Common: Collector Configuration | Analog and Digital Electronics - Electrical Engineering (EE) PDF Download

COMMON – COLLECTOR CONFIGURATION

 Also called emitter-follower (EF). It is called common-emitter configuration since both the signal source and the load share the collector terminal as a common connection point. The output voltage is obtained at the emitter terminal. The input characteristics of common-collector configuration is similar with common-emitter configuration.Common-collector circuit configuration is provided with the load resistor connected from the emitter to the ground. It is used primarily for impedance-matching purpose since it has high input impedance and low output impedance.

For the common-collector configuration, the output characteristics are a plot of I_E vs V_CE for a range of values of I_B.

Limits of opearation:

Many BJT transistors are used as an amplifier. Thus it is important to notice the limits of operations. At least 3 maximum values are mentioned in the data sheet.

They are:

a) Maximum power dissipation at collector: P_Cmax or P_D.

b) Maximum collector-emitter voltage: V_CEmax sometimes named as V_BR(CEO) or V_CEO.

c) Maximum collector current: I_Cmax.

There are few rules which need to be followed for BJT transistor to be used as an amplifier. The rules are: The transistor must be operated in active region!

I_C < I_Cmax 

P_C < P_Cmax 

Note:          V_CE is at maximum and I_C is at minimum (I_Cmax=I_CEO) in the cutoff region. I_C is at maximum and V_CE is at minimum (V_CEmax = V_CESAT = V_CEO) in the saturation region. The transistor operates in the active region between saturation and cutoff.

Refer to the fig. Example; A derating factor of 2mW/°C indicates the power dissipation is reduced 2mW each degree centigrade increase of temperature.  

Step1:

The maximum collector power dissipation,

P_D=I_CMAX x V_CEmax= 18m x 20 = 360 mW

Step 2:

At any point on the characteristics the product of and must be equal to 360 mW.

Ex. 1. If choose I_Cmax= 5 mA, substitute into the (1), we get

V_CEmaxI_Cmax= 360 mW

V_CEmax(5 m)=360/5=7.2 V

Ex.2. If choose V_CEmax=18 V, substitute into (1), we get

V_CEmaxI_Cmax= 360 mW

(10) I_CMAX=360m/18=20 mA 

Derating P_Dmax

P_DMAX is usually specified at 25°C.

The higher temperature goes, the less is P_DMAX

Example;A derating factor of 2mW/°C indicates the power dissipation is reduced 2mW each degree centigrade increase of temperature.