I found the enclosed circuit on the internet. There were plenty more like it so I would have thought it should work.

The output is supposed to be a good sine wave at \$\frac{1}{2.6 RC} = \small 38\mathrm{\ kHz}\$.

What I get is nothing like a sine wave, at 2 MHz.

What could be wrong?

Relatedly, the NE5532 seemed to be getting a little warm. The circuit was consuming 25 mA. I already had 2- and 2+ joined, so I connected them to the virtual ground. Now the circuit consumes 50 mA, and the 5532 seems warmer.

Thanks very much for any help

  • 4
    \$\begingroup\$ your virtual ground impedance is too high - put some bulk capacitance across the lower 10k resistor. \$\endgroup\$
    – Kartman
    Sep 12, 2022 at 11:21
  • \$\begingroup\$ A significant number of circuits on the internet probably don't work. \$\endgroup\$
    – Andy aka
    Sep 12, 2022 at 11:28
  • \$\begingroup\$ Shouldn't that oscillate around 4.5v? \$\endgroup\$ Sep 12, 2022 at 11:38
  • \$\begingroup\$ What is opamp B doing? Nothing? If it's unused, you could use it as a buffer for the virtual ground. \$\endgroup\$
    – GodJihyo
    Sep 12, 2022 at 13:23
  • \$\begingroup\$ As is, the circuit is "working" ... (~20 kHz), but nowhere sinusoidal, de facto square waves ... With capacitor at "center" point (1 uF), no working. EE&O \$\endgroup\$
    – Antonio51
    Sep 12, 2022 at 13:35

2 Answers 2


It looks like you have an unused opamp and are connecting the inputs together to try to stabilize it. Connecting the inputs is not the way to do it. Have a look at this document from TI.

You can use the second opamp to make a buffered virtual ground.

I ran this through LTspice and it oscillated slightly above 30 kHz.

Phase Shift Oscillator Phase Shift Oscillator output

  • \$\begingroup\$ ....as it should! Ok, thanks a lot. I'll make that and see what happens. It should work. I'm fully confident! \$\endgroup\$ Sep 12, 2022 at 15:35
  • \$\begingroup\$ By the way, would you mind running the original circuit through LTspice, just out of interest? \$\endgroup\$ Sep 12, 2022 at 21:01

An alternative amendment to the circuit is to leave the op-amp's non-inverting input connected to the junction between the two 10k resistors and just reconnect the bottom of all 3 capacitors to 0 V. If its reluctant to start oscillating then you could try increasing the gain a little by reducing that 56k resistor a bit.

The theoretical oscillation frequency for the low pass version is fosc = sqrt(6)/(2.pi*RC) = about 39 kHz.

The actual oscillation frequency will be a little way from 39 kHz because of the lagging phase shift across the op-amp. The oscillation frequency will shift slightly away from 39 kHz in search of the frequency where the loop phase is -360 degrees. If the lag across the op-amp is, let's say, -200 degrees (inversion - 20 degrees) then the oscillation frequency will shift to a frequency where the lag across the three RC networks is -160 degrees.

With the low pass version you can increase the op-amp's gain enough to drive the op-amp into saturation without the spiking problem associated with the high pass version. Driving the op-amp's output into saturation reduces the loop gain to unity and now the Barkhausen Criteria for oscillation have been met (loop gain of 1 and loop phase of -360 degrees).

Take the waveform from the output of the phase shift network and buffer or amplify as necessary with a high impedance input non-inverting amplifier. The output at this point should be a nice low distortion sine-wave as the harmonics from the op-amp's saturated output will have been filtered out by the 3 cascaded low pass filters of the phase shift network.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.