A transistor (BJT) is a three-terminal device ( having three terminals namely emitter, base and collector) but we require four terminals - two for the input and two for the output for connecting it in circuit. Hence one of the terminals of the transistor is made common to the input output circuits. Thus there are three types of configurations for operation of a transistor. These configurations are common base (CB), common emitter (CE) and common collector (CC) configurations. The term common is used to denote the electrode that is common to input and output circuits. Since the common terminal is grounded, these configurations are also referred to as grounded base, grounded emitter and grounded collector configurations, as shown in Figure 1. Each configuration has its own merits and demerits. It is to be remembered that regardless of the circuit configurations, the emitter is always forward biased and the collector is always reverse biased.
NPN Common Base Amplifier
This configuration is used for high frequency applications because the base separates the input and output, minimizing oscillations at high frequency. It has a high voltage gain, relatively low input impedance and high output impedance compared to the common collector.
NPN Common Emitter Amplifier
The common emitter configuration lends itself to voltage amplification and is the most common configuration for transistor amplifiers.
NPN Common Collector Amplifier
The common collector amplifier, often called an emitter follower since its output is taken from the emitter resistor, is useful as an impedance matching device since its input impedance is much higher than its output impedance. It is also termed a "buffer" for this reason and is used in digital circuits with basic gates.
Post a Comment