Unit C. Common-Emitter Amplifier Stage

Analog amplifier stages (BJT) generally comprise three possible terminal configurations: common emitter, common base, and common collector (emitter follower). As the names suggest, in each case, the input and output are referred to the common terminal. The MOSFET equivalents are common source, common gate, and common drain (source follower), respectively.

As the most fundamental of transistor amplifier building blocks, the common-emitter stage is the logical configuration to begin a study of signal amplifiers. The common-emitter considered initially in this unit is obtainable with a simple extension of the dc measurement circuit from Unit B (Fig. B.12). Based on this circuit various aspects germane to signal amplifiers in general are discussed. In Unit C.7, the common-emitter stage with active load is explored. Both types of amplifiers, with resistive and active loads, are investigated extensively in projects.

The emitter-follower stage is discussed in Unit C.9 along with a general treatment of the various effects on the common-emitter stage with an emitter-branch resistor. An entire unit (Unit C.7) is devoted to the source-follower stage, the MOSFET equivalent of the emitter-follower stage. The common-gate stage, the MOSFET equivalent of the BJT common-base stage, is considered extensively in conjunction with the role it plays in the differential amplifier stage (Unit 8).