Common Base Transistor Configuration
In this Transistor configuration base is used as a common terminal for both input and output signals. The input is applied between the base and emitter terminals and output is taken between the base and collector terminals with the base terminal grounded. Here the input parameters are base-emitter voltage and emitter current and the output parameters are collector-base voltage and collector current.
The current gain is equal to or less than unity for the Common Base configuration. The amplifier circuit configuration of this type is a non-inverting amplifier circuit. It has a high voltage gain value. This transistor configuration has high output impedance and low input impedance. It has high resistance gain.
The voltage gain for this configuration is given below.
AV = Vout/Vin = (IC*RL) / (IE*Rin)
Current gain in common is given as
α = Output current/Input current = IC/IE
Input characteristics are obtained between Emitter current and Base Emitter voltage with constant output voltage which is Collector Base voltage. The below figure shows the input characteristics of the common base configuration.
The output characteristics are obtained between the output current and output voltage with the emitter current constant. By varying the Collector Base voltage value for different points, the collector current is recorded. The figure below shows the output characteristics of the common base configuration.
Common Collector Transistor Configuration
In this Transistor configuration collector terminal act as common for both input and output signals. It is also known as emitter follower configuration because the emitter voltage follows the base voltage. It is mostly used as a buffer. They are widely used in impedance matching applications because of their high input impedance.
The input signal is applied between the base-collector region and the output is taken from the emitter-collector region. The input parameters are Base Collector Voltage and Base Current and the output parameters are Emitter Collector Voltage and Emitter Current. This has high input impedance and low output impedance.
Ai = output current/Input current
Ai = β + 1
The input characteristics of a common-collector configuration are obtained between the base current and the Collector Base voltage keeping the Emitter Collector voltage constant. The below figure shows the output characteristics of the common collector configuration.
The output characteristics are obtained between the Emitter Collector voltage and emitter current keeping the base current constant. In this, if the base current is zero then the emitter current also becomes zero and no current flows through the circuit. The transistor operates in the active region and finally reaches the saturation region. The below figure shows the output characteristics of the common collector.
Common Emitter Transistor Configuration
In this Transistor configuration, the Emitter is used as a common terminal for both input and output signals. It is an inverting amplifier circuit. The input is applied between the base-emitter region and the output is taken between collector-emitter terminals. Collector current and Base current give the current gain beta. It is mostly used because it has medium input and output impedance. This Transistor configuration has medium current and voltage gains.
- Current gain (α) = IC/IB
- Current gain (β) = IC/IB
- Collector current IC =α IE = βIB
The input characteristics of the common emitter configuration are obtained between Base current and Base Emitter voltage while keeping the Collector-Emitter voltage constant. The below figure shows the input characteristics of the common emitter configuration.
The output characteristics of the common-emitter configuration are obtained between the Collector current and Collector-Emitter voltage while keeping the base current constant.
The below figure shows the output characteristics of the common-emitter configuration.
Comparison Between Transistor Configurations
The table below gives the major properties of the different transistor configurations. It shows the comparison between transistor configurations with respect to voltage gain, current gain, and input and output impedance.
| Characteristics | Transistor Configuration | ||
Common Base | Common Emitter | Common Collector | |
Current Gain | Low | High | High |
Voltage Gain | High | High | Approximately 1 |
Power Gain | Moderate | High | Low |
Input Impedance | Low | Medium | High |
Output Impedance | High | High | Low |
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