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I am having a problem replicating some lab results in the PSPICE/capture CIS program for an assignment. I have this circuit right here: enter image description here which is an oscillator using the uA741 op-amp which is supposed to output an oscillation through its output pin(pin 6). After doing this experiment in the lab i found out that oscillations begin at about 220 ohms but after doing a parametric sweep for r between the values of 10 and 300, with an increment of 10, i am getting only a stable voltage of a few mV and not an oscillation of any kind. Is there any mistake i'm not seeing?

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  • \$\begingroup\$ Increase your gain-set resistors by 100X. The 741 does not produce lots of output current. \$\endgroup\$ – analogsystemsrf May 21 at 15:56
  • \$\begingroup\$ @analogsystemsrf I know it doesn't produce a lot of current. I am trying to replicate lab results so i don't really mind. \$\endgroup\$ – George Tsaki May 21 at 18:25
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The issue you are facing, is that, your oscillator has a stable DC operating point.

Imagine you open-circuit the capacitors. You now have a zero volt input with 100 kOhm source resistance being amplified by U1, which regardless of gain is a 0 V output.

A real oscillator circuit relies on the noise voltage/current generated in circuit to initiate the oscillator.

In a spice simulation you can try:

  • Skipping the DC operating point solution
  • Injecting a small transient current pulse just after t=0
  • Force a non-steady-state value to one of the circuit nodes in the oscillator loop
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  • \$\begingroup\$ Forgive my ignorance because i am new to using pspice(hence the post itself). Would you mind explaining how to do the last 2 solutions? \$\endgroup\$ – George Tsaki May 21 at 18:21
  • \$\begingroup\$ @GeorgeTsaki I'm not familiar with your exact simulator but generally to force an initial condition you would use '.IC' spice directive. \$\endgroup\$ – sstobbe May 22 at 2:26
  • \$\begingroup\$ Incase your instructor hasn't enlightened you on the history of this circuit you can find more information at en.wikipedia.org/wiki/Wien_bridge_oscillator \$\endgroup\$ – sstobbe May 22 at 2:28
  • \$\begingroup\$ Thanks for the info but we were given some brief info about it. Also thanks for the advice. I'll try and see how it goes. \$\endgroup\$ – George Tsaki May 22 at 7:42

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