Test: The Common Base Configuration - Electrical Engineering (EE) MCQ

The AC current gain is denoted by α_ac. The ratio of change in collector current to the change in emitter current at constant collector base voltage is defined as current amplification factor.

 For all practical purposes, αac=αdc=α and practical values in commercial transistors range from 0.9-0.99. It is the measure of the quality of a transistor. Higher is the value of α, better is the transistor in the sense that collector current approaches the emitter current.

 Emitter current I_E=I_C+I_B =100+0.5=100.5mA
α_dc=I_C/I_E=100/100.5=0.995.

Here, I_C=4.9/5K=0.98mA
α = I_C/I_E .So,
I_E=I_C/α=0.98/0.98=1mA.
I_B=I_E-I_C=1-0.98=0.02Ma.

I_C=αI_E + I_CBO
=0.98*3+0.005=2.945mA.
I_E=I_C+I_B . So, I_B=3-2.495=0.055mA=55µA.

Given, I_B=50µA=0.05mA
I_CBO=4µA=0.004Ma
I_C=α/(1- α)I_B+1/(1- α)I_CBO=2.45+0.2=2.65Ma
I_E=I_C+I_B=2.65+0.05=2.7mA.

 When no signal is applied, the ratio of collector current to emitter current is called dc alpha, α_dc of a transistor. α_dc=-I_C/I_E. It is the measure of the quality of a transistor. Higher is the value of α, better is the transistor in the sense that collector current approaches the emitter current.

The terms Alpha and Beta refer to BJTs, Bipolar Junction Transistors. In these devices current leaves the emitter and crosses to the base and most is diverted towards the collector. The remainder leaves the base terminal.

Using a convenient notation, Ie = Ic + Ib

So Ib = Ie - Ic

Alpha is the proportion of Ie that flows to the collector,

ɑ = Ic/Ie ————— Definition

So Ic = ɑ Ie

Also Ib = Ie - ɑ Ie = Ie(1 - ɑ)

Beta is the current gain, the ratio of Ic to Ib.

β = Ic/Ib ————— Definition

Substituting in this definition for Ib and Ic

β = (ɑ Ie)/ Ie(1 - ɑ)

Cancelling Ie leaves:

β = ɑ/(1 - ɑ)

Given, I_E = 0.9mA, α=0.98
We know, α= I_C/I_E
So, I_C=0.98*0.9=0.882mA.

 (I_C – I_CBO)/α=I_E
= (2.945-0.002)/0.98=3mA.
I_E=I_C+I_B . So, I_B=3-2.495=0.055mA=55µA.

Formula Used:

I_E = I_C + I_B = αI_E + I_CBO

Where,

I_E is emitter current

I_C is collector current

I_B is base current

Amplification factor (α) = IC/IE

I_CBO is an open circuit current

Application:

We have,

I_E = 1 mA

I_CBO = 50 μA =  50 × 10-3 mA

α = 0.92

Hence,

IC = αI_E + I_CBO = 0.92 × 1 + 50 × 10^-3 = 0.97 A