Lab 10: Common base and Common emitter Amplifiers
Part1 : Common Base Amplifier
The common-base configuration (CB) shown in figure below is mainly used for
impedance matching, since it has a low input resistance (30 ohms-160 ohms)
and a high output resistance (250 kilohms-550 kilohms). However, two factors
limit its usefulness in some circuit applications: (1) its low input resistance and
(2) its current gain of less than 1. Since the CB configuration will give voltage
amplification, there are some additional applications, which require both a
low-input resistance and voltage amplification that could use a circuit
configuration of this type; for example, some microphone amplifiers. In the
common-base configuration, the input signal is applied to the emitter, the
output is taken from t he collector, and the base is the element common to
both input and output. Unlike the common-emitter, circuit, the input and
output signals in the common-base circuit are in phase.
It is popular in high-frequency amplifiers, for example for VHF and UHF,
because of the relatively high isolation between the input and output. This
high isolation means that there is little feedback from the output back to the
input, leading to high stability.

The open-circuit voltage gain Av of the common Base amplifier of
above figure can be calculated using the following equation (positive
sign means no phase shift )
v 

Vo Rc

Vi re
The output resistance Zo(stage) of the common Base amplifier of
above figure can be calculated using the following equation:
Zo  Rc // Rl

The input resistance Zin of the common Base amplifier of above figure
can be calculat ed using the following equations:
Zin  re // RE  re
Part 2: Common Collector Amplifier
The last important small-signal amplifier configuration of the BJT is the
common collector, or emitter follower, amplifier. It is extremely useful
because it has very high input resistance, high current gain, very small output
resistance, and approximately unity voltage gain. The high input resistance
and low output resistance make the emitter follower an ideal buffer between
a high impedance source and a low impedance load. A buffer is any circuit
that keeps the source from being affected by a load. For example, a
common emitter amplifier with a l0k output resistance could not provide very
much voltage gain to a 50 ohm load resistor.

The open-circuit voltage gain Av of the common Collector amplifier of
above figure can be calculated using the following equations:
v 

Vo
RE

 1 (Emitter Follower ) !!!
Vi RE  re
The output resistance Zo(stage) of the common Collector amplifier of
above figure can be calculated using the following equation:
Zo  re // RE  re

The input resistance Zin of the common Collector amplifier of above
figure can be calculated using the following equations:
Zin  RB // Zb
Zb   (re  RE )
and
Characteristics
Common Base
Very Low(less than
100 ohm)
Common Collector
Very High(750K)
Common Emitter
Low(less than 1K)
1.
Input Dynamic
Resistance
2.
Output dynamic
resistance
High(less than 1M)
Low(50 ohm)
High (less than 45K)
3.
Current Gain
Less than 1
Very High(20-greater
than 100)
High(20-100)
4.
Voltage Gain
High
Less than 1
High About 500
5.
6.
Power Gain
Phase relation
Medium
In Phase
Medium
In Phase
Highest
Out of Phase(180)
7.
Applications
*For High
Frequency apps.
*Matching circuit
*As input stage of
multistage
amplifiers
For impedance
Matching
Apps.(buffer)
Amplifier For Audio
Freq. Apps
Lab Work
Part 1 : Common Base


To find re using Orcad find Ie then re=25m/Ie
𝑉
To find Zin = 𝐼 𝑖𝑛 from orcad plot Iin and find the peak to peak value

then find Zin
𝑉
𝑉
To find 𝑍0 = 𝐼0 = 𝐼0 from Orcad plot Ic and find the peak to peak value of
𝑖𝑛
𝑜
𝑐
Ic then plot Vo and find peak to peak value then find Zo
Mathematically
Using Orcad
re
Zin
Zo
Av
Ai
β=100
v dc
12
0
RC
3k
C3
R1
80k
22u
C1
RL
Q1
10k
Q2N2222
22u
0
C2
0
20k
R2
RE
22u
Vin
VOFF = 0
VAMPL = 1m
FREQ = 1k
1k
0
0