We will consider only the small signal equivalent circuit for this situation.

This is the small signal equivalent of the transistor, which we have yet

to derive, and these are the small-signal voltages shown at the terminal voltages.

The DC bias quantities are replaced by short-circuits,

because this circuit only represents small signal operation.

The corresponding currents are also small-signal currents.

Now, we will assume that the voltages vary sinusoidally,

for example, Vg, the gate small signal voltage,

is supposed to have magnitude M and a phase phi, and

it is varying at radian frequency omega, so it's M cos omega t plus phi.

Now, in circuit analysis, you know that we very often handle this

situation by defining complex phasors that retain the magnitude and

phase correspondingly M is the amplitude of the time function,

phi is the phase of the time function and omega is the same for

all quantities in the circuit and it is understood.

So if instead of voltages and currents you use voltage phasors and current phasors,

you end up with the corresponding circuit that looks like this.

So for example, Vg is this complex number,

Ig is another complex number that represents the magnitude and

phase of the corresponding small signal gate current, and so

on for all of the other quantities.

So we will be using a phasor representation.

But for brevity, instead of saying the voltage, the gate voltage phasor and

the gate current phasor, we will be talking about the gate voltage and

the gate current, and the word phasor will be understood.