I will allow v sub G of t to change with time.

That means I can grab this handle over here and move the separator between the

source and drain up and down. I will do so very, very slowly.

In such a way that at any given time if I could take a photograph of what happens

to the channel and I look at it. It will look the same as if I had dc

operation where VG had been frozen for a long time.

Repeating this over here. If the voltages are varying very very

slowly and I could take a photograph of what happens inside the transistor.

And then I look at that photograph. This, the distribution of charges looks

the same as what I would have had had I frozen VG of t, VS of t, VB of t, and VD

of t at their values at the moment I took the photograph.

When that happens, I can say that I have quasi-static operation.

Which means that essentially the charges are governed by the laws that we

discussed for this operation. Only instead of fixed voltages I have

varying voltages. So, for example, QI, that was before

given by FI of the DC voltages vD, vG, vB, and vS, now it is given as a function

of time. By the same function, and this is key, by

the same function, which has to be the case for quasi-static operation, of VD of

T, VG of T, VB of T, and VS of T. Similarly I can say the same thing for

the gate[INAUDIBLE], I'm still using the same function as before.

And for the body voltage. So, quasi-static operation is operation

of the device slowly enough so that I'm allowed to use my equations for the

charges derived for DC operation. And the only change I have to do is

replacing dc voltages by time varying voltages otherwise I can assume I have

the same functions as before. Of course you might say, does this not

always hold true? No, it does not.

Because If you take for example the handle here and you start moving it up

and down very fast. You can see that the distribution in the

channel will be different than what you expect.

For example, if you move it all the way up fast the car, the water here doesn't

have time to get up. So, it will take some time for DC like

conditions to be established and the chamber and the same falls through sort

of this. Once Quasi-static operation does not go

through, we have what we call non-quasi-static operation, which we will

discuss later on. So, for now and for the next several

videos, we're assuming quasi-static operation.

In the next video we'll talk about terminal currents in quasi-static

operation.