Indian Institute of Technology Ropar
Electrical Engineering Department
Beasic Electronics Lab (GE 108)
Lab exercise # 4
Title: Study output characteristics of BJT
Objectives:
• To understand the opration of BJT.
Components required: - BJT, Resistors (Fixed + Variable), Fixed DC supply (12 V),
bread board, Wires & multimeter for testing
BJT Theory:
A bipolar junction transistor (bipolar transistor or BJT) is a type of transistor that uses
both electron and hole charge carriers. In contrast, unipolar transistors, such as field-effect
transistors, only use one kind of charge carrier. For their operation, BJTs use two junctions
between two semiconductor types, n-type and p-type. BJTs are manufactured in two types,
NPN and PNP, and are available as individual components, or fabricated in integrated
circuits, often in large numbers. The basic function of a BJT is to amplify current. This
allows BJTs to be used as amplifiers or switches, giving them wide applicability in
electronic equipment, including computers, televisions, mobile phones, audio amplifiers,
industrial control, and radio transmitters.
Regions of operation :
Bipolar transistors have four distinct regions of operation, defined by BJT junction biases.
Forward-active (or simply active)
The base–emitter junction is forward biased and the base–collector junction is reverse
biased. Most bipolar transistors are designed to afford the greatest common-emitter current
gain, βF, in forward-active mode. If this is the case, the collector–emitter current is
approximately proportional to the base current, but many times larger, for small base
current variations.
Reverse-active (or inverse-active or inverted)
By reversing the biasing conditions of the forward-active region, a bipolar transistor goes
into reverse-active mode. In this mode, the emitter and collector regions switch roles.
Because most BJTs are designed to maximize current gain in forward-active mode, the βF
in inverted mode is several times smaller (2–3 times for the ordinary germanium
transistor). This transistor mode is seldom used, usually being considered only for failsafe
conditions and some types of bipolar logic. The reverse bias breakdown voltage to the base
may be an order of magnitude lower in this region.
Saturation
With both junctions forward-biased, a BJT is in saturation mode and facilitates high
current conduction from the emitter to the collector (or the other direction in the case of
NPN, with negatively charged carriers flowing from emitter to collector). This mode
corresponds to a logical "on", or a closed switch.
Cut-off
In cut-off, biasing conditions opposite of saturation (both junctions reverse biased) are
present. There is very little current, which corresponds to a logical "off", or an open
switch.
Circuit Diagram and procedure:
Fig. 1
1. Measure beta of BJT using mutimeter.
2. Considering VBE is 0.7 V, calculate RB value, i.e. 11.3 M ohms : for 1uA IB current.
3. Calculate the value of Ic.
4. Vary the value of VCE by changing by changing Rc (consider value of Ic : from step
3).
Calculate values of Rc according to Ic.
5. Record the values of VBE, Rc, VCE and Ic.
6. Repeat step 1 to 5 : for IB = 5uA.
Sr.
no VBE Rc VCE Ic
1.
2.
.
.
.
Observation:
1. Plot Ic and VCE on graph paper for IB = 1uA and 5uA on same scale.
2. Plot straight line projecting on curve for VCE 5V and 7V points. Take a inverse of slope to
find output impedance of given transistor. Do it for IB = 1uA and 5uA
Conclusion: To be written by students.
Post Lab questions:
1. Draw input characteristics of BJT.
2. Draw output characteristics of BJT and mark the regions i.e. Active, Saturation and cutoff.
3. Explain the importance of above-mentioned regions.
4. Explain importance of output impedance which we have calculated from graph.
