1
\$\begingroup\$

I have started studying the topic of "RF" Power Amplifier (PA) operation, and so far I have come across description like, if PAs are non-linear, it does not form a straight line and rather it has a curved line (more so in the higher region). And non-linear region of PAs can cause distortion. This is very vague to me still, specifically in understanding, why do specifically RF Power Amplifier have to be Linear? What happens when they are linear? And then what happens in the non-linear region (or what does it mean by distortion here?)?

Thanks.

\$\endgroup\$

1 Answer 1

3
\$\begingroup\$

By "straight line", they mean if you plotted a graph of "input amplitude" vs. "output amplitude", it would form a straight line. Distortion is a general term for when the output of an amplifier looks different to the signal coming in (if a sine wave went in and a square wave came out, we'd say that the signal had been greatly distorted).

Now, PA's don't specifically have to be linear, it depends on the application. For signals modulated in phase or frequency (FM radios), linearity isn't necessarily a big issue as the output amplitude is not that critical. However, non-linearity can produce harmonics at unwanted frequencies (intermodulation distortion is a good example) and is usually undesirable.

Where linearity is a real issue is in high bandwidth systems with complex modulation schemes (WiFi, 4G, ADSL, cable TV - cable has VERY complex modulation schemes with up to a hundred amplitude levels or so). The reason is that the amplitude and phase carry information and if the amplifier is not very linear it can mess with the phases and the amplitude ratios (signals with amplitudes A and 2A might go in, but only A and 1.1A might come out). In these situations it's important that the signal coming out looks that same as the one coming in (albeit at a higher power). Otherwise, the decoder at the other end won't be able to tell the symbols apart which will lead to data corruption.

So do PA's have to be linear? Well, it depends...

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.