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PA interview question

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Full Member level 3
Jan 20, 2005
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ideal class A and B PA, with 6dB backoff input power and same bias conditions, what are the efficiency?

my answer is
A: 50%/4=12.5%
B: 78.5%

am I right?

I imagine you can tell why you calculate as you did? (Even if you can do it in your head.)

Class A:
I suppose the transistor is never fully On or fully Off? (In other words linear mode.) Therefore the load receives average 1/2 supply V. This suggests efficiency 50 percent.

To tell my sidebar story... I ran simulations of class A with different resistor values in the collector leg. I experiment as I try to obtain maximum voltage swing of a sine wave to a speaker with capacitor in series.

I find it occurs when the collector resistor is about twice ohm value of the load. I was never quite sure why. Could it be that it wastes the least amount of power? Does it yield best efficiency?
What is resulting best efficiency?
Does capacitor behavior improve efficiency in the class A amplifier?

Class A, max PAE 50%, 6dB back off means 1/4 of RF power, with same bias, DC power remain the same. So the efficiency become 50%/4.
Class B, biased on pinch off, so doesn't matter the lower RF power, it is still a class B, so the efficiency is still 78.5%.

This is an interview question I was asked, so it was not supposed to be very accurate or math intensive. It should be just applying some basic principle. The interviewer didn't say anything after I gave the answer, I am not sure if that was the right answer.

I think the interviewer recognized that your answer shows you're familiar with the terminology, and you're conversant with established concepts in the field.

An interviewer looks for signs that an applicant has skills and knowledge to contribute to the company, and is easy to get along with, and doesn't need to be told everything, and isn't mixed up in stuff he has to unlearn.

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