i have a problem to design wien-bridge oscillator

Discussion in 'The Projects Forum' started by flugman, May 21, 2008.

  1. flugman

    Thread Starter New Member

    May 21, 2008
    5
    0
    hi i have a big problem to design wien-bridge oscillator.

    i've made it with ua741 and simulated it with pspice.

    (attached some figures.)

    i wanted to get oscillation freqeuncy in 100kHz and 10V amplitude but i could't have that.

    so i guessed there is a simple problem that ua741 is not ideal.

    however, i don't know why that circuit in not in oscillation frequency 100kHz.


    this circuit oscillated in about 70kHz.

    help me to compose it!!!
     
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  2. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    The lousy old 741 opamp is 40 years old this year.
    Its output drops above only 9kHz.
    Its gain is 200,000 from DC to about 2Hz then drops at 6dB/octave until its gain at 1MHz is 1.

    You need a newer high frequency opamp.
     
  3. flugman

    Thread Starter New Member

    May 21, 2008
    5
    0
    i should use ua741 because the professor designated it

    so i must use ua741 to design it.

    do you know excess phase shift at 100kHz with ua741??

    i just got a hint that excess phase shift would cause distortioness.

    i heard there is about 0.1 rad excess phase shift at 10kHz.

    if i get information about excess phase shift at 100kHz, i can solve

    this problem.
     
  4. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    The amplifier in the Wien Bridge oscillator needs a gain of slightly more than 3. The open loop gain of a 741 opamp at 100kHz is about 8 and its phase shift is 90 degrees so it has a pretty high phase shift of about 40 degrees (just a guess) when its gain is reduced to 3. Your frequency-determining resistors are already a heavy load on the opamp so decrease the value of the capacitors so that it oscillates at 100kHz.

    At 100kHz its max output level is less than only 4V p-p with a horrible 10% distortion.
     
  5. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    If you decide to build such a circuit from actual components, you will need to consider some sort of amplitude stabilization method to maintain a consistent operation of of the oscillator over a wide range of environmental variations (i.e. temperature, supply voltage, component aging)

    hgmjr
     
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