RF PA Distortion and bias point

Hi all,
I have a question on RF Power amplifier design. I understand that choosing the bias point (Class of the amplifier) determines the efficiency and the linearity. I would appreciate if someone could explain how the class of operation impacts the linearity.

Thanks

It depends on the type of data that you want to transmit. If it is frequency modulation based, then you can use switching PA which has a much better efficiency than class A and you will not face linearity issues. But if the data has amplitude modulation you may use class A or AB amplifier and then you may have linearity issues.

It depends on the circuit of your PA. If it has a tuned circuit in its output then you can reduce the angle of conduction to almost 180 degrees ( collector current flows for 1/2 on the rf cycle), this is because the tuned circuit will "generate" the missing half cycle. if you are using a wide band circuit, then you must use 360 degrees of collector current conduction OR have some output filter that will resonate. If you want the RF ouput power to increase with increasing input levels, then you need a linear amp. if you are just switching on and off the carrier then a non linear system (class C) will be the best.
The classes are A, 360 degrees of conduction, most linear, B, 180 degrees of conduction, linear RF or push pull, linear audio. C, conduction less then 180, very efficient, need tuned circuit.
Frank

In ideal case, the class A and B are linear configurations, because the conduction angles of them remain constant during the power sweep. On the contrary, class AB and class C are non-linear, due to the drive-dependence of conduction angles.
However, in real case, the story changes, resulting from the non-linear characteristics of the transistor. For example, a class AB LDMOS PA has a better AM-AM response than the class B, contributed by the non-linear transconductance of the device.
The most important topic about linearity is the AM-PM response, which is difficult to be related to the configuration of PA.

There are reasons why Class A yields the highest linearity.
Class A current is where fT curve is flat, respectively has its peak.
If in datasheets gain would be plotted linearly rather than on a logarithmic scale one would see that gain peaks where fT peaks. So gain is also flat at Class A current.
Slope and curvature of gain have strong impact on linearity.
Imagine like RF voltage and current circle around operation point along the dynamic loadline. Means that during an RF cycle voltage and current vary.
But, gain depends on voltage and current. Means that gain changes during an RF cycle. Means that a sinusidual input signal is not amplified constantly during an RF cycle. Means distortion.
This is why in Class A operation the flat gain helps to keep distortion low.
Look at a datasheet and compare IP3-, gain- and fT-curves over current and voltage, e.g.
Infineon Technologies
Note that this explanation is independent on the technology you use, BJT, GaAs, ...