2. outline
Introduction
Bipolar Junction Trnasistor (BJT)
Construction of BJT
Operation of Transistors
BJT configurations
●
Common- base configuration
●
Common-emmiter configuration
●
Common-collector configuration
3. 2.1 Introduction
The invention of transistors (in 1947)
marked the beginning of revolution in
electronics.
The pace of the revolution was
accelerated a decade later by the
development of the integrated circuit
(IC or silicon chip).
Transistors are broadly classified into
two types: Bipolar Junction Transistor
(BJT) and Field Effect Transistor (FET).
4. 2.2 Bipolar Junction
Transistor (BJT)
Construction: The transistor is a three-layer semiconductor
device consisting of either two n- and one p-type layers of
material or two p- and one n-type layers of material. The
former is called an npn transistor, while the latter is called a
pnp transistor.
The term bipolar reflects the fact that holes and electrons
participate in the injection process into the oppositely
polarized material. If only one carrier is employed (electron or
hole), it is considered a unipolar device.
6. Construction of BJT
In the previous diagrams, the terminals
have been indicated by the capital
letters E for emitter, C for collector,
and B for base.
7. 2.2.2 Transistor Operation
Consider the pnp trnasistor of previous
figure (Fig. (a) ) to describe the basic
operation of a BJT transistor.
The operation of the npn transistor is
exactly the same if the roles played by
the electron and hole are
interchanged.
We investigate the two junctions of
pnp transistor (separately).
8. Forward-biased junction of a
pnp transistor (without base-to-
collector bias).
•
The depletion
region has
been reduced
in width due to
the applied
bias, resulting
in a heavy flow
of majority
carriers from
the p- to the n-
type material.
•
It is similar to
forward biased
diode.
9. Reverse-biased junction of
a pnp transistor (without
base-to-emitter bias).
•
Flow of majority
carriers is zero,
resulting in only a
minority-carrier
flow.
•
Similar to reverse
biased diode.
The number of uncovered negative
ions in the depletion region of the P-
type material will increase due to the
large number of “free” holes drawn to
the negative potential of the applied
voltage. The effect is a widening of the
depletion region.
10. Operation of BJT
In general:
● One p-n junction of a transistor is reverse biased,
while the other is forward biased.
Both biasing potentials are applied (normal
operation), which results in Majority and
minority carrier flow of a pnp transistor, as
follows.
12. Operation of BJT
The widths of the depletion regions, indicating clearly which
junction is forward-biased and which is reverse-biased.
A large number of majority carriers will diffuse across the
forward-biased p-n junction into the n-type material.
Since the sandwiched n-type material is very thin and has a
low conductivity, a very small number of these carriers will
take this path of high resistance to the base terminal.
13. Operation of BJT
The larger number of these majority carriers will diffuse across
the reverse-biased junction into the p-type material connected
to the collector terminal.
Applying Kirchhoff’s current law to the transistor of previous
Fig. : IE = IC + IB
The collector current, however, is comprised of two
components—the majority and minority carriers. The minority
current component is called the leakage current and is given
the symbol ICO (IC current with emitter terminal Open). The
collector current is then given by
IC = ICmajority + ICOminority
14. 2.2.3 BJT Configurations
A) Common base configuration
In common base configuration, the base is
common to both the input and output sides of
the configuration.
The arrow in the graphic symbol defines the
direction of emitter current (conventional
flow) through the device.
17. Common base configuration
To fully describe the behavior of a three-terminal device such
as the common base amplifiers of above figure, it requires
two sets of characteristics—one for the driving point or input
parameters and the other for the output side.
The input set for the common-base amplifier, as shown in Fig.
below, relates an input current (IE) to an input voltage (VBE)
for various levels of output voltage (VCB).
19. Common base configuration
The output set relates an output
current (IC) to an output voltage (VCB)
for various levels of input current (IE)
as shown in following figure.
20. Common base configuration
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Second level
●
Third level
●
Fourth level
●
Fifth level
21. Common base configuration
The output or collector set of characteristics has three basic
regions of interest, as indicated in the previous figure - the
active, cutoff, and saturation regions.
The active region is the region normally employed for linear
(undistorted) amplifiers. In particular : In the active region the
collector-base junction is reverse-biased, while the base-
emitter junction is forward-biased.
22. Common base configuration
The curves clearly indicate that a first
approximation to the relationship
between IE and IC in the active region
is given by IC ≈ IE .
As inferred by its name, the cutoff
region is defined as that region where
the collector current is 0 A. In
addition, in cutoff region the collector-
base and base-emitter junctions of a
transistor are both reverse-biased.
25. B) Common emitter
configuration
It is called the common-emitter
configuration since the emitter is
common or reference to both the input
and output terminals (in this case
common to both the base and collector
terminals).
Notation and symbols used with the
common-emitter configuration (for
both npn and pnp transistors) are
shown in the following figure.
27. Common emitter
configuration
Even though the transistor configuration has changed, the
current relations developed earlier for the common-base
configuration are still applicable . That is, IE = IC + IB and IC
= ᾳIE.
For the common-emitter configuration the output
characteristics are a plot of the output current (IC) versus
output voltage (VCE) for a range of values of input current
(IB). The input characteristics are a plot of the input current
(IB) versus the input voltage (VBE) for a range of values of
output voltage (VCE). Both plots are shown in the following
figure.
29. Common emitter
configuration
The curves of IB are not as horizontal as those obtained for IE
in the common-base configuration, indicating that the
collector-to-emitter voltage will influence the magnitude of the
collector current.
Note on the collector characteristics of the previous Fig. that
IC is not equal to zero when IB is zero. For the common-base
configuration, when the input current IE was equal to zero, the
collector current was equal only to the reverse saturation
current ICO, so that the curve IE = 0 and the voltage axis
were, for all practical purposes, one.
The reason for this difference in collector characteristics can
be derived as follows:
30. Common emitter
configuration
If we consider the case IB = 0 A, and
substitute a typical value of ᾳ such as
0.996, the resulting collector current is
the following:
