UNIT 3

BIPOLAR JUNCTION TRANSISTOR

 Transistors
• Two main categories of transistors:
– bipolar junction transistors (BJTs) and
– field effect transistors (FETs).
• Transistors have 3 terminals where the application of
current (BJT) or voltage (FET) to the input terminal
increases the amount of charge in the active region.
• The physics of "transistor action" is quite different for the
BJT and FET.
• In analog circuits, transistors are used in amplifiers and
linear regulated power supplies.
• In digital circuits they function as electrical switches,
including logic gates, random access memory (RAM), and
microprocessors.
 Bipolar Junction Transistors (BJT)
• A bipolar transistor
essentially consists of a pair of
PN Junction diodes that are
joined back-to-back.
• There are therefore two kinds
of BJT, the NPN and PNP
varieties.
• The three layers of the
sandwich are conventionally
called the Collector, Base, and
Emitter.
 BJT Structure - Planar

The “Planar Structure” developed by Fairchild in

the late 50s shaped the basic structure of the BJT,
even up to the present day.

• In the planar process, all steps are performed from the

surface of the wafer
• BJTs are usually constructed vertically
– Controlling depth of the emitter’s n doping sets the base
width

E B C

n
p
n
Terminals & Operations
• Three terminals:
– Base (B): very thin and lightly doped central region (little
recombination).
– Emitter (E) and collector (C) are two outer regions sandwiching
B.
• Normal operation (linear or active region):
– B-E junction forward biased; B-C junction reverse biased.
– The emitter emits (injects) majority charge into base region and
because the base very thin, most will ultimately reach the
collector.
– The emitter is highly doped while the collector is lightly doped.
– The collector is usually at higher voltage than the emitter.
Terminals & Operations
Operation Mode
 Operation Mode
• Active:
– Most importance mode, e.g. for amplifier operation.
– The region where current curves are practically flat.
• Saturation:
– Barrier potential of the junctions cancel each other out
causing a virtual short.
– Ideal transistor behaves like a closed switch.
• Cutoff:
– Current reduced to zero
– Ideal transistor behaves like an open switch.
Operation Mode
Circuit Symbols
Circuit Configuration
 I-V Characteristics
IC
IC VBE3

VCE VBE2
VBE
VBE1
VBE3 > VBE2 > VBE1

VCE

• Collector current vs. vCB shows the BJT looks like a current
source (ideally)
– Plot only shows values where BCJ is reverse biased and so BJT in active
region
• However, real BJTs have non-ideal effects
 I-V Characteristics

Base-emitter junction looks like a Collector-emitter is a family of

forward biased diode curves which are a function of
base current.
I-V Characteristics
 Common-emitter

It is called the common-emitter configuration because (ignoring the power

supply battery) both the signal source and the load share the emitter lead as
a common connection point.
 Common-collector

It is called the common-collector configuration because both the signal source

and the load share the collector lead as a common connection point. Also called
an emitter follower since its output is taken from the emitter resistor, is useful as an
impedance matching device since its input impedance is much higher than its output
impedance.
 Common-base

This configuration is more complex than the other two, and is less common
due to its strange operating characteristics.
Used for high frequency applications because the base separates the input and
output, minimizing oscillations at high frequency. It has a high voltage gain,
relatively low input impedance and high output impedance compared to the
common collector.
 BJT Analysis

• Here is a
common
emitter BJT
amplifier:
• What are the
steps?
 Input & Output

• We would want to know the collector current (iC), collector-

emitter voltage (VCE), and the voltage across RC.
• To get this we need to fine the base current (iB) and the base-
emitter voltage (VBE).
 Input Equation

• To start, let’s write Kirchoff’s voltage law (KVL) around

the base circuit.
 Output Equation

Likewise, we can write KVL around the collector circuit.

 Use Superposition:
DC & AC sources
• Note that both equations are written so as to calculate the transistor
parameters (i.e., base current, base-emitter voltage, collector current,
and the collector-emitter voltage) for both the DC signal and the AC
signal sources.
• Use superposition, calculate the parameters for each separately, and
add up the results:
– First, the DC analysis to calculate the DC Q-point
• Short Circuit any AC voltage sources
• Open Circuit any AC current sources
– Next, the AC analysis to calculate gains of the amplifier.
• Depends on how we perform AC analysis
– Graphical Method
– Equivalent circuit method for small AC signals
 BJT - DC Analysis
• Using KVL for the input and output circuits and
the transistor characteristics, the following
steps apply:
1. Draw the load lines on the transistor characteristics
2. For the input characteristics determine the Q point for
the input circuit from the intersection of the load line and
the characteristic curve (Note that some transistor do not
need an input characteristic curve.)
3. From the output characteristics, find the intersection of
the load line and characteristic curve determined from the
Q point found in step 2, determine the Q point for the
output circuit.
Base-Emitter Circuit Q point

The Load Line

intersects the
Base-emitter
characteristics at
VBEQ = 0.6 V and IBQ
= 20 µA
 Collector-Emitter Circuit Q point

Now that we have

the Q-point for the
base circuit, let’s
proceed to the
collector circuit.

The Load Line intersects the Collector-emitter characteristic, iB = 20 µA at VCEQ = 5.9 V

and ICQ = 2.5mA, then β = 2.5m/20 µ = 125
 BJT DC Analysis - Summary
• Calculating the Q-point for BJT is the first step in analyzing the
circuit
• To summarize:
– We ignored the AC (variable) source
• Short circuit the voltage sources
• Open Circuit the current sources
– We applied KVL to the base-emitter circuit and using load line analysis
on the base-emitter characteristics, we obtained the base current Q-
point
– We then applied KVL to the collector-emitter circuit and using load
line analysis on the collector-emitter characteristics, we obtained the
collector current and voltage Q-point
• This process is also called DC Analysis
• We now proceed to perform AC Analysis
 BJT - AC Analysis
• How do we handle the variable source Vin(t) ?
• When the variations of Vin(t) are large we will
use the base-emitter and collector-emitter
characteristics using a similar graphical
technique as we did for obtaining the Q-point.
• When the variations of Vin(t) are small we will
shortly use a linear approach using the BJT
small signal equivalent circuit.
 BJT - AC Analysis
• Let’s assume that Vin(t) = 0.2 sin(ωt).
• Then the voltage sources at the base vary from a
maximum of 1.6 + 0.2 = 1.8 V to a minimum of 1.6 -0.2 =
1.4 V
• We can then draw two “load lines” corresponding the
maximum and minimum values of the input sources
• The current intercepts then become for the:
– Maximum value: 1.8 / 50k = 36 µA
– Minimum value: 1.4 / 50k = 28 µA
 AC Analysis Base-Emitter Circuit

Note the asymmetry around the Q-point

From this graph, we find:
of the Max and Min Values for the base
At Maximum Input Voltage: current and voltage which is due to the
VBE = 0.63 V, iB = 24 µA non-linearity of the base-emitter
At Minimum Input Voltage: characteristics
VBE = 0.59 V, iB = 15 µA
Recall: At Q-point: ∆iΒmax = 24-20 = 4 µA;
VBE = 0.6 V, iB = 20 µA ∆iBmin = 20-15 = 5 µA
AC Analysis Base-Emitter Circuit
 AC Characteristics-Collector Circuit

Using these max and min values for the base current on the collect
circuit load line, we find:
At Max Input Voltage: VCE = 5 V, iC = 2.7mA
At Min Input Voltage: VCE = 7 V, iC = 1.9mA
Recall: At Q-point: VCE = 5.9 V, iB = 2.5ma
AC Characteristics-Collector Circuit
 The pnp Transistor
• Basically, the pnp transistor is similar to the
npn except the parameters have the opposite
sign.
– The collector and base currents flows out of the
transistor; while the emitter current flows into the
transistor
– The base-emitter and collector-emitter voltages
are negative
• Otherwise the analysis is identical to the npn
transistor.
The PNP Transistor

Current flow in a pnp transistor biased to operate in the active

mode.
 The pnp Transistor
• Two junctions
– Collector-Base and Emitter-Base
• Biasing
– vBE Forward Biased
– vCB Reverse Biased
 IE IC

In p u t E pnp C O u tp u t
p+ n p c ir c u it c ir c u it (c)
B IB
(a)
E m itte r B a se C o lle c t o r
E B C V EB V C B
E E B C
x
E
p n (0)
IE IC E le c t r o n
p n (x ) IE D iff u s io n IC
H o le
n p(0 ) H o le d iffu s io n d rift

(b) n p (x ) pno
n po R e c o b in a tio n
(d)
W E B
W B W E le c tro n s L eakage c u rre n t
B C
IB V
V EB C B

IB
(a) A schematic illustration of pnp BJT with 3 differently doped regions. (b) The
pnp bipolar operated under normal and active conditions. (c) The CB
configuration with input and output circuits identified. (d) The illustration of
various current component under normal and active conditions.
 The pnp Transistor

Current flow in an pnp transistor biased to operate in the

active mode.
Frequency Response Curve
The frequency response curves of a circuit is commonly
described in terms of the effect that a change in frequency
has on the ratio of its output amplitude
the ratio of a circuit’s output amplitude to its input amplitude
is referred as gain.
a frequency response curve is a graph that show the effect
that frequency has on circuit gain.
A frequency-response curve is a graphical representation of
the relationship between amplifier gain and operating
frequency
 There are four basic types of filters.
The low-pass filter is designed to pass all frequencies below its
cutoff frequency.
The high-pass filter is designed to pass all frequencies above its
cutoff frequency.
The bandpass filter is one designed to pass the band of
frequencies that lies between two cutoff frequencies.
The band-stop (or notch) filter is designed to block the band of
frequencies that lies between two cutoff frequencies.
DesiBell (dB)
The ratio of circuit output amplitude to input amplitude is
normally expressed using decibels (dB). Decibels are used because
they allow us to easily represent very large and very small values.

Gain (Av)
is a value that indicates the magnitude relationship between the
circuit’s input and output signals. If the gain of an amplifier is 100,
then the output is 100 times greater than the input signal
The ratio of circuit output voltage to input voltage is generally
referred to as voltage gain (Av).

The voltage gain of a circuit equals 70.7% of its maximum

value at the cutoff frequencies. Expressed mathematically: Av
= 0.707 Av(max) when ƒ = ƒC. This relationship is based on the fact
that power gain equals 50% of its maximum value when
voltage gain equals 70.7% of its maximum value.
The dB voltage gain of a circuit is found as twenty times the
common log of Av. By formula:

When dB voltage gain drops to 70.7% of its maximum value,

the change in dB voltage gain is –3 dB.
Bandwidth is the difference between the upper and lower
frequencies in a contiguous set of frequencies. It is typically
measured in hertz, and may sometimes refer to passband
bandwidth, sometimes to baseband bandwidth, depending on
context.

Passband bandwidth is the difference between the upper and

lower cutoff frequencies

baseband bandwidth always refers to the upper cutoff

frequency, regardless of whether the filter is bandpass or low-
pass.
The bandwidth of an amplifier is the range of frequencies for
which the amplifier gives "satisfactory performance". The
definition of "satisfactory performance" may be different for
different applications. However, a common and well-accepted
metric is the half power points (i.e. frequency where the power
goes down by half its peak value) on the output vs. frequency
curve. Therefore bandwidth can be defined as the difference
between the lower and upper half power points. This is
therefore also known as the −3 dB bandwidth.
The gain of a good quality full-range audio amplifier will be
essentially flat between 20 Hz to about 20 kHz (the range of
normal human hearing). In ultra high fidelity amplifier design,
the amp's frequency response should extend considerably
beyond this (one or more octaves either side) and might have
−3 dB points < 10 and > 65 kHz.
Definitions
In small-signal amplifiers the main factors are:
• Amplification
• Linearity
• Gain
Since large-signal, or power, amplifiers handle relatively large
voltage signals and current levels, the main factors are:
• Efficiency
• Maximum power capability
• Impedance matching to the output device
Amplifier Types
Class A
The amplifier conducts through the full 360° of the input. The Q-point is
set near the middle of the load line.
Class B
The amplifier conducts through 180° of the input. The Q-point is set at
the cutoff point.
Class AB
This is a compromise between the class A and B amplifiers. The amplifier
conducts somewhere between 180° and 360° . The Q-point is located
between the mid-point and cutoff.
Class C
The amplifier conducts less than 180 of the input. The Q-point is located
below the cutoff level.
 Class A Amplifier

The output of a class A amplifier conducts for the full 360° of the cycle.
The Q-point is set at the middle of the load line so that the AC signal can
swing a full cycle.

Remember that the DC load line

indicates the maximum and minimum
limits set by the DC power supply.
 Class B Amplifier

A class B amplifier output only conducts

for 180° or one-half of the AC input signal.

The Q-point is at 0V on the load line, so that

the AC signal can only swing for one-half cycle.
 Class AB Amplifier

This amplifier is a compromise between the class A and class B amplifier

—the Q-point is above that of the Class B but below the class A.

The output conducts between 180° and 360° of the AC input signal.
 Class C

The output of the class C

conducts for less than 180° of the
AC cycle. The Q-point is below
cutoff.
Efficiency refers to the ratio of output to input power. The lower the
amount
of conduction of the amplifier the higher the efficiency.

Class C usually not used for delivering large amount of power, thus the
efficiency is not given here.