Common Emitter Configuration-Basic Transistor Electronics-Lecture Slides, Slides of Basic Electronics

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Basic Electronic Engineering

Lecture 10

• The emitter is common to both input and output terminals
• The input terminal is the base and the output terminal is the

collector

• The current relation developed for common-base

configuration are valid for common emitter configuration

I E  I C  IB I C  IE

• Relation of output current(IC) with output voltage(VCE) at

different values of input current(IB)

• IB is in microamperes and IC is in milliamperes.
• The curves of IB are not horizontal as IE in common base

configuration.

• The collector to emitter voltage will effect the magnitude of

collector current.

• The active region lies to the right of VCE(SAT) and above IB=
• The active region for the common-emitter can be employed

for voltage current and power amplification

• In cut-off region IC is not zero for IB=

• ICEO is the collector to emitter current when base is open
• Cut-off in common-emitter configuration is defined for IC=ICEO

Approximation

• For common-emitter configuration the

voltage drop across the forward biased

emitter-base junction remains 0.7 V at

any value of base current

Beta(β)

• In dc mode Beta is the ratio of IC and IB
• In ac mode Beta is defined as
• Beta is common-emitter forward current amplification factor
• βdc is also referred to as hFE (forward current gain in common

emitter configuration)

C dc B

I

I

CE constant

C ac B V

I

I



Calculation of β

C dc B

I mA

I A

 

1 2

1 2

C C C ac B B B

I I I

I I I

C ac B

I mA mA mA

I A A A

 ac  100

Example

Using the characteristic curves of common-emitter configuration

• Find the value of βdc at IB = 80μA and VCE = 5V
• Find the value of βdc at IB = 10μA and VCE = 15V
• Find the value of βdc at IB = 30μA and VCE = 10V

Relation between α and β

Using the relation between currents we can find

C B

I

I

 C

E

I

I

^ I^ E ^ I^ C  IB

^1

I CEO  (^)    (^1) ICBO I E  (^)    (^1) IB

Relation between β and α

• A small change in α results in a large change in the

value of β

• β can vary from transistor to transistor of the same

type

α = 0.98, β = 49

α = 0.99, β = 99

α = 0.995, β =