26a)
Common Emitter Amplifier
The common emitter amplifier circuit is shown in the figure below which consists of voltage divider biasing and is used to supply the transistor base bias voltage as per requirement. The voltage divider biasing circuit consists of the potential divider with two resistors connected such that their mid point is used for supplying base bias voltage.
The common emitter amplifier consists of different electronic components that include resistors R1 used to determine forward bias, R2 is used to develop bias, RL is used for developing the output called as a collector load resistor, and RE is used for thermal stability. Capacitor C1 which is called as a coupling capacitor (including capacitor C2 that can be connected at the output terminal) which is used for separating AC signals from DC biasing voltage. As shown in the figure above bias vs gain common emitter amplifier transistor characteristics, if R2 increases, then forward bias will increase and R1 & bias are inversely proportional to each other.
If a weak signal (AC) is given to the transistor base of the common emitter amplifier circuit, then a small base current (AC) will start flowing. Thus, much larger current will flow through the collector through Rc (which is equal to β-beta times the base current). The voltage across resistance Rc is very this is because the resistor Rc value is very high (in the range of 4 to 10kOhm). Thus, a very high current appears in the collector circuit in the amplified form with the applied weak base signal. Hence, common emitter transistor acts as an amplifier circuit.
Voltage gain
voltage gain is given by the product of current gain and ratio of output resistance of the collector circuit to the input resistance of base circuit.
The ratio between collector current (change in collector current) and base current (change in base current) is equal to current gain
Regards