NPN transistor in common emitter configuration (a) Schematic circuit diagram (b) Circuit symbol iii) Common-Collector (CC) configuration Here, the collector is common to both the input and output circuits as shown in Figure . . The base current I B is the input current and the emitter current I E is the output current. The input signal is applied between base and collector while the output is measured between emitter and collector.
As the output is taken from the emitter in common collector configuration, it is called an emitter follower. Note - - - - - - - - Unit electronics and Communication milliamperes for emitter and collector currents. It is to be noted that if the emitter current is zero, then the collector current is almost zero. It is therefore imperative that a BJT is called a current controlled device.
Applying Kirchoff’s law, we can write the emitter current as the sum of the collector current and the base current. E C ( . ) Since the base current is very small, we can write I E C ≈ . There is another component of collector current due to the thermally generated electrons called reverse saturation current, denoted as I CO .
This factor is temperature sensitive. Therefore, care must be taken towards the stability of the system at high temperatures. The ratio of the collector current to the emitter current is called the forward current gain α ( ) of a transistor. α = I C E ( .
) The a of a transistor is a measure of the quality of a transistor. Higher the value of a, better is the quality of the transistor. It means that the collector current is closer to the emitter current. The value of a is less than unity and it ranges from .
to . . This indicates that the collector current is % to % of the emitter current. Working of a PNP transistor The working of a PNP transistor is similar to that of the NPN transistor except for the transistor, the emitter-base junction is forward biased by a DC