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 IE vs VCE for a range of values of IB.
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.
a) Maximum power dissipation at collector: PCmax or PD.
b) Maximum collector-emitter voltage: VCEmax sometimes named as VBR(CEO) or VCEO.
c) Maximum collector current: ICmax.
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!
IC < ICmax
PC < PCmax
Note: VCE is at maximum and IC is at minimum (ICmax=ICEO) in the cutoff region. IC is at maximum and VCE is at minimum (VCEmax = VCESAT = VCEO) 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.
The maximum collector power dissipation,
PD=ICMAX x VCEmax= 18m x 20 = 360 mW
At any point on the characteristics the product of and must be equal to 360 mW.
Ex. 1. If choose ICmax= 5 mA, substitute into the (1), we get
VCEmaxICmax= 360 mW
VCEmax(5 m)=360/5=7.2 V
Ex.2. If choose VCEmax=18 V, substitute into (1), we get
VCEmaxICmax= 360 mW
(10) ICMAX=360m/18=20 mA
PDMAX is usually specified at 25°C.
The higher temperature goes, the less is PDMAX
Example;A derating factor of 2mW/°C indicates the power dissipation is reduced 2mW each degree centigrade increase of temperature.