The PI network, commonly used to drive etching systems

The 'PI' Network

The PI network's name comes from the arrangement of the three impedances C1, C2, L1. The PI network is able to match impedances below and above 50Ω. It is primarily used for driving plasma etching systems, though in modified form it is also one of the most common inter-stage matches in R.F. amplifiers. It has a curious property that for a certain range of loads there are two possible settings that will give a match. The PI network is not as useful as might be thought as it does not operate well with low output resistance. Also it can behave unpredictably when automated.