I am trying to design a phase shift oscillator, using a CE Amplifier. I have a Vc of approximately 12V, gain of approximately 250 I aim for an oscillation frequency of 60 Hz.

But whenever I start the simulation, no oscillations take place. Can anyone tell me what is wrong with the circuit? When I convert into hardware it works perfectly, but not in simulation. I am using NI Multisim circuit design suite 14.NI Multisim


The third phase shift stage (that is closest to Q1's base) is "ruined" by the biasing resistor R2. R2 is in parallel with R8 and it greatly reduces the value of R8. Instead, remove R2 and make R5 much bigger so that the appropriate base DC voltage is produced (circa 1 volt).

A test you can do is to open the feedback loop and inject an AC signal into C1 (from the left) and do a bode plot to see what output voltage you get at the collector. At the point you have output and input in-phase there should be a gain greater than 1.

  • \$\begingroup\$ I tried it , it indicates at 1Khz gain starts increasing above 1 but dips below after 10 ghz. How could i calculate the output voltage \$\endgroup\$ – Bala Subramanian Mar 16 '18 at 12:15
  • \$\begingroup\$ That tells you the problem. You have a gain less than 1 at 60 Hz hence the circuit won't oscillate. This is because you need to get rid of R2 and increase R5 to something like 330 kohm. Try the sim again. \$\endgroup\$ – Andy aka Mar 16 '18 at 12:19
  • \$\begingroup\$ Wow , that works . Can you explain why it works? There is a huge voltage drop across the R5, transistor won't turn on right? But hardware with same circuit above works well why? \$\endgroup\$ – Bala Subramanian Mar 16 '18 at 12:23
  • \$\begingroup\$ There is meant to be about 11 volts DC across R5 to set the base at about 1 volt. This in turn sets the emitter at about 0.3 volts DC. \$\endgroup\$ – Andy aka Mar 16 '18 at 12:24
  • \$\begingroup\$ But when i remove the source connect it as a positive feedback it's not working \$\endgroup\$ – Bala Subramanian Mar 16 '18 at 12:30

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