TRANSISTOR AMPLIFIER DESIGN Ampli®ers are among the basic building blocks of an electronic system. While vacuum tube devices are still used in high-power microwave circuits, transistorsÐ silicon bipolar junction devices, GaAs MESFET, heterojunction bipolar transistors (HBT), and high-electron mobility transistors (HEMT)Ðare common in many RF and microwave designs. This chapter begins with the stability considerations for a two-port network and the formulation of relevant conditions in terms of its scattering parameters | Radio-Frequency and Microwave Communication Circuits Analysis and Design Devendra K. Misra Copyright 2001 John Wiley Sons Inc. ISBNs 0-471-41253-8 Hardback 0-471-22435-9 Electronic 10 TRANSISTOR AMPLIFIER DESIGN Amplifiers are among the basic building blocks of an electronic system. While vacuum tube devices are still used in high-power microwave circuits transistors silicon bipolar junction devices GaAs MESFET heterojunction bipolar transistors HBT and high-electron mobility transistors HEMT are common in many RF and microwave designs. This chapter begins with the stability considerations for a two-port network and the formulation of relevant conditions in terms of its scattering parameters. Expressions for input and output stability circles are presented next to facilitate the design of amplifier circuits. Design procedures for various small-signal single-stage amplifiers are discussed for unilateral as well as bilateral transistors. Noise figure considerations in amplifier design are discussed in the following section. An overview of broadband amplifiers is included. Small-signal equivalent circuits and biasing mechanisms for various transistors are also summarized in subsequent sections. STABILITY CONSIDERATIONS Consider a two-port network that is terminated by load ZL as shown in Figure . A voltage source Fs with internal impedance Zs is connected at its input port. Reflection coefficients at its input and output ports are Tin and Fout respectively. The source reflection coefficient is Ts while the load reflection coefficient is rL. Expressions for input and output reflection coefficients were formulated in the preceding chapter Examples and . 385 386 TRANSISTOR AMPLIFIER DESIGN Figure A two-port network with voltage source at its input and a load terminating the output port. For this two-port to be unconditionally stable at a given frequency the following inequalities must hold irs 1 i irL 1 i irin 1 s21 s12rL 5 1 - rL 1 .
