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.