Basic Types of Transistor Configurations

 Three main types of configurations in which transistors are used are the common emitter, common base and common collector. While two leads each are needed for input and output of an amplifier circuit, the transistor has only three. So one lead has to be common between input and output. The transistor configurations are named after the lead which is common to both input and output in the given configuration. In actual circuits, there will be resistors and capacitors in addition. Radiofrequency circuits will have inductors and transformers as well. Values of each component will be calculated according to the purpose of the circuit. In all these configurations, the emitter is forward biased and the collector reverse biased.

The most common type is common emitter configuration in which emitter is grounded, base acts as the input and collector as the output. So the input is given between the base and the emitter which is grounded. Current gain in this configuration, which is the ratio of collector current to the base current is known as beta or hFE, which stands for Hybrid parameter Forward current gain, common Emitter. Beta is always more than 1. This configuration has medium input and output impedance levels. Both current and voltage gains are medium. There is a 180 degree phase change between input and output, so that output is the inverse of the input.

In common base configuration, emitter is the input terminal and collector is the output terminal while base is the common terminal. This configuration is also known as grounded base configuration. Alpha is the current gain in common base configuration. It is the ratio of collector current to emitter current. Alpha is always less than 1. When there is no AC input, the alpha is known as DC Alpha. Low input impedance and high output impedance are features of common base configuration. Voltage gain is high, but current gain and overall power gain is low compared to other configurations. The input and output signals are in phase in common base configuration. Low input impedance can provide a good match to 50 Ohms, which may be useful in RF linear amplifiers. Grounding of the base can also reduce unwanted feedback between input and output stages in RF circuits.

Base is the input terminal in common collector configuration, also known as grounded collector configuration and emitter-follower. It is called emitter follower because the emitter voltage follows that of the base, though the emitter voltage is lower by an amount equal to the turn on voltage of the base emitter junction. Here output terminal is the emitter. Gamma is the current gain in common collector circuit. Though the current gain in this configuration is high, voltage gain is unity. Input and output signals are in phase. Emitter follower configuration has high input impedance and low output impedance. High input impedance is useful in preventing loading of the previous stage. Thus it is an ideal buffer amplifier.

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