Power amplifiers and inductors

[juliog]

Hi guys,

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?

Thanks in advance

 

[pancho_hideboo]

There is a DC voltage drop if you use resistor.

 

[juliog]

And why can we tolerate a DC voltage drop in the case of a small signal amplifier but not for power amplifiers?

And even if we can tolerate it for small signal, why not to use the inductor anyway?

Thanks for ur help!

 

[pancho_hideboo]

And why can we tolerate a DC voltage drop in the case of a small signal amplifier but not for power amplifiers?

Consider voltage swing at drain.
For small signal application, voltage swing is small.

And even if we can tolerate it for small signal, why not to use the inductor anyway?

In RF, inductor is used intensively as load for small signal LNA.

 

[juliog]

Thanks again...

But wouldn't be a better approach to use active loads for small signal?

 

[pancho_hideboo]

If we use inductor as load, frequency characteristics of amplifier has to be bandpass.
We can never realize lowpass characteristics.

So if you want to realize broadband amplifier or lowpass amplifier, you have to use resistor as load.

But wouldn't be a better approach to use active loads for small signal?

Active inductor generates large noise.

And there is a DC voltage drop for active inductor since we have to feed DC bias for active devices such as MOS.

 

[juliog]

Thanks for ur answers... really helpful!

 

[Miguel Gaspar]

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.

 

[pancho_hideboo]

the high frequencies passes to the core of the choke that disipates this energy in form of heat.

What on earth do you want to mean ?

 

[Miguel Gaspar]

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.

 

[pancho_hideboo]

Maybe you mean ripple rejection effects by using large inductor which is mandatory in switching mode power amplifier such as Class-D amplifier.

Your claim is true for low frequency amplifier.

But it is not true for RF Power Amplifier.

 

[dick_freebird]

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.