Notes of Bipolar Junction Transistor

1. Unit 2: Bipolar Junction
Transistors (BJT)
for
Open Educational Resource
on
Electronic Devices and Circuits (EC201)
by
Dr. Piyush Charan
Assistant Professor
Department of Electronics and Communication Engg.
Integral University, Lucknow
This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.

2. PNP Transistor
11/2/2020 Dr. Piyush Charan, Dept. of ECE, Integral University, Lucknow 2
In the p-n-p transistor, we have a p-
type semiconductor region followed by
an n-type region and then a p-type
region again.

3. NPN Transistor
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In an n-p-n transistor on the other hand,
we have an n-type region then p-type
region and again an n-type region. And in
both cases, there are three contacts
emitter, base and collector.

4. BJT Symbol and direction of
Conventional Currents
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5. Simple Numerical Example
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Q. In a certain transistor, the emitter current is 1.02 times as large as the collector
current. If the Emitter current is 12mA, find the base current.

6. Why BJT is called bipolar
junction transistor?
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→Bipolar because both electrons and holes contribute to conduction in these devices as
opposed to for example, MOS transistors in which only one type of carrier contributes.
→Junction, because the device includes two p-n junctions one is E-B Junction and the
other is C-B Junction. and this should be contrasted with the “point-contact” transistor
which was the first transistor to be invented by Shockley and others in 1947. This point
contact transistor is not used anymore.
→And finally, this word transistor stands for transfer resistor, and you can read more
about that at this website.

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15. Ebers-Moll Model for pnp
transistor
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Lucknow
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Ebers-Moll Model for pnp
transistor

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Ebers-Moll Model

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Ebers-Moll Model in Active Mode

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BJT I-V Characteristics

20. Common-Emitter NPN Transistor
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Dr. Piyush Charan, Dept of ECE, Integral University,
Lucknow
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Forward bias the E-B Junc.
Reverse bias the C-B Junc.

21. Input Characteristics
• Plot IB as f(VBE, VCE)
• As VCE increases, more
VBE required to turn the
BE on so that IB>0.
• Looks like a p-n junction
volt-ampere characteristic.
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Dr. Piyush Charan, Dept of ECE, Integral
University, Lucknow
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22. Output Characteristics
• Plot IC as f(VCE, IB)
• Cutoff region (off)
– both BE and BC reverse
biased
• Active region
– BE Forward biased
– BC Reverse biased
• Saturation region (on)
– both BE and BC forward
biased
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Dr. Piyush Charan, Dept of ECE, Integral
University, Lucknow
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23. Transfer Characteristics
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Dr. Piyush Charan, Dept of ECE, Integral University,
Lucknow
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Numerical Problem 1: Simple
BJT Circuit

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Numerical Problem 1: Continued……..

26. Darlington Pair Amplifier
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Dr. Piyush Charan, Dept. of ECE, Integral University,
Lucknow
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A Darlington transistor acts as a single
transistor with high current gain, it means
that a small amount of current is used from
a microcontroller or a sensor to run a
larger load. For instance, the following
circuit is explained below. The Darlington
circuit is built with two transistors shown
in the circuit diagram, alongside.

27. Darlington Pair
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Dr. Piyush Charan, Dept. of ECE, Integral University,
Lucknow
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A Darlington Transistor configuration,
also known as a “Darlington pair” or
“super-alpha circuit”, consist of two NPN
or PNP transistors connected together so
that the emitter current of the first
transistor TR1 becomes the base current of
the second transistor TR2. Then
transistor TR1 is connected as an emitter
follower and TR2 as a common emitter
amplifier as shown below.

28. Working of Darlington Pair
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Dr. Piyush Charan, Dept. of ECE, Integral University,
Lucknow
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Using the NPN Darlington pair as the example, the
collectors of two transistors are connected together, and
the emitter of TR1 drives the base of TR2. This
configuration achieves β multiplication because for a
Base current ib, the collector current ic is β*ib where the
current gain is greater than one, or unity and this is
defined as:

29. DP working contd…
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Dr. Piyush Charan, Dept. of ECE, Integral University,
Lucknow
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But the base current, IB2 is equal to transistor TR1 emitter current, IE1 as the emitter of TR1 is
connected to the base of TR2. Therefore:
Then substituting in the equation 2 :
Where β1 and β2 are the gains of the individual transistors.

30. DP working contd…
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Dr. Piyush Charan, Dept. of ECE, Integral University,
Lucknow
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This means that the overall current gain, β is given by the gain of the first
transistor multiplied by the gain of the second transistor as the current gains of
the two transistors multiply.
In other words, a pair of bipolar transistors combined together to make a single
Darlington transistor pair can be regarded as a single transistor with a very high
value of β and consequently a high input resistance.

31. Numerical on Darlington Pair
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Dr. Piyush Charan, Dept. of ECE, Integral University,
Lucknow
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❖ Q1. Two NPN transistors are connected together in the form of a Darlington Pair to
switch a 12V 75W halogen lamp. If the forward current gain (β1)of the first transistor is 25 and
the forward current gain (β2) of the second transistor is 80. Ignoring any voltage drops across
the two transistors, calculate the maximum base current required to switch the lamp fully-ON.

32. Numerical on Darlington Pair
contd…
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Dr. Piyush Charan, Dept. of ECE, Integral University,
Lucknow
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33. Special Case Darlington pair
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Lucknow
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