SINGLE –STAGE BJT AMPLIFIER CONFIGURATIONS

Three different amplifier circuit configurations can be obtained by selecting one of the transistor terminals as a common between input circuit and output circuit. In the BJT circuits, figure 3 shows these configurations, which are known as Common Base (CB), Common Emitter (CE), and Common Collector (CC). These amplifier circuit configurations lead to significant changes in the amplifier characteristics. The most noticeable changes in CC (emitter follower) configurations are: the input resistance becomes very high and the gain is close to the unity. These specific characteristics are translated into a useful application known as buffer amplifier.

Figure 1

Therefore amplifier configurations are employed to widen the scope of the amplifier circuit applications. Table1 summarizes the main characteristics of each configuration. The model used in the analysis is the T-model with transistor parameters, : transconductance, : emitter resistance, : common-emitter current gain, and : common-base current gain. , , are the collector, the emitter, and the load resistors NOTICE THAT in CE configuration the presence of has a very large impact on the input resistance and the voltage gain. Most device manufacturers specify as and as . From table1 we observe that the input resistance of the CB configuration is much smaller than the input resistance of CE or CC configuration. The input and output resistances of the above amplifier configurations limit the use of the amplifier to certain applications. Figure 2 below shows a single stage CE and CB amplifiers. Notice the difference between the relative positions of the input-output signals in both configurations.