EXPERIMENT 7
BIPOLAR JUNCTION TRANSISTOR CHARACTERISTICS
1.0 OBJECTIVES
To become familiar with the theory of operation of bipolar junction transistors (BJT)
and to examine the V-I characteristics of BJT’s
2. INTRODUCTION
The transistor type (NPN or PNP) can be determined using a multimeter. The test
checks the polarity of the base-emitter and base-collector junctions.
For the BJT, there are three regions of operation;
a. Active region: In this region, the base emitter junction is forward biased and the
base-collector junction is reverse biased. This region is the normal transistor
operation mode for amplification, and is characterized by the transistor current gain
value, beta.
b. Cut-off region: In this region, both base-emitter and base-collector junctions are
reverse biased and the transistor acts like an open switch. (IC = 0)
c. Saturation region: In this region, both base emitter and base-collector junctions are
forward biased and the transistor acts like a closed switch. (VCE = 0)
In the active region of the transistor, a figure of merit has been defined to quantify the
capability of the transistor to amplify the input signal. This parameter is defined as
the ratio between IC to IB and typically called the β factor. Similarly an α factor is
defined as the ratio between IC to IE. Thus;
β = IC / IB
and α = IC / IE
It can be easily shown that β = α /(1 − α) and α = β /(β + 1).
As a rule of thumb, the larger the value of β, the higher the gain obtainable from the
transistor, i.e. the better the transistor. Typical values for β ranges from about 80 to
300 or higher.
3. LAB WORK
3.1 Determine Transistor Types DC Junction resistance of the transistor:
Verify the transistor type for each unit by checking the polarity of the base-emitter
junction. Use a Fluke DMM in diode-test mode. Tabulate your measured data.
For the given transistor (2N3904), measure the forward and reverse bias resistance
between Base and Emitter, Base and Collector, and Collector and Emitter. Use a
Fluke DMM in diode-test mode. The lead connection of this transistor is as shown in
Fig. 1.
Figure 1 - Simplified outline and leads connection of transistor 2N3904.
3.2 VBE - IC Characteristic of the bipolar junction transistor:
Connect the transistor test circuit as shown in Figure 2. Set The DC voltage (VB) to
zero Volt and VCC to 10 V. Increase VB in 0.1 V steps and measure the DC voltage
between the base and emitter (VBE), the DC current through the collector IC, and the
current through the base IB. Tabulate your readings in clear table and plot the VBE
versus IC. Make sure that you take enough data points to plot a typical characteristic
of a BJT. Calculate β for every data point measured and tabulate the calculated
values of β with the measured data. Plot βversus VBE.
Figure 2 - Test circuit for measuring the VBE versus IC characteristic
of the bipolar transistor.
3.3 Measurement of IC versus VCE characteristic of the bipolar transistor
Using the test circuit of Figure 2, adjust VB to generate 50 µA current into the base of
the transistor. Vary VCC in reasonable steps (small steps at lower voltages larger steps
at higher voltages) to measure VCE and IC.
Repeat the above measurement for IB = 100 µA, 150 µA and 200 µA. Plot a set of
curves for VCE versus IC for constant IB.
From the measured data, determine the range of VCE over which IC is close to zero
Amp.
Can you find the value of α from the set of data obtained from this measurement?
How much is α
for the transistor under test.
Find the value of α from this set of measured data then calculate β. Compare the
value of β obtained from this measurement with that obtained in the measurement
performed in 3.2.