path reduces the gain as the diodes turn on (about 700mV). This is why the output is limited to +/-700 mV.

Yes this is what I expected and got for the (simple) diodes. However for the back to back zener diodes I get peaks of 10V when they are powered at 12. Assuming a swing to within 2V to the rail then 10 seems ok. But when the zener starts conducting shouldn't the voltage change? Shouldn't it be clipped at the zener's 6.2 volts?

TIA for the feedback.

 

R3 and R4 act as a potential divider, so when the 741 output is 10V the voltage across the series-connected zeners is ~6.8V, i.e they are just beginning to conduct.

 

R3 and R4 act as a potential divider, so when the 741 output is 10V the voltage across the series-connected zeners is ~6.8V, i.e they are just beginning to conduct.

Ok, I think I understand. Just going to make a few more simulation to confirm.

Thank you.