Common Emitter Configuration

The common-emitter (CE) configuration is the most commonly used transistor configuration.

Common-emitter configuration has emitter terminal common to both the input and the output sections. The input signal is applied to the base–emitter section and the output is taken from the collector–emitter section. Figures below show the CE configuration for the NPN and the PNP transistors, respectively. Salient features of CE transistor configuration are high values of voltage and current gains, medium values of input and output impedances.

Common-emitter configuration for NPN transistor

Common-emitter configuration for PNP transistor

The input characteristics of a CE transistor (refer to figure below) relate the base current (I_B) to the base–emitter voltage (V_BE) for different values of the collector–emitter voltage (V_CE). We can infer the following from the input characteristics of a CE transistor.

• Magnitude of base current (I_B) is in range of several tens of microamperes
• An increase in the value of collector–emitter voltage (V_CE) results in a decrease in the value of the base current (I_B), for fixed values of base–emitter voltage (V_BE). This is because of the early effect which results in reduction of the base width with increase in the collector–emitter voltage (V_CE).

Input characteristics of a CE transistor

Output characteristics of the CE configuration (refer to figure below) relate the collector current (I_C) to the collector–emitter voltage (V_CE) for different values of base current (I_B).

Output characteristics of a CE transistor

As we can see from the figure, the output curves for CE configuration are not as horizontal as that for CB configuration. This indicates that the collector–emitter voltage has an influence on the value of collector current.

For any transistor, the emitter, collector and base currents are related to each other as given below

As the value of I_CO is very small,

Therefore, β is defined as the DC forward current transfer ratio or the DC current gain of the transistor. It is also denoted as h_FE. Typical value of β is in the range of 50–100.

α and β are related to each other by the equation

Therefore, a very small change in α is reflected as a very large change in β.

The value of β varies considerably with changes in both the operating temperature and collector current (Refer to figure below). It also varies from one transistor to another transistor of the same type number.

Variation of β with change in temperature and collector current

As in the case of CB transistor configuration, for CE transistor configuration also, the collector–base junction is reverse-biased while the emitter–base junction is forward-biased. The active region characteristics (refer to figure below) corresponds to the portion of the graph to the right of the line at V_CE(sat) and above the curve for I_B = 0.

Output characteristics of a CE transistor

Transistors in the CE configuration have high value of current gain, voltage gain and power gain. Hence, they are used as voltage, current and power amplifiers.

The term ac beta is defined for AC applications. For AC applications ac beta ((β_ac) is defined as

β_ac is referred to as the CE forward-current amplification factor and is also denoted by h_FE.

A CE transistor is in the saturation region when both the collector–base and the emitter–base junctions are forward-biased. Therefore, collector–base voltage (V_CB) and emitter–base voltage (V_EB) are equal to the cut-in voltages of the base–collector and the base–emitter junctions, respectively. The value of V_CE (V_CB-V_BE) is few tenths of volts in the saturation region. Saturation region corresponds to the region left of V_CE = V_CE(sat) line in the output characteristics. In the saturation region, the value of the collector current is independent of the base current and depends on the value of resistor connected between the collector terminal and the supply terminal.

Transistor in CE configuration

For the given figure, value of collector current in the saturation region is given by I_C = V_CC/R_L. The minimum base current required to saturate the transistor is given by I_C/h_FE.

CE saturation resistance (R_CE(sat)) is defined as the ratio of the collector–emitter voltage at saturation to the collector current (V_CE(sat)/I_C). The curves to the left of the V_CE = V_CE(sat) line can be approximated as straight lines whose slope can be determined using the value of R_CE(sat). Therefore, the CE saturation resistance parameter is of utmost importance in the saturation mode of a CE configuration transistor.

In the cut-off region, both the collector–base and the emitter–base junctions are reverse-biased. The base current is equal to zero (I_B = 0) in this region. The collector current flowing through the transistor is then given by

This current is denoted by the symbol I_CEO.

For Silicon transistors, the value of α near cut-off region is nearly zero and therefore, the value of collector current is equal to I_CO. Hence, the Silicon transistor is in the cut-off region when I_B = 0 both for short circuit (V_BE = 0) and reverse-biased base–emitter junction.

For Germanium transistors, the value of α near cut-off region can be as large as 0.9. Therefore, the value of the collector current flowing through the transistor can be as large as 10 times the value of leakage current I_CO. This leads to Germanium transistor not being in the cut-off region for I_B = 0. A reverse bias needs to be applied to the base–emitter junction to bring the value of collector current (I_C) less than or equal to reverse saturation current (I_CO) and hence driving the transistor to cut-off. Reverse-bias voltage of 0.1 V is sufficient to reduce the collector current to this value and bring the transistor to cut-off. Therefore, a Germanium transistor is in cut-off region when I_B = 0 for reverse-biased base–emitter junction with V_BE greater than 0.1 V.