TRANSISTOR RF IMPEDANCE MATCHING

با سلام و عرض تبريك ايام و ليالي فجر بر فجر افرينان

TRANSISTOR RF IMPEDANCE MATCHING
by Harry Lythall - SM0VPO

Introduction

In recent months I have been asked to provide more basic information, such as FET biasing, and before this; "how one can accurately calculate the impedance matching to power RF transistors?". In this article I will discuss transistor impedance maching but the article will not be entirely theoretical. There are a few points that are not so obvious from a purely mathematical point of view, for example bandwith.

As a matter of interest, when I took my basic RF engineers training it was all valves (tubes), where a high impedance of typically 5000 Ohms is matched to an output impedance of 50 Ohms. It was not until Cambridge 1975 to 1980 that I learned about transistors.

At first sight one knows that the input circuit to the output transistor has a pre-determined Q value, which in turn affects the RF drive to the transistors with frequency. BUT RF transistor specifications also give a specific power gain, which is subject to tolerance. In our example circuit I will assume the output transistor manufacturer has quoted a gain specification of 22dB. I don't believe that all transistors of one type can be relied upon to have the same gain, so I will overload the input circuit by 3dB to 5dB. This means that if the transistor does indeed fulfil its gain promise, then as the input circuit signal falls at band edges, the transistors will still be fed with 100% of the required signal instead of 300% (the 5dB extra drive).

The overall effect is that the output will remain flat(ish) and have a more predictable output power; independant if small transistors variations. The output matching circuit will, however, affect the output signal at band edges, but in commercial designs this is corrected by designing for a higher output power and using Automatic Gain Control (AGC) to keep the output flat.