power amplifier drive design

I am trying to design a power amp. and driver, I get couple of questions:
1) do I need to let PA operate near @ P1dB point, why?
2) why the driver should be designed to have high linearity, that means should operate @about 9dB lower than its P1dB?
3) is there any power loss if I use balun to convert diff. to single?

Thanks!

lijulia said:

I am trying to design a power amp. and driver, I get couple of questions:
1) do I need to let PA operate near @ P1dB point, why?

Click to expand...

This produces a high efficiency. Do you need high efficiency?

2) why the driver should be designed to have high linearity, that means should operate @about 9dB lower than its P1dB?

Click to expand...

This is the simple way to improve linearity of the combined two stages. The driver operates at lower power level so the reduction in its efficiency will not impact the two stage efficiency very much.

3) is there any power loss if I use balun to convert diff. to single?

Click to expand...

Losses in balun and other similar circuits are very low if designed properly.

flatulent,Thanks for the reply, but can you give me more detail about the first two question? why I get higher efficiency if PA operates @P1dB? and how to calculate the total efficiency for the two stages?

How should I start the design?

any good paper or link or other stuffs?

Thanks!

flatulent, also, there is off -chip matching network between driver and PA, should I use power matching instead of conjugate match?

In response to your several questions. The reason that higher output produces better efficiency is that the collector or drain current flows from the power supply voltge. For a small signal you could have used lower voltage for the same current.

Efficiency is output power compared to DC input power.

Innerstage matching is a complex (sorry for the pun) situation. The driver requires a certain load and the input impedance to the next stage is a certain amount. The network between them transforms the second stage input impedance to the impedance required to load the first stage.

This subject fills an entire book. The one by Cripps is probably easiest to find in a library or from a used book site.

Motorola once put out many application notes on power amplifier design. Someone reading this can probably tell us where to find them on school sites.

Higher efficiency of the PA you get AFTER P1dB.
Higher conduction angle means lower efficiency and vice-versa.
Take a look to this document:

**broken link removed**

The driver usually is working in the linear region (below P1dB), for all types of PA’s (linear or nonlinear).

For a Power Amplifier usually is better to use Power Added Efficiency, that take also into account the input power.
PAE = (Pout-Pin)/Pdc

regards