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DEPARTMENT OF ELECTRICAL & ELECTRONIC ENGINEERING
BANGLADESH UNIVERSITY OF ENGINEERING & TECHNOLOGY
COURSE NO.: EEE 212
EXPT. NO. 04
NAME OF THE EXPERIMENT: STUDY OF N-P-N CE AND CB TRANSISTOR
CHARACTERISTICS.
OBJECTIVE
To determine the CE and CB characteristics of a transistor.
MATERIALS REQUIRED
n-p-n transistor (C829)
resistors 2k,1k,500,100
multimeter
bread board
one piece
one piece each
two pieces
THEORY
Transistor has two p-n junctions (see figure below). One junction is called emitter
junction and other is called collector junction. When transistor is used as an amplifier, it
is operated in active mode. In active mode, emitter junction is forward biased and
collector junction is reverse biased.
JnE
JnC
p
E
iE
nC
nE
JpE
iC
n
p
n
C
C
iB
B
B
E
Emitter current is given by
IE = InE + IpE
we can also write
IE = IC + IB = [(1 + )/]IC
Where  = IC /IB is called common emitter current gain.
In good transistor IC>>IB i.e. >>1. IC can also be expressed as IC =  IE .
where  = /(1+) .  is called common base current gain. For good transistor,  is close
to unity.
Proper dc biasing of a transistor is a prerequisite for proper operation as an amplifier. The
purpose of the biasing is to fix the IC (dc) and VCE (dc) . But IC is a function of
temperature, VBE and . It is always desirable to design a biasing circuit where IC is
insensitive to change in .
When E-B junction is forward biased and C-B junction is reverse biased, the transistor
operates in active mode. For saturation mode of operation, both junction are forwardbiased. Cut-off region operation requires that both E-B and C-B junctions be reverse
biased. The inverted active operation occurs when E-b is reverse-biased and C-B is
forward biased.
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CIRCUIT DIAGRAMS
100
IE
C829
IC 500
VEE
(0-5V)
VCC
(0-20V)
Fig. 1(a) Common base configeration
IC
1k
IB
2k
C829
VCC
(0-20V)
VBB
(0-5V)
Fig. 1(b) Common-emitter configuration
PROCEDURE
For CB Configuration refer to Fig. 1(a)
1.
Construct the circuit shown in Fig. 1(a)
2.
Set VCC to 10V. vary VEE so that IE is around 2 mA. Adjust VCC to get VCB as
zero. Measure the value of VBE
3.
Keeping VCB=0, vary IE by changing VEE and measure the value of VBE and IE
4.
Set IE=3mA. Adjust VCC to get VCB=1V. Repeat step 3.
5.
Adjust VEE so that IE is zero. Vary VCC from zero to a suitable value and
measure VCB and IC.
6.
Vary VEE so that IE = 1mA. Repeat step 5.
7.
Repeat step 5 for IE=2mA.
8.
Assemble the circuit shown in Fig. 1(b).
9.
First set VCC=10V. Apply voltage to B-E junction. Adjust the value of VCC so
that VCE is zero. Vary VBB and measure VBE for different values of IB.
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10.
Repeat the above experiment for VCE=1Volt.
11.
Vary VBB so that IB is about 10A. Vary VCE in steps of 2.5 volts and measure
VCE and IC.
12.
Repeat the experiment for IB=20A and 30A.
REPORT
1.
(a) Plot output and input characteristics of the n-p-n CB transistors.
(b) Indicate active, linear/saturation and cut-off regions of the characteristics.
2.
(a) Plot output and input characteristics of CE transistor.
(b) Calculate the early voltage from the output characteristic.
(c) Discuss the effect of changing  on the output characteristics.
(d) Discuss the effect of changing VCE on the input characteristics
3.
What are the role of the 2K and 1K fixed resistors in the circuits?
4.
Plot (=Ic/IB) as a function of IC for VCE=7.5V.
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