The name itself implies that the Emitter terminal is taken as common terminal for both input and output of the transistor. The common emitter connection for both NPN and PNP transistors is as shown in the following figure.

Just as in CB configuration, the emitter junction is forward biased and the collector junction is reverse biased. The flow of electrons is controlled in the same manner. The input current is the base current I_B and the output current is the collector current I_C here.

Base Current Amplification factor (β)

The ratio of change in collector current (ΔICΔIC) to the change in base current (ΔIBΔIB) is known as Base Current Amplification Factor. It is denoted by β

β=ΔICΔIBβ=ΔICΔIB

Relation between β and α

Let us try to derive the relation between base current amplification factor and emitter current amplification factor.

β=ΔICΔIBβ=ΔICΔIB

α=ΔICΔIEα=ΔICΔIE

IE=IB+ICIE=IB+IC

ΔIE=ΔIB+ΔICΔIE=ΔIB+ΔIC

ΔIB=ΔIE−ΔICΔIB=ΔIE−ΔIC

We can write

β=ΔICΔIE−ΔICβ=ΔICΔIE−ΔIC

Dividing by $$

β=ΔICΔIEΔIEΔIE−ΔICΔIEβ=ΔICΔIEΔIEΔIE−ΔICΔIE

α=ΔICΔIEα=ΔICΔIE

We have

α=ΔICΔIEα=ΔICΔIE

Therefore,

β=α1−αβ=α1−α

From the above equation, it is evident that, as α approaches 1, β reaches infinity.

Hence, the current gain in Common Emitter connection is very high. This is the reason this circuit connection is mostly used in all transistor applications.

Expression for Collector Current

In the Common Emitter configuration, I_B is the input current and I_C is the output current.

We know

IE=IB+ICIE=IB+IC

And

IC=αIE+ICBOIC=αIE+ICBO

=α(IB+IC)+ICBO=α(IB+IC)+ICBO

IC(1−α)=αIB+ICBOIC(1−α)=αIB+ICBO

IC=α1−αIB+11−αICBOIC=α1−αIB+11−αICBO

If base circuit is open, i.e. if I_B = 0,

The collector emitter current with base open is I_CEO

ICEO=11−αICBOICEO=11−αICBO

Substituting the value of this in the previous equation, we get

IC=α1−αIB+ICEOIC=α1−αIB+ICEO

IC=βIB+ICEOIC=βIB+ICEO

Hence the equation for collector current is obtained.

Knee Voltage

In CE configuration, by keeping the base current I_B constant, if V_CE is varied, I_C increases nearly to 1v of V_CE and stays constant thereafter. This value of V_CE up to which collector current I_C changes with V_CE is called the Knee Voltage. The transistors while operating in CE configuration, they are operated above this knee voltage.

Characteristics of CE Configuration

·        This configuration provides good current gain and voltage gain.

·        Keeping V_CE constant, with a small increase in V_BE the base current I_B increases rapidly than in CB configurations.

·        For any value of V_CE above knee voltage, I_C is approximately equal to βI_B.

·        The input resistance r_i is the ratio of change in base emitter voltage (ΔVBEΔVBE) to the change in base current (ΔIBΔIB) at constant collector emitter voltage V_CE.

ri=ΔVBEΔIBatconstantVCEri=ΔVBEΔIBatconstantVCE

·        As the input resistance is of very low value, a small value of V_BE is enough to produce a large current flow of base current I_B.

·        The output resistance r_o is the ratio of change in collector emitter voltage (ΔVCEΔVCE) to the change in collector current (ΔICΔIC) at constant I_B.

ro=ΔVCEΔICatconstantIBro=ΔVCEΔICatconstantIB

·        As the output resistance of CE circuit is less than that of CB circuit.

·        This configuration is usually used for bias stabilization methods and audio frequency applications.