why does the power amplifier introduce cross-over distortion??

Answer 1

PAs usually have push-pull output stages, which are class B amplifiers. Some of the better PA output stages have modified bias networks so the cut-off half of the "push" or "pull" transistor is not completely cut-off, thereby creating a class AB.

The reason there is crossover distortion in transistor push-pull outputs is, that the base-to-emitter diode has a forward voltage drop of anywhere between 0.5 and 1 volt. When the signal is near the zero crossover point, and the "push" transistor is nearing cutoff (so the "pull" transistor can start sinking current for the negative voltage swing of the output signal), it cuts off approx. 0.5 volts above zero -- very non-linear, causing the crossover jump.

Likewise, when the signal is coming out of the negative swing and going positive, near zero volts, the "pull" transistor cuts off at about 0.5 volts while the "push" transistor starts to turn on -- both transistors are operating non-linearly, causing distortion.

In an AB amplifier, since the transistor that is going to be turned on soon is sort of half-biased, it can start to pick up some of that current from the transistor that is in the process of turning off, but there will still be a partial dead band around zero volts where neither transistor is in its linear portion of operation. The distortion will be reduced over a pure class B amplifier.

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Answer 2

Amplifier circuits have to trade off between fidelity - the ability of the amplifier to match the output with the input and efficiency - the power consumed versus the amplification achieved. Higher fidelity amps (audio, for example) are not as efficient as those whose primary purpose is simply signal amplification.

Class B fall somewhere in between. This class actually uses a pair of amplifiers, each amplifying only one-half of the waveform; this is commonly referred to as a "push-pull" amplifier. One amplifier handles the negative portion of the signal, the other handles the positive portion. During the zero point transition, distortion is induced as the amplification "switches over" from one to the other. This is crossover distortion.

Answer 3

each part of the transformer must work into a proper termination.