Course Content

Introduction to transistors.
Types of transistor(BJT and FET).
Different between BJT and FET.
Principle operation of BJTs.
Biasing and Application areas of BJT.

6/4/2023 By Alemie Assefa 1

 TRANSISTORS
TRANSISTORS are three-layered, three- terminal and two-junction
electronic devices constructed of doped semiconductor.
TRANSISTOR =TRANsfer and reSISTOR (transfer and resistor)
which means transfer of electrical power from a low resistive
circuit to a high resistive circuit.
Application of Transistors are:-
Signal amplifiers
Electronic switches
Oscillators
Design of digital logics
Memory circuits etc.
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 Cont..
Classification of transistors based on their majority and minority
charge carriers are:-
1. Bipolar transistor(BJT)
The charge carriers are both electrons and holes (majority charge
carriers and minority charge carriers).
They are principally called Bipolar Junction Transistors (BJTs).
2. Unipolar transistor(FET)
The charge carrier is either electrons or holes as majority charge
carriers only.
The unipolar junction transistors are mainly known as Field Effect
Transistors (FETs).
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 Cont..
Most of the BJT collector current is due to the flow of charges
injected from a high-concentration emitter into the base where
they are minority carriers that diffuse toward the collector.
BJTs are current controlled devices.
FETs are said to be voltage controlled devices.

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 Bipolar Junction Transistors
BJT consists of three differently doped semiconductor regions
namely:-
Emitter region
Base region
Collector region
Based on doping concentration Emitter region>Collector region
>Base region
Based on Area Collector region>Emitter region >Base region

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 Cont..
Types of BJT transistors are:-
NPN (Not Point iN) Transistor
PNP(Point iN) Transistors

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 Principles of Operation
Both PNP and NPN transistors, the emitter-base junction (EB) is
forward-biased while the collector-base junction (CB) is reverse-
biased to use the transistor as:-
An amplifier,
Oscillator,
Mixer, detector etc.

NPN transistors

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 Cont..
In the NPN transistor the EB junction is forward-biased by VBE, so
that the majority charge carriers (electrons) are emitted from the
emitter into the base because the negative potential of the battery
of VBE repels the electrons from the N-type material (emitter).
The CB is reverse-based by VBC to collect or attract the most of
emitted electrons (say, about 99%) crossing the CB junction as
collector current (IC).
 Some of the charge carriers from the emitter, which do not reach
the collector (say, about 1%), entering the base (recombination) as
base current(IB).
•
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 Cont..
The emitter current is the total transistor current which is the sum
of base current and collector current.
IE=IB+IC
IB is very much small, IE and IC are almost equal(IC = αIE ).
𝜶 is current amplification factor for CB modes which flows to
collector (0.98 - 0.998).

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 Cont..
The CB junction is reverse-biased, a very small minority charge
carrier, called Reveres Saturation Current, flows through the junction.
This current is termed as collector-base-leakage-current (ICBO).
IC = 𝜶IE + ICBO
ICBO means, current flowing form collector to the base when the
emitter junction is open.

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 Modes of BJT Configurations
For applications such as amplifier circuit, four terminals are required;
two for the input section and two for the output section.
One of the three terminals of the transistor should be made common
for both the input and for the output terminals.
Common - Base (CB) - The base is common for the input and the
output
Common-Emitter (CE)-The emitter is common for the input & the
output
Common - Collector (CC) - The collector is common for the input and
the output
Q. Why the common emitter (CE) is the most typical configuration?
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 Cont..
The current amplification factor for CB mode is known as alpha (𝛼)
 DC current gain(𝜶𝒅𝒄 )
𝐼𝐶
𝐷𝐶 𝐶𝑢𝑟𝑟𝑒𝑛𝑡 𝑔𝑎𝑖𝑛(𝛼𝑑𝑐 ) =
𝐼𝐸
Ac current gain(𝜶𝒂𝒄 )

𝐶ℎ𝑎𝑛𝑔𝑒 𝑖𝑛 𝑜𝑢𝑡𝑝𝑢𝑡 𝑐𝑢𝑟𝑟𝑒𝑛𝑡 ∆𝐼𝐶

𝛼𝑎𝑐 = = , 𝑤ℎ𝑒𝑟𝑒 𝑉𝐶𝐵 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡
𝐶ℎ𝑎𝑛𝑔𝑒 𝑖𝑛 𝑖𝑛𝑝𝑢𝑡 𝑐𝑢𝑟𝑟𝑒𝑛𝑡 ∆𝐼𝐸
Note: 𝜶𝒂𝒄 and 𝜶𝒅𝒄 are approximately equal and their values lie
between 0.95 and 0.998.

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 Cont..
The current amplification factor for CE mode is called beta (𝛽).
DC current gain(𝜷𝒅𝒄 )
𝐼𝐶
𝐷𝐶 𝐶𝑢𝑟𝑟𝑒𝑛𝑡 𝑔𝑎𝑖𝑛(𝛽𝑑𝑐 ) =
𝐼𝐵
Ac current gain(𝜷𝒂𝒄 )

𝐶ℎ𝑎𝑛𝑔𝑒 𝑖𝑛 𝑜𝑢𝑡𝑝𝑢𝑡 𝑐𝑢𝑟𝑟𝑒𝑛𝑡 ∆𝐼𝐶

𝛽𝑎𝑐 = = , 𝑤ℎ𝑒𝑟𝑒 𝑉𝐶𝐸 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡
𝐶ℎ𝑎𝑛𝑔𝑒 𝑖𝑛 𝑖𝑛𝑝𝑢𝑡 𝑐𝑢𝑟𝑟𝑒𝑛𝑡 ∆𝐼𝐵
Note: 𝛽𝑑𝑐 are always much greater than unity(20 to 50)
𝛽𝑎𝑐 = ℎ𝑓𝑒

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 Cont..
The current amplification factor for CC mode is also called
gamma(𝛾).
DC current gain(𝛾𝒅𝒄 )
𝐼𝐸 (𝐼𝐵 +𝐼𝐶 )
𝐷𝐶 𝐶𝑢𝑟𝑟𝑒𝑛𝑡 𝑔𝑎𝑖𝑛(𝛾𝑑𝑐 ) = = =𝛽+1
𝐼𝐵 𝐼𝐵
Ac current gain(𝛾𝒂𝒄 )

𝐶ℎ𝑎𝑛𝑔𝑒 𝑖𝑛 𝑜𝑢𝑡𝑝𝑢𝑡 𝑐𝑢𝑟𝑟𝑒𝑛𝑡 ∆𝐼𝐸

𝛾𝑎𝑐 = = , 𝑤ℎ𝑒𝑟𝑒 𝑉𝐸𝐶 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡
𝐶ℎ𝑎𝑛𝑔𝑒 𝑖𝑛 𝑖𝑛𝑝𝑢𝑡 𝑐𝑢𝑟𝑟𝑒𝑛𝑡 ∆𝐼𝐵
Note: 𝛾𝑑𝑐 are always much greater than unity.

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 Cont...
The relation between 𝜷𝒅𝒄 and 𝜶𝒅𝒄

Example: A transistor has a 𝛼𝐷𝐶 of 0.9 and a base current, IB, of 20

μA.
Calculate the collector current(IC)?

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 Transistor Regions of Operation
There are four different junction-biasing combinations to
have four distinct regions of operation.

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 BJT Characteristic Curves
The behaviour of a particular transistor is recommended that the
relationship between its:
Input current and its input voltage
Output current and its output voltage
A transistor may have two characteristic curves known as input
characteristic curves and output characteristic curves respectively.
Common Base(CB) Characteristic Curves
Input characteristic curves: The input voltage VEB versus input
current IE for various values of output voltage VCB as a constant
parameter.
The forward bias VEB is increased, the input current IE increases
similar to diode characteristics. If VCB is increased, then IE increases
slightly.

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 Cont..
Output characteristic curves: Output voltage VCB versus output
current IC for various values of input current IE as a parameter.
What is early effect ?
Answer: When VCB is increased, depletion region width at C-B
junction increases, so effective base width decreases and IB
decreases. Hence IC increases.

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 Common Emitter(CE) Characteristic Curves
Input characteristic curves: The input voltage VBE versus input
current IB for various values of output voltage VCE as a parameter.
As the forward bias VBE is increased, the input current IB increases
similar to diode characteristics. If VCE is increased, then IB
decreases slightly. This is due to early effect.
Output characteristic curves: Output voltage VCE versus output
current IC for various values of input current IB as a parameter.
VCE = VCB + VBE , VCE VCB IC that is IB

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 Common Collector(CC) Characteristic Curves
Input characteristic curves: The input voltage VBC versus input
current IB for various values of output voltage VEC as a parameter.
Output characteristic curves: Output voltage VEC versus output
current IE for various values of input current IB as a parameter.
Since IC is approximately equal to IE, the common collector output
characteristic curve is the same as that of the common emitter
circuit.

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 BJT Biasing Circuit Types
The most common applications of transistor is in amplifier circuits.
For a faithful amplification we require that a transistor should be
operated in active region throughout the duration of input signal.
To ensure this, proper dc voltages should be applied which result to
a situation called biasing.
The operating point(bias point, quiescent point, or Q-point ) is
expected to remain almost at the center of the active region of the
device.
The point on the output characteristics that shows the DC collector
emitter voltage (VCE) and the collector current (IC) with no input
signal applied.
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 BJT DC Biasing Circuit Types
The most common five biasing circuits used in small signal BJT
amplifiers are :-
Fixed Bias Circuit
Fixed Bias with Emitter resistor
Collector-to-Base Bias Circuit
Voltage Divider Bias
Emitter Bias

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 Cont..
Fixed Bias (Base Bias)
In this biasing circuit, the base current (IB), remains relatively
constant (fixed) regardless of variations in the collector current (IC)
by the collector circuit bias (VCC) and the base resistor (RB).
Since VCC, VBE and RB are constant; IB remains constant at a
particular level.
Base bias circuit is most useful in switching circuit.
Input section
Applying the KVL around the supply base ground

Output section
Applying the KVL around the supply collector ground
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 Cont..
Merits of fixed bias Circuit
It is simple to shift the operating point anywhere in the active
region by merely changing the base resistor (RB)
Simple circuit; very small number of components are required.
Demerits of fixed bias Circuit
The collector current does not remain constant with variation in
temperature or power supply voltage. Therefore the operating point
is unstable.
When the transistor is replaced with another one, considerable
change in the value of β can be expected. Due to this change the
operating point will shift causes thermal runaway.

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 Cont..

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 Cont..
Fixed Bias with Emitter Resistor
It is also called Emitter Feedback Bias, is a fixed bias circuit modified by
attaching an external resistor to the emitter.
This resistor introduces negative feedback that stabilizes the Q-point.
This is due to that the emitter current (IE) flowing through the emitter
resistor (Re) is equal to IC + IB. The collector current is brought from the
output circuit back to the input. This current helps to stable the circuit
operation. Input section
Applying KVL to the input circuit

Output section
Applying KVL to the output circuit
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 Cont..
Merits of fixed bias with Emitter Resistor Circuit
The circuit has the tendency to stabilize operating point against
changes in temperature and β-value.
Demerits of fixed bias with Emitter Resistor Circuit
In this circuit, to keep IC independent of β the following condition
must be met:

In addition to the above, RE causes ac feedback which reduces the

voltage gain of the amplifier.
As β-value is fixed for a given transistor, this relation can be
satisfied either by keeping RE very large or making RB very low.
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 Cont..
Voltage Divider Bias
Case1:Approximate Analysis
The voltage divider is formed using external resistors R1 and R2.
By proper selection of resistors R1 and R2, the operating point of
the transistor can be made independent of β.
Input section
The base voltage VB = VR2 is ,VB = (VCC x R2) / (R1 + R2)
VB = VBE +IERE= VBE +RE (𝜷 + 𝟏)I𝐁
Output section
Applying KVL to the output circuit
VCEQ = VCC – ICRC - IERE = VCC – IE (RC + RE)
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 Cont..
Case2:Exact Analysis
In this method can be obtained by applying Thevenin's theorem
to the circuit.

Thevenin voltage (VTh)

Thevenin Resistance (RTh)

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 Cont..
Merits of Voltage Divider Bias
Only one dc supply is necessary.
Operating point is almost independent of β variation.
Operating point stabilized against shift in temperature.
Demerits of Voltage Divider Bias
to keep IC independent of β the following condition must be met:

As β-value is fixed for a given transistor, this relation can be

satisfied either by keeping RE fairly large or making R1||R2 very low.
AC as well as DC feedback is caused by RE, which reduces the AC
voltage gain of the amplifier.
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Small Signal BJT Amplifiers and Parametric Representations
 An amplifier is a circuit using an active device such a transistor or
an integrated circuit (IC) to increase
the intensity (strength) of current, voltage or power of a signal
without changing the shape of the waveform and the frequency.
Amplification means the process of increasing the intensity
(amplitude) of a signal.
Amplifiers are necessary in most applications because the desired
signal is usually too weak to directly useful.
Example, audio output from a microphone may be as little as one
millivolt, whereas the loudspeaker needs at least a few volts of audio
signal.
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 Cont..
Two port networks (system) are widely used to model transistors
amplifier circuit.

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 Cont..
Parameters of an Amplifier
The three most important parameters of any amplifier are:
Input Impedance(Zi): or ac resistance

 Output Impedance (Zo): AC output resistance

Amplification factor (Gain): determines how much the input

signal is increased.

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 Cont..
Voltage Gain (AV):the voltage gain is the ratio of the output
voltage to the input voltage with constant output current

Current Gain (AI): The ratio of output current to the input current
with constant output voltage.

Power Gain (AP): The ratio of output power to the input power or
the product of a voltage gain and a current gain of each stage is a
power gain.
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 Cont..
Note! A negative sign of a linear gain indicates a 1800 phase shift
between the input and the output signals.
In most circuits, active devices such as, Transistors and ICs are
used as a main amplifier part.
In addition, resistors, inductors and capacitors are required to form
a complete amplifier circuit.
These passive components provide paths for the input and output
signals.

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 Cont..
Typical Small Signal Amplifier Circuit

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 Analysis of Small Signal BJT Amplifier Circuits
The simplest way to analyse the circuit are;
DC Analysis
AC Analysis
The DC Analysis is used to determine the dc operating values.
Setting all ac sources to zero and replacing them by an open-circuit
equivalent.
 Replacing all capacitors by an open-circuit equivalent.
Removing all elements bypassed by the open-circuit equivalents
introduced by steps 1 and 2.
 Redrawing the network in a more convenient and logical form.
Calculate all the dc operating values.

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 Cont..
The AC Analysis is used to determine the ac values of the amplifiers.
Setting all dc sources to zero and replacing them by a short-circuit
equivalent
 Replacing all capacitors by a short-circuit equivalent
Removing all elements bypassed by the short-circuit equivalents
introduced by steps 1 and 2
Redrawing the network in a more convenient and logical form
Calculate all necessary ac values (parameters)

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 Cont..

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 Cont..
THE re TRANSISTOR MODEL

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 Cont..

BJT Equivalent Circuit

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 Cont..
Common-base BJT transistor equivalent circuit

Common base re
equivalent circuit.

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 Cont..
Common-emitter fixed-bias configuration

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 THE HYBRID EQUIVALENT MODEL
The parameters of the hybrid equivalent circuit are defined in
general terms for any operating conditions.

Two-Port hybrid model

Short-circuit input-impedance
parameter

Open-circuit reverse transfer

voltage ratio parameter
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 Cont..
Short circuit forward transfer
current ratio parameter

Open-circuit output admittance

parameter

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 Cont..
hr is normally a relatively small quantity, its removal is approximated
by hr ≅0 and hrVo = 0, resulting in a short-circuit equivalent for the
feedback element.
The resistance determined by 1/ho is often large enough to be
ignored in comparison to a parallel load, permitting its replacement
by an open-circuit equivalent for the CE and CB models.

Effect of removing hre and hoe from the hybrid Approximate hybrid equivalent model
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 Cont..
The hybrid equivalent circuit is quite similar to the general structure
of the CB and CE equivalent circuits obtained with the re model.
The hybrid equivalent and the re models for each configuration are:

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 Cont..

Approximate common-emitter
hybrid equivalent circuit

Approximate common-base
hybrid equivalent circuit

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 Cont..
Fixed-Bias Configuration
The hybrid ac equivalent circuit for fixed bias configuration

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 Cont..
Example: For the network shown below, determine:
 Zi
 Zo
 Av
 Ai

Answer

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