RF and Microwave Circuit Design

9Microwave Small-Signal Amplifiers

9.1 Introduction

Transistors used for small-signal amplification at microwave frequencies are usually supplied by the manufacturer in beam-lead packages, with the transistor performance data in an S-parameter format. The transistors are normally GaAs MESFETs (gallium arsenide metal semiconductor field effect transistors) because these offer better noise performance at microwave frequencies, compared to silicon bipolar devices.

The design procedure for microwave small-signal amplifiers is largely that of designing suitable input and output matching networks to enable the transistor to give the required performance when connected between specified source and load impedances. If the source and load impedances are conjugately matched to the input and output, respectively, of the amplifier, then it follows from Eq. (5.22) of Chapter 5 that the maximum unilateral transducer power gain of the amplifier will be

(9.1)

where the S-parameters are those of the transistor.

In addition to gain requirements, the required performance will normally include noise specifications and these also impinge on the design of the matching networks. The main consideration of this chapter will be on the design of suitable matching networks, and since the frequencies are in the microwave region the primary focus will be on distributed networks.

One of the major problems ...