What are the criteria for judging the nonlinearity and linearity of a power amplifier?

The criterion for judging nonlinearity and linearity can be: use a fixed frequency f1 and level input to the amplifier, observe the output of the amplifier, if its output contains additional frequency components (usually its harmonics 2f1, 3f1...) Then the amplifier is non-linear.

In fact, the linear/non-linear area of any amplifier depends on the input signal level. When the input signal level is greater than a certain value (P1dB), the non-linear frequency component of the output will increase sharply. In order to reduce non-linear products, the input signal is generally required to be 3~6dB lower than the amplifier P1dB.

There is also a problem with the signal modulation method. For example, the PAR (peak/average) of the GMSK modulated signal is ~=0dB, and its amplifier has relatively low linearity requirements, because its signal will not exceed the P1dB point and push the amplifier into the saturation zone; QPSP's PAR=6~10 dB, its amplifier has relatively high linearity requirements, otherwise the signal will easily exceed the P1dB point and push the amplifier into the saturation zone. OQPSK has a lower peak value than QPSK, so the power amplifier used in CDMA mobile phones does not have to be completely linear.

In fact, the most fundamental explanation of this problem is to read books on low-frequency and high-frequency electronic circuits. The amplification curve of the three-base tube is linearly amplified only in the lower section of the operating point. When the operating point is raised, it enters the nonlinear amplification zone. The usual Class A and Class B power amplifiers work in the linear range, but the efficiency is very low, usually only 30% to 40%, most of the energy is consumed on the internal resistance of the electrode, and the flow angle is large; and C The class amplifier works in the non-linear range, and the efficiency is very high, which can reach 70%~80%, but the waveform of the output amplified signal will produce distortion, thereby producing harmonic components. Therefore, the class C amplifier must be connected to the LC oscillation circuit. Because the LC tank can play a certain filtering role, filter some harmonic components and retain the fundamental components.

Therefore, if you use a constant envelope modulation method, you can use a nonlinear amplifier; and if you use a non-constant envelope modulation method, especially when the peak-to-average ratio is relatively large, you must use a linear method.