4.1 Introduction

As we saw in the preceding chapter, a bipolar junction transistor requires a forward-bias of the base-emitter junction and a reverse- bias of the collector-base junction in order to operate properly. One of the prime BJT parameters is the current gain, β. It can have a considerable impact on the operation of the circuit. Unfortunately, β also varies with changes in temperature, collector-emitter voltage, etc., and this can lead to circuit instability. In this chapter we shall investigate a variety of circuit topologies to bias the BJT, always with an eye toward stability.