Confirmation of calcs for Wien Bridge Oscillator Circuit

Discussion in 'Homework Help' started by jg80, Oct 27, 2011.

  1. jg80

    Thread Starter New Member

    Mar 30, 2011

    Iwas hoping someone could confirm the results I have for a Wien Bridge Oscillator Circuit?

    The objective is to calculate a suitable value for Resistor 2 (see attached circuit), so that the circuit oscillates (sinusoidally). R1 = 100kΩ.
    Vout = 0.4v. No other information is provided.

    My attempt is as follows:

    Vout = 0.4v; x 1/3* = 0.132v (Vin)

    Amp Gain A = Vout/Vin = 0.4v/0.132v = 3.

    Aβ must equal 1 when the phase shift equals 0 degrees. For maximum output voltage.
    Therefore β = 1/A = 1/3 = 0.33v.

    To find the value of R2:
    β = R1/R1+ R2 where β =0.33v and R1 = 100kΩ.

    R2 = (100kΩ/0.33v)-100kΩ = 203kΩ.


    *my understanding from my study material that this is a 1/3 of the Vout. I have also seen this as Vout is 1/3 of Vin???

    Also can anyone confirm that my working is sound?

    Thanks for your time.
  2. Audioguru

    New Member

    Dec 20, 2007
    The extremely simple Wien Bridge oscillator has NOTHING to set its output level.
    If it has enough gain to start (a gain of slightly more than 3) then its output will keep increasing until its opamp "hits the rails" and clips.
    Most Wien Bridge oscillators use a non-linear network in the negative feedback (two diodes) or an automatic-gain-control system (an incandescent light bulb, a thermistor or a Jfet) to set the output level.
  3. jg80

    Thread Starter New Member

    Mar 30, 2011
    Thanks Audioguru.