TABLE OF CONTENTS An active device such as a transistor in an amplifier is necessary to give gain to the circuit. These transistors may need matching circuits because they lack the correct input or output impedance. Some devices are well matched to the characteristic impedance of the transmission line connecting the circuit. However, many transistors need matching circuits at the input and output.
This chapter introduced many useful analysis and design techniques to optimize transistors. The first section discussed information published by manufacturers of high-frequency transistors. The S-parameters are given as two-port magnitude and phase information that are dependent on frequency and bias point. The S-parameters of a transistor vary with the voltage and current of the transistor. Sometimes it is necessary to operate a transistor at various bias points for different applications. The difference between the various bias points are discussed in Chapter 8.
Section 6.3 showed how to calculate the stability of a transistor. Most transistors are potentially unstable at some frequencies. This potential instability is characterized by an input or output reflection coefficient greater than 1. This means that the circuit will exhibit reflection gain, i.e., it will reflect more power than the incident power. Under certain conditions, a growing standing wave can develop and the amplifier can break into oscillations, which will render the circuit useless. Section 6.3 also presented equations for all possible load and source impedances that result in a reflection coefficient greater than 1 at the input or output of the transistor. We can...
