Configurations of BJT
There are three possible ways to connect a BJT:
- Common Base configuration- has Voltage Gain but no Current Gain.
- Common Emitter configuration- has both Current and Voltage Gain.
- Common Collector configuration- has Current Gain but no Voltage Gain.
Ideal characteristics of transistor configuration
Characteristics Common Base Common Emitter Common Collector
Input impedance Low Medium High
Output impedance Very High High Low
Phase Angle 0 degrees 180 degrees 0 degrees
Voltage Gain High Medium Low
Current Gain Low Medium High
Power Gain Low Very High Medium
Common Base configuration
In CB configuration grounded base is common to both input side and output side.
In this configuration, current gain, α is given by
α = Ic/Ie
Since Ie > Ic, α < 1 i.e., current gain of the circuit is less than 1, i.e., it attenuates the signal.
Common Base amplifier circuit
- This is a non inverting voltage amplifier circuit.
- Signal voltages Vin and Vout are in phase.
- Input characteristics represent that of a photo diode.
- Output characteristics represent a forward biased diode.
- Voltage gain is given by Av = (Ic*RL / (Ie*Rin)
- They are used in single stage amplifier circuits such as RF radio amplifiers due to its very good high frequency response.
Common Emitter configuration
In this configuration grounded emitter is common to both input and the output. This type of configuration is most commonly used circuit for transistor based amplifiers. This configuration produces highest power gain.
Common emitter amplifier circuit
- This is a inverting amplifier.
- Signal voltages VIN and VOUT are out of phase.
- Current gain of the this configuration, β is given by, β = Ic/Ib
- Since, Ic>>Ib, β>>1 i.e. current gain of a transistor in CE configuration is much greater than 1. Hence it is used as a voltage, current and power amplifier.
- They are used as class A power amplifiers.
Common Collector configuration
In this configuration grounded collector is common to both input and output. This type of configuration is commonly called voltage follower or emitter follower
Common Collector amplifier circuit
- This is a non inverting amplifier circuit.
- Voltages VIN and VOUT are in phase.
- This provides a very good current amplification but voltage gain is less than 1.
- Current gain Ai is given by Ai = β+1
- Used in applications where impedance matching is necessary.
Relations between α and β
α = β/ (β+1)
β = α/ (1-α)