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A simple Question abouT Class B RF Power Amplifier

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eoncu

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Hello all,
I am trying to learn some RF power amplifier theory. I am confused in the very begining. Maybe it is a funny question.In Class B RF PA, the drain current is zero for half of the input cycle.(the conduction angle is 180 degrees) That means the transistor does not conduct for the half of the cycle. There is no output voltage.But in PA books i see a pure sinusiod as output voltage.(for single-ended PA)I was expecting zero output voltage instead.(because of zero drain current) How can it be?Can anyone help me?
Thank you in advance...
 

The output current from the transistor is not sinusoidal. However, if the output is loaded with a tuned circuit, then the output voltage will be almost sinusoidal. The tuned circuit acts as a filter to remove most of the distortion. There may be extra LC filtering at the output as well, to reduce the distortion further.

In a push-pull class B amp, both transistors conduct for half the cycle, so there is less distortion to start with.
 

Thank you for your reply godfreyl. Can you explain how the tuned circuit operates to produce the other half of the sinusoid?
Thank you very much...
 

The current versus time plot for class B contains the first harmonic component (the pure sinewave) and lots of harmoncs. The higher harmonic components are the reason that the waveform doesn't look like a pure sine wave.

When you remove all the harmonic components, you will only see the first harmonic, and that is a pure sine wave. The low pass or band pass filter remove all harmonics and pass the wanted first harmonic component.

If you make a pendulum from a rope and a weight, you can get a nice sinusoidal back and forth movement, though you give only a very small push each period.

You will have the same situation in a class C amplifier where the conduction angle is less then 180 degrees.
 

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