When transistor used in practical circuits such as amplifiers, oscillators etc., we require four terminals, two for the input and two for the output port. Since the transistor has only three terminals this difficulty is over come by making one terminal common for the input and output ports.

• The input is applied between the common terminal and one ot the other two terminals. The output is taken between the common terminal and the remaining terminal.
• The term common is used to denote the region that is common to the input and  output circuits.
• Accordingly, a transistor can be connected in the following three different configurations.
  

Transistor Circuit Configuration

1. Common – Base (CB) connection
2. Common – Emitter (CE) connection
3. Common – Collector (CC) connection

• Principle and operation of Transistor

It may be noted that each circuit configuration has its own specific advantages and disadvantages. However regardless of the type of connection, the emitter-base junction is always forward biased and collector-base junction is always reverse biased, when used as an amplifier.

The figure below shows that transistor circuit configuration with n-p-n transistors:

• Forward and Reverse biased PN Junction

Biased Transistor

The application of suitable dc voltage to the different terminals of the transistor is called biasing.

Biasing is to produce power amplification in a transistor circuit. Each junction of the transistor may be independently forward biased. Accordingly there are three different modes of transistor operation.

1. Forward active mode
2. saturation mode
3. cut-off mode

Forward Active Mode

In this mode, the emitter-base junction of the transistor is forward biased and the collector-base junction is reverse biased. This mode is used when the transistor has to be used as an amplifier.

Saturation Mode

In this mode, both the emitter-base junction and collector-base junction are forward biased and the transistor has a very large value of current. The transistor is operated in this mode, when it is to be used as a closed switch.

Cut-off mode

In this mode, both emitter-base junction and collector-base junctions are reverse biased. In this case the current in the transistor is practically zero. The transistor is operated in this mode, when it is to be used as an open switch. For switching application of a transistor, saturation and cut-off modes are used alternatively.

• Types of Semiconductors

Comparison among different types of configuration

	Common Emitter	Common Collector	Common Base
Voltage Gain	Medium	Low	High
Current Gain	Medium	High	Low
Input Impedance	Medium	High	Low
Output Impedance	Medium	Low	High

CSV Jun 21, 2016

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