PART 1: AMPLIFIER LINEARITY P1DB COMPRESSION
Amplifier linearity is focused on the amplifier's linear relationship of input power to output power. In the most optimal case, it would be directly related by the gain of the amplifier. Technically, this would mean an amplifier with a gain response of “X” dB across the frequency range of the amplifier would be consistent, but most lose gain with increasing frequency and inevitably suffer a gain loss at higher frequencies.
There are two important measurements in determining rf power amplifier linearity: the third-order intercept IP3 point and the 1- dB compression (P1dB) point. These two elements allow you to evaluate and compare power amplifier specifications and performance.
RF Amplifiers are specified by many characteristics including gain, frequency bandwidth, power output, linearity, efficiency, noise, and input/output impedances. In wireless applications, linearity is key because of the broadband modulation schemes used today such as wideband code division multiple access (WCDMA), and others.
Linear amplifiers operate in class A or class AB. Class A operation is preferred if maximum linearity is desired, but its downside is poor efficiency, typically less than 20% in practice. To achieve greater efficiency, class AB is used. The disadvantage is that class AB biasing introduces signal distortion and produces harmonics and intermodulation products. The IP3 and the P1dB points can provide a means of determining amplifier linearity.
Most linear amplifiers have a fixed gain for a specific frequency band such as the Elite RF Gold series RF amplifier shown below. When you plot output power versus input power, you will see a linear relationship (Fig. 1). The slope of the line is the gain. As the input power continues to increase, at some point the gain starts to decrease. The amplifier goes into compression where no further output increases occur for an input increase. The gain flattens, meaning the amplifier becomes saturated. Its response becomes non-linear and produces signal distortion, harmonics, and inter-modulation products.