The simulation problem of RC phase shift oscillator using op amp(741) in Multisim

Thread Starter

UMID Narziyev

Joined Mar 17, 2017
8
Hi everybody! I have question about RC phase shift oscillator using op amp(741).
I designed circuit using Rf=29R, so Rf=60k and R=1k. But There is no oscillation at the output. Can you help me? I attached picture of circute. Simulation program multisim.
Capture.JPG
 

crutschow

Joined Mar 14, 2008
27,701
Simulators normally do a DC bias calculation before doing a transient analysis, which can put an oscillator circuit in a quasi-stable DC state with no oscillation.
Normally circuit noise would start the oscillation but the simulator has no noise so you have to start the oscillation by some other means.

One way is the use the .ic command which avoids the DC calculation before the Transient analysis.

Another is to add a voltage source in series with the feedback loop and have it output a single small pulse at startup.
After the pulse, the voltage source looks like a short-circuit and has no effect on the circuit's operation.
 

Kjeldgaard

Joined Apr 7, 2016
453
I see two possible problems:
  • The old 741, may not be fast enough for the chosen frequency, or it can not supply enough current to the feedback network.
  • The final phase shift is formed by R1 / C1, then R4 must be removed.
 

Thread Starter

UMID Narziyev

Joined Mar 17, 2017
8
Simulators normally do a DC bias calculation before doing a transient analysis, which can put an oscillator circuit in a quasi-stable DC state with no oscillation.
Normally circuit noise would start the oscillation but the simulator has no noise so you have to start the oscillation by some other means.

One way is the use the .ic command which avoids the DC calculation before the Transient analysis.

Another is to add a voltage source in series with the feedback loop and have it output a single small pulse at startup.
After the pulse, the voltage source looks like a short-circuit and has no effect on the circuit's operation.
I see two possible problems:
  • The old 741, may not be fast enough for the chosen frequency, or it can not supply enough current to the feedback network.
  • The final phase shift is formed by R1 / C1, then R4 must be removed.
When I changed R5(Rf) to 70k I got the result. As I understood from above answers R1 alone must be used. Then, what is the equivalent of R1 and R4.Capture.JPG
 
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