Recapping from our previous post on Amplifier Linearity: 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.
As we have discussed, there are two important measurements in determining power amplifier linearity: the third-order intercept (abbreviated TOI or IP3) point and the 1-dB compression (P1dB) point. These two elements allow you to evaluate and compare amplifier specifications and performance. In the first article, we discussed the P1dB, which you can read more about here. Let’s get to the Third Order Intercept.
When an amplifier becomes non-linear, it will begin to produce harmonics of the amplified inputs. The second, third, and other harmonics are outside of the rf amplifier bandwidth, they are usually easy to filter out. However, non-linearity will also produce a mixing effect of two or more signals.
If the signals are close together in frequency, some of the sum and difference frequencies called intermodulation products produced can occur within the bandwidth of the amplifier. These cannot be filtered out, so they will ultimately become
interfering signals to the main signals to be amplified.Figure 2 shows two signals f1 and f2 occurring within the amplifier bandwidth. With distortion, new signals f1 – f2 and f1 +f2 are produced. However, these signals will also mix with the second, third, and higher harmonics to produce a wide range of potentially interfering signals within the amplifier pass band. The most troublesome are the third-order products, which are 2f1 ± f2 and 2f2 ± f1. Those possibly occurring in the amplifier frequency range are2f1 – f2 and 2f2 – f1.