PRINCIPLES OF ELECTRONIC
ENGINEERING
EN 23001
MRM Rouzin Azar
BSc Eng. (Hons)
Transistor Amplifiers and Switches with BJT
Contents:
•Bipolar Junction Transistor (BJT) behaviour
•CE, CB and CC configurations
•CE transistor
•CE amplifier
What is a transistor?
A transistor is a 3 terminal device made of semiconductor material.
In the simplest view, a transistor is a device which can generate an amplified version of any given
input signal.
Generally, the input current or voltage signal is much smaller than the output, thus there is an
amplification of the signal in terms of voltage, current or power.
Symbol of a transistor,
What is a transistor?
• There are mainly two types of transistors available.
• Bipolar Junction Transistor (BJT)
• Field Effect Transistor (FET)
• BJTs are used in two broad areas of electronics.
• As a linear amplifier to boost an electrical signal
• As an electronic switch
• BJTs are manufactured as three layer devices, made in two different ways,
• NPN
• PNP
NPN transistor
• Heavily doped, n-type emitter (E),
and moderately doped n-type
collector (C) with thin, lightly
doped, p-type base (B)
• This arrangement forms two PN
junctions,
• EB junction
• BC junction
Working principle - Unbiased
Note the
region widths
Working principle – Properly biased
E
Working principle – Summary
• BE junction acts as a control valve
• BE voltage controls the amount of electrons entering the base (just like in a diode)
• Since base is lightly doped only a few electrons find holes to recombine with
• Most of the uncombined electrons wander around in the base, until they get closer to the
depletion region of the BC junction, when they will get quickly accelerated by the electric
field (made by external reverse bias voltage + diffusion effect) and get dumped into the
collector region
• Most of the electrons coming into the base will be forced to go to the collector, hence
keeping the base current at a very low value
• The collector current can be precisely controlled by adjusting the base voltage in very
small amounts
You can see the link below for a clear understanding with the aid of animations.
• https://www.youtube.com/watch?v=DXvAlwMAxiA
Transistor modeling
• Significant voltages and currents,
• Base current – IB
• Collector current – IC
• Emitter current – IE
• Base-emitter voltage – VBE
• Collector-emitter voltage – VCE
• Base-collector voltage – VBC
• Kirchhoff's laws gives us,
• IB + IC = IE
• VCE = VBE - VBC
Transistor modeling
• Most important relationship,
𝐼𝐶 = 𝛽𝐼𝐵
• Applicable in active mode
• 𝛽 is called current gain
• 𝛽 is a ‘bad parameter’ since it varies ±50% or more
• Also varies heavily on IC, VCE and temperature
Transistor configurations
Common Common Common
Base Collector Emitter
Voltage gain High Low Medium
Current gain Low High Medium
Power gain Low Medium High
Phase change 0 0 180
Input resistance Low High Medium
Output resistance High Low Medium
We only study common emitter configuration here,
since it is the most prominently used
Common-Base configuration
Common-Collector configuration
Common-Emitter configuration
Relation between α and β
Relation between Transistor currents
Leakage Currents in a Transistor
• Due to minority carriers, transistors have some leakage current.
• One leakage current in transistor is called ICBO (The symbol I stands for
current, CB stands for collector base junction and O tells us the emitter is
open).
• ICBO is the current that flows across collector base junction under reverse
bias condition with emitter lead open.
• Another transistor leakage current is ICEO (Where, I stands for current, CE
stand for collector emitter terminal and O tells us that base terminal is
open).
Leakage Currents in a Transistor
Leakage Currents in a Transistor
Leakage Currents in a Transistor
Leakage Currents in a Transistor
Leakage Currents in a Transistor
From now, consider common emitter configuration, since
it is most prominently used.
Transistor characteristics
• Remember the I-V characteristics of the diode
• Need to make similar curves for the transistor
• There are mainly two important curves,
• VBE vs IB curve – Input characteristics
• VCE vs IC curve – Output characteristics
• Graphs are not simple like the diode because of the
many parameters involved
Transistor input characteristics
Transistor output characteristics
Transistor output characteristics
Quiescent Point:
• It is a point on the DC load line, which represents the
values of IC and VCE that exist in a transistor circuit
under no input conditions. It is also known as the DC
operating point / the working point.
• The DC load line is also called as static load line.
The operating point or the Q point must lie on this
line as well as on the IB curve determined by the
biasing circuit. Therefore, you can locate the Q-point
by taking the intersection of these two lines.
• AC load line (dynamic load line) is drawn considering
the AC quantities only. The intersection of DC load
line and AC load line also gives the Q-point, whereas
Q-point can be selected at different positions on the
DC load line.
Transistor operating regions
• There are two junctions BE and BC. Either of
these could be in forward bias or in reverse bias
during operation.
• Hence the transistor can work in 3 different
modes,
• Cutoff – Both junctions are reverse biased
• IC is very small, IB is very small, VCE can be large
depending on source
• Less power consumption
• Active – BE is forward biased, BC is reverse biased
• IC is significant, IB is small, VCE can be large
depending on source
• Significant power consumption
• Saturation – Both junctions are forward biased
• IC is significant, IB is larger, VCE is kept small because
of short circuit between CE
• Less power consumption
Modes of BJT Operation
Modes of Operation - Active
Modes of Operation – Saturation
Modes of Operation – Saturation
Uses of transistors
• Transistor acts as a switch when it is in cutoff and saturation regions as OFF
and ON switches respectively.
• Transistor acts as an amplifier when it is operating in the active
region.
• It is clear that we should properly maintain the voltages of the three
terminals in order to drive the transistor to whatever mode we require.
• This process is called Biasing the transistor.
• In active mode, a transistor acts as an ‘active device’ since it draws
power from its DC source and feeds it into the input signal,
amplifying it in the process.
Transistor biasing
• To prepare the transistor for amplification, we need to make sure that it is in
the active region. (i.e. BE junction is FB, BC junction is RB)
• There are several possible ways of doing this, which are collectively called
Biasing.
• Simplest way is to use two separate power supplies. But is this practical?
• There are many ways of biasing a transistor, we will detail out two of the
simplest and the self biased arrangement.
• Remember biasing is just preparation, no amplification is done yet.
Transistor biasing – Fixed bias
• Now transistor will have to obey the laws of the
circuit as well as its own characteristic curves.
• Method of analysis – VCC, 𝛽, RC and RB known
• Voltage law on RC path,
𝑉𝐶𝐶 = 𝑉𝐶𝐸 + 𝐼𝐶 𝑅𝐶
𝑉
𝐼𝐶 = − 1 𝑉 𝐶𝐸 + 𝐶𝐶 << Load line
𝑅𝐶 𝑅𝐶
• Voltage law on RB path,
𝑉𝐶𝐶 = 𝑉𝐵𝐸 + 𝐼𝐵 𝑅𝐵
• Since active mode,
𝑉𝐵𝐸 = 0.7𝑉
• During active mode,
𝐼𝐶 = 𝛽𝐼𝐵
Transistor biasing – Fixed bias
• Now transistor will have to obey the laws of the
circuit as well as its own characteristic curves.
• Method of design – VCC, 𝛽 known
• Choose a proper IB and set,
𝑉𝐶𝐸 = 0.5 𝑉𝐶𝐶
• Since active mode,
𝑉𝐵𝐸 = 0.7𝑉
• Voltage law on RB path,
𝑉𝐶𝐶 = 𝑉𝐵𝐸 + 𝐼𝐵 𝑅𝐵
• During active mode,
𝐼𝐶 = 𝛽𝐼𝐵
• Voltage law on RC path,
𝑉𝐶𝐶 = 𝑉𝐶𝐸 + 𝐼𝐶 𝑅𝐶
1 𝑉
𝐼𝐶 = − 𝑉 𝐶𝐸 + 𝐶𝐶
𝑅𝐶 𝑅𝐶
<< Load line
Transistor biasing – Fixed bias
• When no other signals are applied to the BJT, IC
and VCE are determined by the biasing circuit.
• These values correspond to a specific point on
the load line.
• Derive expressions for IB and IC, look at the
parameters that can change this bias point.
• A disadvantage of this technique is that the bias
point is very much dependent on the 𝛽, which we
know to be variable.
• Stability factor (S) is defined as the change in
collector current per unit change in leakage
current and the best possible biasing circuit has
S ≈ 1. But S ≈ 1+β for the fixed bias circuit,
which is disadvantageous in terms of stability too.
Transistor biasing – Fixed bias
Transistor biasing – Fixed bias
Transistor biasing – Collector feedback
Transistor biasing – Emitter feedback
Transistor biasing – Self biased arrangement
Transistor biasing – Self biased arrangement
Transistor biasing – Self biased arrangement
Transistor biasing – Self biased arrangement
Transistor biasing – Self biased arrangement
Transistor biasing – Self biased arrangement
Transistor biasing
Transistor biasing
Transistor biasing
125
Transistor biasing
Transistor biasing
Transistor biasing
Transistor biasing
Transistor biasing
Transistor biasing
Transistor operation – As switch
Transistor operation – As switch
Transistor operation – As switch
Transistor operation – As amplifier
Transistor operation – As amplifier
Transistor operation – As amplifier
Transistor operation – As amplifier
Transistor operation – As amplifier
Transistor operation – As amplifier
Transistor operation – As amplifier
• The capacitor C1 is called as a coupling or blocking capacitor and performs two
functions.
• It couples the input signal to the base of the transistor.
• It blocks the DC voltage from reaching the input signal from Vcc.
• It is necessary to prevent the DC voltage from reaching the input signal source,
because it disturbs the biasing conditions of the circuit that supplies the input
signal source.
• The capacitor C2 is also called as a coupling or blocking capacitor. It couples the
output of collector voltage to the load resistor .
• The third capacitor CE is connected in shunt(parallel) with the emitter resistor RE.
It is called the emitter by pass capacitor. Because, it by passes the resistor RE and
provides an easy path for the AC signal. If RE were not by passed, the AC signal
would flow through RE and result an AC voltage drop across it.
THANK YOU
