Theory - 33 :- Transistor configuration ‐ CB, CE, CC,

 

Transistor configuration ‐ CB, CE, CC,  

CE, CB, CC Configurations:       

    Transistor Configuration:

We know that transistor has three terminals namely emitter (E), base(B), collector(C).  

• However, when a transistor is connected in a circuit, we require four terminals (ie) two terminals for
• input and two terminals for output.  This difficulty is overcome by using one of the terminals as common terminal.
• Depending upon the terminals which are used as a common terminal to the input and output
• terminals, the transistors can be connected in the following three different configurations.

1. Common base configuration                            
2.  Common emitter configuration
3.  Common collector configuration

Transistor configuration basics

The terminology used for denoting the three basic transistor configurations indicates the transistor terminal that is common to both input and output circuits. This gives rise to the three terms: common base, common collector and common emitter. The term grounded, i.e. grounded base, grounded collector and grounded emitter may also be used on occasions because the common element signal is normally grounded. The three different transistor configurations are:

1.  Common base configuration:                      

• In this configuration base terminal is connected as a common terminal.
• The input is applied between the emitter and base terminals. The output is takenv between the collector and base terminals.

Characteristic of Common base: 

 This transistor configuration provides a  low input impedance while offering a high output impedance. Although the voltage is high, the current gain is low and the overall power gain is also low when compared to the other transistor configurations available. The other salient feature of this configuration is that the input and output are in phase.

1.      2 Common collector configuration:

• In this configuration collector  terminal is conncted as a common terminal.
• The input is applied between the base and collector terminals.The output is taken  between the    emitter and collector terminals

Characteristic of Common collector:  

This transistor configuration is also known as the emitter follower because the emitter voltage follows that of the base. Offering a  high input impedance and a low output impedance it is widely used as a buffer. The voltage gain is unity, although current gain is high. The input and output signals are in phase.  

3 Common emitter configuration:

• In this configuration emitter terminal is conncted as a common terminal.
• The input is applied between the base and emitter terminals.
• The output is takenv  between the collector and base terminals.

Characteristic of Common emitter:   

This transistor configuration is probably the most widely used. The circuit provides a medium input and output impedance levels. Both current and voltage gain can be described as medium, but the output is the inverse of the input, i.e. 180° phase change. This provides a good overall performance and as such it is often thought of as the most widely used configuration.  

Transistor circuit configuration summary table

The table below gives a summary of the major properties of the different transistor configurations.

Transistor Configuration Summary Table

Transistor Configuration		Common Base	Common Collector (Emitter Follower)	Common Emitter
Voltage gain		High	Low	Medium
Current gain		Low	High	Medium
Power gain		Low	Medium	High
Input / output phase relationship		0°	0°	180°
Input resistance		Low	High	Medium
Output resistance		High	Low	Medium