There are three leads in a transistor, i.e., the emitter, base, and collector terminals. When a transistor is to be connected in a circuit, we require four terminals: two for the input and two for the output. This difficulty is overcome by making one terminal of the transistor common to both input and output terminals. The input is fed between this common terminal and one of the other two terminals. The output is obtained between the common terminal and the remaining terminal. A transistor can be connected to a circuit in the following three ways:
- Common base(CB) connection
- Common emitter(CE) connection
- Common collector(CC) connection
Common Base (CB) connection
In a common base connection, an input signal is applied between the emitter and the base, and the output is taken from the collector and the base. The base of the transistor is common to both input and output circuits, hence the name common base connection. A common-base npn transistor circuit is shown in Fig. (i), and Fig. (ii) shows the common-base pnp transistor circuit.
(i) current amplification factor α is given by:
(ii) collector current,
(iii) Input resistance,
Common emitter (CE) connection
In this arrangement, the input signal is applied between the base and emitter, and the output is taken from the collector and emitter. The emitter of the transistor is common to both input and output circuits and hence the name is common emitter connection. Fig (i) shows the npn transistor circuit and Fig (ii) shows pnp transistor circuit.
(i) current amplification factor β is given by:
Out of the three transistor connections, the common emitter circuit is the most efficient. It is used in about 90 to 95 percent of all transistor applications. It is used because of:
- High current gain: The current gain is very high in the range of 20 to 500. In a common emitter connection, Ic is the output current and IB is the input current. The collector current is given by:
As the value of β is very high, the output current Ic is much higher than the input current IB. Hence, the current gain is very high in the CE arrangement.
- High voltage and power gain: Due to the high current gain, the common emitter circuit has the highest voltage and power gain of three transistors connections.
- Moderate output to input impedance ratio: The ratio of output impedance to input impedance is small. Hence the coupling becomes efficient in this arrangement.
Common collector (CC) connection
In this circuit arrangement, the input signal is applied between the base and collector, while the output is taken between the emitter and collector. Here, the collector of the transistor is common to both input and output circuits and hence the name is common collector connection. Figure (i) shows the common collector npn transistor circuit, and Figure (ii) shows the common collector pnp circuit.
(i) current amplification factor γ is given by;
Current gain is the same as the common emitter circuit i.e
But the voltage gain is always less than 1.
(ii) The relation between γ and α is,
(iii)The collector current is;
(iv)The common collector circuit has very high resistance (about 750 kΩ) and very low output resistance (about 25Ω) as a result the voltage gain provided by this circuit is always less than 1. This arrangement is used for amplification. Due to relatively high input resistance and low output resistance, this circuit is primarily used for impedance matching i.e. for driving a low impedance load from a high impedance source.
Comparison of Transistor connections
|S.N||Characteristics||Common Base||Common Emitter||Common collector|
|1||Input resistance||Low(about 100 Ω )||Low (about 750Ω)||Very high (about 750Ω)|
|2||Output resistance||Very high(about 450kΩ)||High(about 45kΩ)||Low(about 50Ω)|
|3||Voltage gain||About 150||About 500||Less than 1|
|4||Applications||For high frequency applications||For audio frequency applications||For impedance matching|
|5||Current gain||No(less than 1)||High(β)||Appreciable|