Wien oscilator: amplitudes of a stabalized bridge

Discussion in 'General Electronics Chat' started by hugo23, Apr 26, 2014.

  1. hugo23

    Thread Starter Member

    Mar 3, 2010
    15
    0
    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.
     
    Last edited: Apr 26, 2014
  2. w2aew

    Member

    Jan 3, 2012
    219
    64
    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.
     
  3. hugo23

    Thread Starter Member

    Mar 3, 2010
    15
    0
    Ok. I will have to learn to read the data-sheet. I'll try and see where that information is given.

    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.
     
  4. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,783
    1,103
    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.
     
  5. hugo23

    Thread Starter Member

    Mar 3, 2010
    15
    0
    Ok, I think I understand. Just going to make a few more simulation to confirm.

    Thank you.
     
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