6.1 INTRODUCTION

In Chapter 4, we discussed the basic construction, characteristics, various modes and principle of operation of bipolar junction transistor (BJT) in detail. Subsequently, in Chapter 5, we dealt with biasing techniques of the transistor in an elaborate manner. In addition, we understood the fundamental concept of BJT acting as an amplifier in a qualitative manner. While biasing a transistor our main goal is to fix a proper operating point, also known as Q point, based on the specific applications. This is achieved by proper choice of the bias resistor and DC voltages. Once this Q-point is finalized, we apply the AC signal in the input side of the biased transistor circuit. Thus, complete analysis of an amplifier consists of DC analysis followed by an AC analysis. In this chapter, we will focus on the AC analysis of the BJT circuits in an elaborate manner with special emphasis on design of the BJT-based linear amplifiers. One important ingredient in the AC analysis and design of linear amplifiers is to replace the BJT with proper equivalent circuits. Such replacement of BJTs with equivalent circuits, known as ‘modeling’ of the BJT, is applicable under small signal approximations. This assumes the amplitude of the applied AC signal is small enough to consider that BJT is essentially operated in active region and its characteristics can be approximated by linear relations. Such linear relations between input and output signals of the BJT allow superposition theorem applicable in BJT circuits and we can obtain the total response of the BJT circuit by separately determining the response due to i) DC sources alone (called biasing), and ii) AC sources. Figure 6.1 shows a schematic diagram of an electronic circuit driven by both DC and AC input. While the purpose of the DC input is to ensure that the device operates in the desired region of operation, i.e. bias the device properly, AC input provides the required functionality like amplification, switching, etc. By applying the small-signal models of the device, thereby linearizing their performance, we apply the superposition theorem to obtain the overall response of the circuit by separately obtaining the individual responses due to DC input and AC input, as shown in Figure 6.1.