Experiment 09: The Common Base

Experiment 09
The common base terminology is derived from the fact that the base is common to both the input and
output sides of the configuration.The base is usually the terminal closest to, or at, ground potential.
diagram of a transistor.
In CB configuration, the base is common to both input (emitter) and output (collector). For normal
operation, the Emitter Base junction is forward biased and Collector Base junction is reverse biased.
In CB configuration, IE is +ve, IC is –ve and IB is –ve. So,
VEB=f1 (VCB, IE) and IC=f2 (VCB, IB)
With an increasing the reverse collector voltage, the space-charge width at the output junction increases
and the effective base width ‘W’ decreases. This phenomenon is known as “Early effect”. Then, there will
be less chance for recombination within the base region. With increase of charge gradient within the base
region, the current of minority carriers injected across the emitter junction increases. The current
amplification factor of CB configuration is given by,
α= ∆IC/ ∆IE
The input or driving point characteristics and the output or collector characteristics for a common-base
transistor amplifier is shown below
A diagram showing the input and the output characteristics of a common Base.
The common-base amplifier can provide a reasonable level of voltage gain but suffers from low
input impedance and a current gain of less than one. However, this circuit is used extensively for
high-frequency applications because its terminal characteristics at high frequencies are better
than those of a common-emitter configuration using the same transistor.
The standard common-base configuration also requires two DC supplies rather than the one
needed for the common-emitter and common-collector configurations.
NPN transistor is one of the Bipolar Junction Transistor (BJT) types. The NPN transistor consists
of two n-type semiconductor materials and they are separated by a thin layer of p-type
semiconductor. Here the majority charge carriers are the electrons. The flowing of these
electrons from emitter to collector forms the current flow in the transistor. Generally, the NPN
transistor is the most used type of bipolar transistors because the mobility of electrons is higher
than the mobility of holes. The NPN transistor has three terminals – emitter, base and collector.
The NPN transistor is mostly used for amplifying and switching the signals.
A diagram showing the symbol and structure of NPN transistor

Figure 4 shows the symbol and structure of NPN transistor. In this structure we can observe the
three terminals of transistor, circuit currents and voltage value representations. The above figure
shows the NPN transistor circuit with supply voltages and resistive loads. Here the collector
terminal always connected to the positive voltage, the emitter terminal connected to the negative
supply and the base terminal controls the ON/OFF states of transistor depending on the voltage
applied to it.
Diagram
A diagram to show the common Base characteristics of an NPN transistor.
1. Voltmeter 5. Emitter current meter

2. VBB variable power supply 6. NPN transistor
3. Base ammeter 7. Breadboard
4. VCC variable power supply 8. Resistor
Work Sheet
Sr number VCC = OV VCC = +5V VCC = +10V
Veb Ie Veb Ie Veb Ie

0 0.10 0.00 0.00 0.10 0.00 0.10
2 0.68 0.33 0.62 1.39 0.61 1.37
4 0.72 3.22 0.64 3.30 0.63 3.34
6 0.74 5.19 0.67 5.25 0.64 5.25
8 0.75 7.18 0.72 7.19 0.66 7.29
10 0.76 9.15 0.74 9.15 0.67 9.18
12 0.77 11.16 0.76 0.71 11.20
11.09
14 0.78 13.10 0.77 0.74 13.13
13.14
16 0.79 15.09 0.78 0.76 15.13
15.07
18 0.79 17.12 0.78 0.77 17.14
17.13
20 0.80 19.12 0.79 0.78 19.18
19.11
Sr IE = 0mA IE = 1MA IE = 5MA IE = 10MA

number
Vcb IC Vcb IC Vcb IC Vcb IC
0 0.00 0.65 -0.73 -0.75
0.00 0.46 0.51 0.53
2 1.99 0.92 -0.70 -0.74
0.00 1.01 2.57 2.64
4 4.02 2.87 -0.66 -0.73
0.00 1.01 4.34 4.53
6 5.93 4.83 0.52 -0.72
0.00 1.01 5.04 6.39
8 7.96 6.85 2.58 -0.69
0.00 1.02 5.04 8.31
10 9.92 8.86 4.54 -0.52
0.00 1.02 5.04 10.03
12 11.88 10.75 6.52 1.44
0.00 1.03 5.04 10.07
14 13.80 12.74 8.44 3.34
0.00 1.03 5.05 10.08
16 15.88 14.69 10.24 5.39
0.00 1.03 5.05 10.05
18 17.73 16. 66 12.32 7.13
0.00 1.04 5.05 10.07
20 19.68 18.62 14.37 9.54
0.01 1.04 5.05 10.08
CALCULATIONS AND GRAPH
Experiment 10

Experiment 09: The Common Base

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Experiment 09

A diagram showing the symbol and structure of NPN transistor

A diagram to show the common Base characteristics of an NPN transistor.

1. Voltmeter 5. Emitter current meter

Veb Ie Veb Ie Veb Ie

Sr IE = 0mA IE = 1MA IE = 5MA IE = 10MA

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