I would like to get some help with the calculation and modeling of the attached circuit. this circuit is supposed to generate a 159.15 Hz sine wave(according to formula), i want to connect the output of the circuit to a transformer with minimal losses of voltage, for that i need that the output impedance of the circuit(which is also the output of the op-amp 741) would be as low as possible.

I know that ideal op-amp output impedance is 0, but how do i calculate the actual impedance of the circuit(Rout) ? and how to i model the opamp in the circuit ?ainw wave circuit

if i suppose to model the opamp according to the pic below how do i calculate the inputs?(v±) enter image description here

when solving these circuits i usually start with KCL from a voltage source summing up all elements until i get to ground but how do start the calculation if it is a feedback circuit like this.

frequency formula: 1/2*pi*R1*R2*C1*C2 while C1=C2=0.1*10^6 [F], R1=1K[ohm],R2=100K[Ohm]

the impedance of C1 and C2 is 1000 [Ohm],

Thanks for your help

  • \$\begingroup\$ Negative feedback lowers the output impedance of the opamp. You cannot simply apply KCL etc and get the correct result. In this case there's more to it. Study feedback circuits if you want to understand how this works. \$\endgroup\$ Oct 24, 2015 at 14:49

1 Answer 1


The circuit is a very poor wien bridge oscillator and will not be stable - amplitude adjustments will need to be made and distortion is quite likely to be high and vary with time. There are better sinewave oscillators that can be found so I urge you to do some google image research on this. This is the sort of circuit I'd be considering for a decent sine wave oscillator: -

enter image description here

The basic Wien bridge oscillator has nothing to define or control the sinewave amplitude whereas this circuit uses a JFET to do that. Here is a google image page for sine wave oscillators.

As for the output impedance of the op-amp, because negative feedback is used you can assume the output impedance is less than an ohm when driving loads that a 741 is capable of. This won't give you much power into a transformer so don't expect miracles. The 741 isn't capable of delivering (relatively cleanly) any more than a few handful of tens of mA.

If you want to model this circuit I have no hesitation in suggesting LTSpice (free version) but I think you'll find you'll need a transistor buffer between op-amp and transformer if you expect to drive powers more than a few tens of milli watts.


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