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.

P1dB Compression

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.

It is important to know at what point compression begins to occur to prevent distortion. That point the input power that causes the gain to decrease 1 dB from the normal linear gain specification. The 1-dB decrease may be specified as the input level that produces it or the output power where the 1-dB drop occurs.

The 1-dB compression point is more commonly used in class A power amplifier specifications, but may also be used to give some indication of linearity in class AB amplifier as well.

The 1-dB point is measured by driving the amplifier with a CW signal at the desired frequency of interest. The input level is increased while the output power is plotted. The output can be measured with RF test equipment such as anElite RF S-Series all in one piece of equipment shown below which has all the devices needed to perform this test all in one unit.

Summary

Typical Elite S-Series RF test equipment

The 1-dB compression point is important since it shows you the input power point where compression begins and distortion will occur. Amplifiers should be operated below the compression point for best linearity. Next, let’s tackle Third-Order Intercept how that plays a role in amplifier linearity. For all your questions around Amplifier Linearity, RF Amplifiers, or RF Testing Equipment needs, contact Elite RF today.