Phase shifter switch with Op-Amp

In electronics circuit design sometimes we need to phase shift the input signal by 180 degree. How can we phase shift the phase of the input signal? Here it is shown how one can use op-amp(operational amplifier) to do either 0(that is no phase shift) or 180 degree phase shift of the phase of the input signal using manual switch or FET switch. Such phase shifter circuit is also called sign changer circuit.

Consider the following Phase shifter switch with Op-Amp with manual switch circuit diagram.

Phase shifter switch with Op-Amp manual switch
A manual switch is connected to the non-inverting terminal of the op-amp such as LM358N. The switch can either be connected to the ground so that non-inverting terminal is grounded or the switch can connect the input signal Vin to the non-inverting terminal. Irrespective of the position of the switch the inverting terminal will always receive the input signal Vin. 

Consider what happens at the output with the switch in either of the position.

When the switch is connected to ground then the op-amp operates as inverting amplifier configuration with the voltage gain given by the following equation.

\(A_v(invert) = -\frac{R_2}{R_1}\)

and since the resistors are equal we have,

\(A_v(invert) = -1\)

In this case the output signal will be 180 out of phase with the input signal. That is the input signal is phase shifted by 180 degree.

When the switch position is changed, both the inverting and the non-inverting terminal gets Vin input signal. That is, it is a circuit wherein both the input terminal of the op-amp is driven by the same input signal. In such case, input get amplified by both the inverting and non-inverting terminal and we get superposition of the signals from the two terminals. The overall voltage gain of such amplifier is given by the following relation.

\(A_v = A_v(invert) +A_v(non-invert)\)   -------->(1)

The voltage gain due to inverting channel is still, \(A_v(invert) = -1\).

The voltage gain due to non-inverting channel is,

\(A_v(non-invert) = 1+\frac{R_2}{R_1}=1+1=2\) since R1=R2 

And therefore the total voltage gain from equation(1) is,

 \(A_v = -1 + 2=1\)

And hence there is no phase shift of the input signal in this case. That is there is 0 phase difference between the input and output signal when switch is in the upper position. 

In other words, the above circuit is a switchable phase shifter producing output signal with equal amplitude as the input signal but with phase difference of either 0 or 180 depending on the position of the switch. This is illustrated in the animation below showing how the Op-Amp phase shifter works.

Phase shifter switch with Op-Amp

In this tutorial we showed how a simple phase shifter circuit works with manual switch which generates either 0 phase shift or 180 phase shift. We can also use JFET switch to control the switching action instead of manual. And instead of just 0 or 180 degree phase we can use phase shifter circuit to get arbitrary phase shift.

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