10. INVESTIGATION OF A BIPOLAR TRANSISTOR
10.1. Objective of the test
Knowledge of bipolar junction transistors (BJT), their structure,
principles of operation, basic configurations, characteristics and
parameters; learning to measure static volt-ampere characteristics
(VACs) and major parameters of a BJT.
10.2. Important notes
A BJT consists of two back-to-back pn junctions closely coupled
electrically by narrow region of material common to both. In normal
operation one of the junctions is forward-biased and the other –
reverse-biased.
The BJT can be used in three basic configurations: common base,
common emitter and common collector.
At low frequency and small signal a BJT may be represented as
the two-port linear network that is described by h parameters that are
given by
h11  U 1 I 1 when
h12  U 1 U 2 when
U2  0,
I1  0 ,
h22  I 2 U 2 when
h21  I 2 I 1 when
U2  0,
I1  0 .
So, h11 is the input impedance with output shorted (short-circuit
input impedance), h12 – reverse voltage feedback ratio (open-circuit
reverse voltage gain), h21 – forward current transfer ratio (short-circuit
forward current gain), h22 – output admittance with input open (opencircuit output admittance).
When BJT is used in common emitter configuration, h parameters
can be found using equations:
h11E 
U BE
,
I B U CE  const
h12E 
U BE
,
U CE I B  const
h21E 
I C
,
I B U CE  const
h22E 
I C
.
U CE I B  const
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10.3. Preparing for the test:
Using your lecture-notes and referenced literature [2, p. 20–57],
consider BJT structure, principles of operation, basic configurations, characteristics and parameters; clear up how h
parameters can be found.
Consider section “10.4. In laboratory” of this test.
Prepare squared millimetre paper for graphs. Sketch the coordinate systems I versus U on the paper. The scale must be
chosen taking into account types of the characteristics. Draw the
lines corresponding to the BJT ratings (highest permissible
parameters): collector current ICmax = 20 mA, collector-emitter
voltage UCEmax = 30 V, power PCmax = 150 mW, base current
IBmax = 0.2 mA (when the UCE = 10 V).
Prepare to answer the questions:
Sketch structures of npn and pnp transistors. Draw their
schematic symbols.
Explain the principles of BJT operation.
Name the components of base, emitter and collector currents and
explain their nature. Draw the section of a pnp (or npn) transistor
and explain how the currents and their components flow.
Draw BJT in its (common-base, common-emitter, commoncollector) configuration. Characterise the properties of the
transistor.
Define dc and ac emitter current gains. On what, how and why do
they depend?
Define dc and ac base current gains. On what, how and why do
they depend?
Draw input VACs of a BJT in the common-emitter configuration
(3 curves as minimum). Comment on them.
Draw output VACs of a BJT in the common-emitter configuration (3 curves as minimum). Explain on what, how and why
the output current of a transistor depends.
Write equations describing a BJT and including h parameters.
Explain the meanings of the parameters.
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10. How can we find h parameters using static characteristics of a
BJT? Demonstrate this (the teacher will point the parameter).
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10.4. In laboratory:
Answer the test question.
Familiarize with measurement devices and laboratory model.
Before any measurements the control knobs of all the
potentiometers of the laboratory model must be turned counterclockwise to the final position. During measurements voltage and
current must not exceed the highest values allowed for the used
devices. In the case of this laboratory work: collector current
ICmax = 20 mA, collector-emitter voltage UCmax = 30 V, power
PCmax = 150 mW, base current IBmax = 0.2 mA (when UCE = 10 V).
Connect the measurement circuit (Fig 10.1). Turn on the current
source and set voltages as shown in Fig 10.1 (+ 3 V and + 60 V).
After the teacher has checked the circuit, connect the measurement circuit to the current source.
After any change of the measurement circuit ask the teacher to
check it.
Carry out the measurements of the static VACs:
Fig 10.1. Measurement circuit
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a) Measure input characteristics I B (U BE ) of a BJT in the
common-emitter configuration when the collector-emitter
voltage UCE is of 0, 5 and 10 V.
To this end turn the control knobs of potentiometers R2, R3,
R5 and R6 counter-clockwise to the final position.
Note values of IB and UBE while increasing the voltage UBE
and sketch the characteristic. During this measurement the
output voltage UCE must be constant (0, 5 or 10 V respectively).
b) Measure three output characteristics I C (U CE ) of the BJT in
the common emitter configuration at the values of the input
base current IB pointed by the teacher.
To this end turn the control knobs of the potentiometers R2,
R3, R5 and R6 counter-clockwise to the final position.
Note values of IC and UCE while increasing the voltage UCE
and sketch the characteristic. During this measurement the
input base current IB must be constant.
Examine the results. Consider the measured characteristics.
Repeat the measurements if it is necessary.
Using the measured characteristics find BJT’s parameters h11E,
h12E, h21E, h22E in a Q point indicated by the teacher.
10.5. Contents of the report
Objectives.
Input and output characteristics of the BJT.
Calculation of h parameters.
Conclusions (comments on the measured input and output
characteristics and h parameters of the BJT).
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