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power amplifier bias voltage_ making power amplifier in the saturation region

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shanmei

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I am told that power amplifier should be biased at saturation region.

For the bipolar process, as shown in the figure, the saturation region( linear region) is at the left side, which is different from the saturation region of CMOS.

1.png

So I need to bias the power amplifier to the B node, right? It is at the center of the ac loadline. The ac signal will have large swing.


Bias the base of the BJT to node A is not right? Since it is the DC loadline center point, but it is not good for ac signal( large signal ) has large swing, which will clip the signal swing.

Is my understanding right? Thanks.
 

Both BJT or CMOS, the Class-A Power Amplifiers are always biased for symmetric swing around the OP.So that OP is selected in according with Load Line theory.
 
Yes, class A PA should be biased at the center of the loadline.

For better power efficiency, it should biased at class AB, or class B.
 

You are wrong, twice:
1) An amplifier with junction transistors is NEVER operated in the Saturation Region.
2) Class-B is more efficient because both output transistors are turned off near the Q-point which causes awful crossover distortion. Class-AB is used in an amplifier because it has no crossover distortion.
 

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