I know it's a very elemental question, but I was having a look at several book and I couldn't find a clear explanation
Why is it normal to place an inductor between NMOS drain and power supply in a power amplifier? I know it's a choke inductor... but I cannot see the point. Why is it "needed"?
And why don't we do the same in small signal amplifiers instead of using a resistor?
The real problem is that the power amplifier affect the power supply, in order to avoid it we use the choke to reduce it.
Remenber the inductor has low impedante for DC an low frequencies. but for high frequencies presents high impedance, the high frequencies passes to the core of the choke that disipates this energy in form of heat.
When You put for example adigital device like logic gates, please put your osciloscope to the source and you will see transicions named PARD. For a low power signals you can reduce them bye conect a capacitor between source and earth.
But for power device capacitors are not enough. So you put a choke in series between the source an your application to reduce noise.
The most popular application of it is in the autostereos. If you put your autostereo to the batery you will ear your music and . . . . a lot of noise due to the spark plugs and other fenomenos. To eliminate the noise they put . . . sorprise . . a CHOKE and the noise doesn't affect your device.
A key figure of merit for PAs is power added efficiency.
The inductor load is a lossless (roughly) element that does
not cost PAE. Inductors can also resonate and get you
a higher gain for narrowband amps.