Driver of Power Amplifiers

In designing power amplifiers.
Why we are using driver stage.
Is that for increasing the over all gain of the PA.
or does it have other purposes?
what is its effect on Psat and OP1dB?

In class B or AB amplifier it's usually easier to work with if we put the N-device at high side, P-device at bottom. To get a certain output amplitude you must apply the same input amplitude. No need for voltage gain but the power amp applies current gain.

It's for a driver stage to apply voltage gain.

If we put a P-device at high side (and N-device at bottom), we can apply both voltage and current gain in one power amplifier. However in that situation the transistors must be biased properly. To do so requires extra design effort and experimentation. Careful attention is needed, to minimize distortion as well as minimize power being wasted.

do You mean that the driver stage is just for getting higher voltage gain ?
if we increased the voltage gain, the power gain would increase as well, is not right?

Power stages in Amplifiers generally do not have such high gain as driver stages often do, and are usually selected in a particular form because they are more efficient, rather than for their gain. Because of that it is sometimes necessary to use a driver stage to obtain the drive level to the power amplifier to get the output power that you want, to achieve the overall gain.

So yes, a driver stage is there to increase the overall gain. Sometimes it also provides a bit more isolation of the source of the drive signal from the load (e.g. antenna) which may vary somewhat in impedance.

Usually a driver stage has minimal impact on parameters such as Psat and OP1dB of the final stage, as they are parameters describing what is happening in the output stage.

Input levels to achieve 1dB compression and saturation will of course be affected, as now there is more gain in the amplifier circuit. I hope that is a better aswer to your question.

Thanks for your evident answer.
I understand that, the main purpose of the driver stage is to improve the overall gain.
because the PA usually does not offer the required gain especially in higher watt levels.