10.1 INTRODUCTION

In Chapter 6, we derived various small-signal models of BJT, followed by detailed analysis of the BJT amplifiers for CE, CB and CC configurations. In designing such small-signal amplifier the main focus is on amplification linearity and magnitude of gain. Since signal voltage and current in the small-signal amplifier is restricted to small amplitude for maintaining linearity, their power handing capability or the power efficiency with which it delivers AC power to the load at the expense of DC power are of secondary concern. A voltage (current) amplifier provides voltage (current) amplification primarily to increase the applied input voltage (current). Thus, in general, these small-signal amplifiers are not capable of delivering large amount of power to a passive load. But most of the practical electronic amplifiers (starting from audio amplifier to switching power supply) require a final output stage capable of delivering large power to the load. Such amplifier/amplifying stage at the output stage of a practical amplifier circuit is known as power amplifier. It can be designed using BJTs, MOSFETs and various special semiconductor devices like thyristors. However, we will consider only BJTs while discussing the design of such amplifiers.

10.2 PERFORMANCE CHARACTERISTICS OF POWER AMPLIFIERS

The quality of power amplifiers is characterized by the following parameters:

• • Power handling capability: The amplifier must be capable of delivering the required amount of power to the load in an efficient manner. This demands that the power dissipation in the transistors be as low as possible.

• • High power conversion efficiency: Power conversion efficiency is another important figure of merit of a power amplifier defined as the ratio of AC power delivered to the load to the DC input power. It is expressed as,

We will calculate the power conversion efficiency η of different types of power amplifiers. A high value of η is desired as it utilizes less DC power to deliver same output AC power to the load.

• • Low total harmonic distortion: Since a power amplifier deals with relatively large signals, the small-signal approximations and resultant linear equivalent circuit models are not applicable here due to the nonlinearities of the transistors while dealing with large signals. Even though power handling capability and high conversion efficiency are major design criteria in power amplifiers, linearity still remains an important criterion.