Yesterday I wrote about how a switchable op-amp based signal inverter can be built. It can also be called a phase shifter circuit because the output signal is 180 degree out of phase w.r.t the input signal. The design of actual phase shifter circuit that can create any angle phase shift between the output and the input was explained in what is phase shifter circuit and how to build it?. These circuits are useful in electronics communication systems, modulation circuit etc.
The last signal inverter circuit had a manual Single Pole Double Throw(SPDT) switch. When one side of the switch was grounded the output signal was inverted and when the switch was connected to the input signal then the output signal was not inverted.
Today, this same phase shifter circuit is modified into a active device controllable phase inverter circuit. This is achieved by connecting a MOSFET or JFET in place of the manual as shown below.
In the circuit, the three resistors R are equal and the resistor RG is the gate pull-down resistor which is used for stable and reliable operation, preventing unwanted switching or floating behavior when the gate drive signal is not active. It keeps the gate voltage at a known state, ensuring the MOSFET stays off when it should be. The input signal is Vin, the output signal is Vout and Vc is the control signal that turns on or off the MOSFET. This control signal Vc can be from timers like 555 Timer or PWM signal from microcontroller board like Arduino. It is the same circuit used to drive dc motor speed with pwm with Arduino. Actually the whole op-amp and other components connected to the drain can be considered as a load. The load is not a rotating physical motors or something like that but a signal converter.
Basically when there is no control signal the gate is low,which keeps the gate off or open. In this case, the input signal Vin is connected to the +ve terminal of the op-amp. The same signal is also applied to the -ve terminal of the op-amp. That is same signal are applied to both the terminals of the op-amp. When this happens, the output signal is in phase with the input signal.This can be proved mathematically as follows.
The op-amp acts as both a non-inverting and inverting amplifiers with gains A+ and A-. The op-amp total gain is,
\(A = A_+ + A_- \)
where,
\(A_+ = 1 + \frac{R_1}{R_3} = 1 + \frac{R}{R} = 1 + 1 =2 \)
\(A_- = -\frac{R_1}{R_2} = -\frac{R}{R} = -1\)
So, \(A = A_+ + A_- = 2 -1 =1\)
And since, \(A = \frac{V_{out}}{V_{in}} \), we have,
\(V_{out} =V_{in}\)
Similarly, when a +ve voltage greater than the \(V_{GS}\) of the transistor, the gate turns on the transistor and in this case, the input signals goes to ground instead of reaching the +ve terminal of the op-amp. That is the +ve terminal is grounded and so that circuit in this case becomes an inverting op-amp amplifier with a gain of,
A = -1
And so the input signal gets phase changes such the output signal is 180 out of phase of the input signal.
So in this way the phase inversion or phase shift of 180 degree is achieved. is achieved.
