5.1 INTRODUCTION

In the previous chapter, we studied about the fundamental operation of the BJT and got acquainted with various configurations and characteristics of a transistor. We have seen that a transistor can be operated in three regions, namely active region, saturation region and cut-off region. We learned that transistor is to be operated in its active region for amplifying action. The saturation and cut-off regions, on the other hand, are mainly used for switching applications. Each of these applications requires the DC operating point (Q point) of the transistor to be fixed in a desired location of the output characteristics before the application of the alternating signal. The entire process of selecting proper voltage sources and external resistors to bias the transistor junctions (EBJ and CBJ) with suitable amplitude and polarity of voltage and ensuring stability of the DC operating point (without any applied signal) is known as transistor biasing. It ensures suitable positioning of the Q point, i.e. choosing proper set of input current, output current and output voltage. Once the transistor is properly biased, we can apply the alternating voltage in the transistor circuit for various applications. This normally causes symmetrical swing of the input and output voltage and currents about their DC level, fixed previously by biasing.

Figure 5.1 shows a block diagram demonstrating the influence of biasing in a transistor amplifier, indicated by symbol A. Let the amplifier is a CE amplifier with a 180° phase difference between input and output. Figure 5.1(a) corresponds to a biasing scheme where the Q point is positioned in active region and its entire swing due to the application of AC signal is confined to the active region. This yields a perfect amplification, i.e. entire input signal is amplified. On the other hand, if the Q point enters into cut-off or saturation region due to the applied AC signal, we get a distorted output signal, as indicated in Figure 5.1(b) and (c). Here, a portion of the desired output signal is clipped off as the Q point enters into cut-off or saturation region for some portion of the input signal. This produces distorted output signal. This particular example tells us the profound impact of transistor biasing on the operation of the device and its desired application.