When we did the CE amplifier experiment or JFET or multistage amplifier experiment,we see that there is a lower 3dB frequency and upper 3dB frequency.
Have you ever thought why one side it is low pass and other side it is high pass?
ACTION OF HIGH PASS
Now when you use a coupling capacitor after the source the biasing resistance,input impedance of amplifier form a net resistance.Which makes the input circuit look like a high pass circuit.
So now we conclude that the amplifier attenuates low frequencies and allows mid band frequencies.
Now when the frequency range crosses the mid-band then these blocking capacitors act as short circuits.
Then what the heck can attenuate the frequencies?.
The answer is the internal capacitances of the transistors
INTERNAL CAPACITANCES
Now the question is why did these capacitances didn't come into picture for lower frequencies?
This may be explained like this,
Now there are two PN junctions in the BJT or 3 In FET
When there is sudden change in the voltage from forward to reverse it takes some time to come into its nature.Suppose there is a forward bias initially and there is an instantaneous change from forward to reverse has happened.Now there are moving carriers which have to be stopped and they have to move in opposite direction. So it takes some time.
Suppose your going to your home and somebody calls you from the back then you can't just vanish and go to him,you have to run/walk.It's the same way here.
Now when you increase the frequency the time taken for recovery is not satisfied.So some of the charges get stored which is a capacitive effect.
Now for low frequencies there is a lot of time left after the recovery.
ACTION OF LOW PASS
All the internal capacitance are parallel to the internal resistances.
So the output circuit looks like a low pass filter,which attenuates higher frequencies and allows mid-band frequencies.
I am glad to say that i have reached an explanatory level of my subject......
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