Op-amp phase shifter

Finished 2025

An op-amp circuit with a potentiometer that shifts the phase of a 500 Hz sine without changing its amplitude, worked out by hand, simulated in LTspice and measured on a scope at three pot settings.

Close view of a breadboard with an op-amp chip, a few resistors and a capacitor wired up, with scope probe clips on the right

The circuit

It’s one op-amp, two 10k resistors, a 100 nF capacitor and a pot. The pot and the capacitor form an RC network on the non-inverting input, and turning the pot slides the output’s phase against the input. The amplitude doesn’t change, only the timing.

Falstad schematic of an op-amp with two 10k resistors, a 5k and 5k potentiometer labeled Rp and a 100 nF capacitor to ground, driven by a 500 Hz source — The circuit, drawn in Falstad

I worked the transfer function out by hand. With the two resistors equal it simplifies to (1 - sCRp) / (1 + sCRp), which is an all-pass filter: gain of one at every frequency, with a phase that depends on Rp. At 500 Hz the capacitor’s impedance is about -3.2k j ohms, which is what the pot fights against.

Calculated, simulated, measured

I picked three pot values, worked out the phase shift for each, ran each in LTspice and then built it and measured on the scope.

Pot	Calculated	Simulated	Measured
100 ohm	-3.6 deg	-1.97 deg	no reading
5k	-114.6 deg	-115.5 deg	-110.29 deg
10k	-145.0 deg	-145.0 deg	-149.85 deg

LTspice plot of two nearly overlapping sine waves with a cursor measuring a tiny time difference — 100 ohm, simulated

LTspice plot of two sine waves with a noticeable shift between them and a cursor measuring the time difference — 5k, simulated

LTspice plot of two sine waves shifted by a large fraction of a cycle with a cursor measuring the time difference — 10k, simulated

At 100 ohms the shift is only about two degrees, so the traces sit on top of each other. The scope couldn’t find an edge to measure between them and reports “No edges”, so that row has no number. At 5k I measured -110.3 degrees against -115.5 simulated, and at 10k I measured -149.9 against -145.0. The scope read 502 Hz for the input.

Oscilloscope trace where the yellow and green sine waves overlap almost exactly, with the phase reading showing No edges — 100 ohm on the bench

Oscilloscope trace of two sine waves offset in time, with a phase reading of minus 110.29 degrees — 5k on the bench

Oscilloscope trace of two sine waves nearly opposite each other, with a phase reading of minus 149.85 degrees — 10k on the bench