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Файл:Basics of electronics. Study aid
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E
I
I
200
150
i
B
(A)
i
(mA)
C
5
i
=200 A
4
3
B
i
=150 A
B
100
50
v
00.20.4
BE
(V)
2
1
0
123456789101112
i
=100 A
B
i
=50 A
B
v
CE
(V)
(a) (b)
Fig. 5.3. The bipolar transistor d.c. characteristics: (a) the input characteristic;
(b) the output characteristic
You can see from fig. 5.2 that the emitter current is the sum of the base
and collector currents, that is,
CB
. The ratio
I
is called , the
CE
II
common-base current gain, and its value is slightly less than one.
1
If
20
then
20 1 20 0.95
.
.
In this n-p-n device, most of the current that flows through the forwardbiased emitter-base junction consists of electrons passing from the heavily
doped emitter region to the more lightly doped base region. (Remember that
electrons, because of their negative charge, actually travel in the direction
that is opposite to the conventional current flow shown in fig. 5.2) There is a
much smaller hole current flowing from the base region to the emitter. But
V
notice in fig. 5.2 that the polarity of
is such that the collector-base
CE
junction is reverse-biased. That seems to be strange since with a simple p-n
junction diode reverse bias means a very small current. However bipolar
junction transistors are not that simple. The large emitter current consists of
electrons that flow through the forward-biased emitter-base junction, cross
the very thin base region, and pass through the reverse-biased collector-base
junction. This current flow is controlled not simply by the bias voltages on
the junctions, but by the distribution of both types of carrier (electrons and
holes) within the base region. The much smaller base current consists mainly
of holes flowing from the base to the emitter.
A more complete explanation is not given in this textbook. The
important point to remember is that small changes in the forward bias voltage
on the base-emitter junction cause large changes in the current flowing
through the emitter and collector terminals.
71
Another graph to note is the variation of collector current with the
collector-emitter voltage
to a different value of base current
For a value of
only slightly with
VCE. We should take account of this slight variation when
V
, as shown in fig. 5.3(b). Each curve corresponds
CE
I
.
B
VCE greater than about 0.6 V, the collector current varies
calculating the small-signal parameters, but for now we consider the variation
to be negligible.
5.2. MODEL OF TRANSISTOR
Figure 5.4 shows small-signal a.c. equivalent circuits. These represent
the action of the n-p-n transistor and its circuit components to the signal
components of the voltages and currents in the amplifier.
Equivalent circuit of the complete amplifier
Figure 5.4(b) shows the small-signal a.c. equivalent circuit of the
complete amplifier. Fixed d.c. voltage, such as the power supply voltage, has
a zero a.c. voltage component. They are at the same a.c. voltage as the
emitter connection (commonly taken as earth, ground or 0 V rail), so
components connected to them go to the 0 V rail in the equivalent circuit.
(a)
(b)
V
BE
V
n-p-n
in
i
B
r
i
B
V
CE
V
BE
E
VT
I
in
R
B
i
b
B
r
r
e
i
E
r
b
B
r
e
i
E
c(e)
r
c(e)
i
C
C
V
CE
E
i
B
i
C
R
C
V
out
R
L
Fig. 5.4. The small-signal a.c. equivalent circuits: (a) an n-p-n transistor;
(b) the complete common-emitter amplifier
72
At the input, the equivalent circuit includes the signal source and the
resistor
V
CC
equivalent circuit, this point is connected to the 0 V rail, and
parallel with the input resistance of transistor
R
. In the real circuit, the ‘top’ end of R
B
goes to the supply voltage
B
. This is a fixed voltage and its a.c. component is zero. So, in the a.c.
RB appears in
VT. The signal source is usually
connected via a capacitor of negligible reactance at signal frequencies, so it
also appears to be in parallel with R
Similarly, the ‘top’ end of the collector resistor
R
so
appears in parallel with the collector-emitter terminals of the transistor.
C
and the input resistance of BJT.
B
RC is connected to V
CC
,
The load resistor is connected via a capacitor of negligible reactance at signal
frequencies, so it also appears to be in parallel with R
.
C
Typically, the input resistance of transistors equals a few hundreds ohms
R
or a few k’s. The base resistor
commonly has higher resistance, tens or
B
hundreds of k’s.
r
The emitter resistance
few ohms or more at room temperature. The base resistance
hundreds ohms, while
r
c(e)
of the transistor is relatively small and equals a
e
r
is about a few
b
may be tens of k’s or more.
The range of possible values of is very wide, even for a given type of
transistor. Typically may be anything from tens to hundreds or more.
The total load on the output current source is the effective load
resistance ,
R'
L
the resistance of r
, RC and RL all in parallel.
c(e)
Note that, with collector resistors and load resistors of a few kilohms,
the effect of the transistor output resistance is quite small and can be ignored
for a good approximation to the gain.
5.3. Types of Configuration. Modes of Operation
Any electrical device can be described by sets of graphs showing how
the input and output voltages and currents vary with respect to each other and
with respect to certain external parameters, of which temperature is usually
the most important. Transistors are no exception to this rule.
Transistors are three-terminal devices and can therefore be connected in
three different ways. The common-emitter, common-base and commoncollector modes are illustrated in fig. 5.5 and the common-collector mode is
more usually termed ‘emitter-follower’. The terms ‘grounded’ or ‘earthed’
emitter, base, or collector are also used. These terms are rather confusing
since they refer to the fact that the electrode mentioned is common to both
input and output circuits, but it does not mean that these electrodes are
necessarily connected to ground.
In fig. 5.5, the arrows show the current flow for an n-p-n
transistor, i. e.
the positive direction of currents.
73
i
i
C
i
B
V
in
i
E
R
load
V
CC
V
in
i
E
i
C
i
B
R
load
V
CC
V
in
C
i
B
i
E
V
CC
V
bias
V
bias
V
bias
(a) (b) (c)
Fig. 5.5. The bipolar transistor configurations: (a) common emitter;
(b) common base; (c) common collector
The current-gain parameter for these three configurations can be written
as follows.
I
C
Common-base direct current gain
Common-emitter direct current gain
Common-collector direct current gain
I
E
I
C
I
I
B
E
I
B
It is therefore apparent that three sets of characteristics are necessary to
describe the operation of a transistor connected in each of its three modes.
However, since the common-emitter connection is the most widely used and
the only one which produces both voltage and current amplification, the
others are very often omitted.
Modes of Operation
Bipolar transistors have five distinct regions of operation, defined by
BJT junction biases. The modes of operation can be described in terms of
junction biasing:
Forward-active (or simply, active): The base-emitter junction is
forward biased and the base-collector junction is reverse biased. Most bipolar
transistors are designed to produce the greatest common-emitter current gain,
, in forward-active mode. In this case, the collector-emitter current is
approximately proportional to the base current, but many times larger.
Reverse-active (or inverted): By reversing biasing conditions of the
forward-active region, a bipolar transistor goes into reverse-active mode. In
this mode, the emitter and collector regions switch roles. As most BJTs are
designed to maximize current gain in forward-active mode, the in inverted
mode is several (2–3 for the germanium BJT) times smaller. This transistor
mode is seldom used.
74
Saturation: With both junctions forward-biased, a BJT is in the
saturation mode and facilitates high current conduction from the emitter to
the collector (or the other direction in the case of n-p-n, with negatively
charged carriers flowing from emitter to collector). This mode corresponds to
a logical ‘on’, or a closed switch.
Cutoff: In cutoff, biasing conditions opposite of saturation (both
junctions reverse biased) are present. There is very little current, which
corresponds to a logical ‘off’, or an open switch.
Avalanche breakdown region: Breakdown is not generally a useful
mode for transistor operation, and so we will avoid that mode.
Thus, three basic operating modes of a BJT are Active, Cutoff, and
Saturation.
Self-Assessment Questions
1. Classify each of the following components as passive or active:
resistor; bipolar transistor (BJT); capacitor; battery; transformer; fieldeffect transistor (FET); inductor. Draw its symbols.
What are two types of bipolar junction transistors?
2.
For a silicon transistor, when a base-emitter junction is forward-
3.
biased, what the nominal voltage drop does it have?
Draw and explain input and output characteristics of common
4.
emitter configuration of a BJT.
In which region are both the base-collector and base-emitter
5.
junctions forward-biased?
Describe the modes of operation of a BJT in terms of junction
6.
biasing.
75
CHAPTER 6. FIELD EFFECT TRANSISTORS
6.1. Construction. Characteristics
Field-effect transistors (FETs) are three-terminal devices with terminals
called
JFET, and the metal-oxide-silicon FET, or MOSFET.
form. In p-channel FETs the current is carried by holes, whilst in n-channel
FETs it is carried by electrons.
source, drain and gate. There are two types, the junction FET, or
Both types of FET can be made in either p-channel form or n-channel
SG
D
drain, D
p-type
gate, G
n-type
channel
p-type substrate
source, S
Fig. 6.1. Symbol and typical structure of an n-channel JFET
silicon
dioxide
An n-channel JFET is shown in fig. 6.1. The p-n junction transition
region is shown in white, just below the p-type material under the
gate
terminal. The electron current flows through a channel of silicon which crosssectional area is controlled by the width of the p-n junction transition region,
which intrudes into the channel, as illustrated in the figure. The two ends of
the channel are called
source and drain. The application of a reverse bias
between gate and source causes the transition region of the gate to widen.
Therefore the width of the channel through which the current flows is
V
reduced. Thus the applied gate-source voltage
I
source-drain current
. Figure 6.2 shows a family of characteristic curves.
D
can be used to control the
GS
The gate of a JFET is a p-n junction which must be reverse-biased. Thus
V
the gate-source voltage
negative voltage. Suppose, for example,
close the channel when the drain-source voltage
of the n-channel type must be held at zero or a
GS
V
is held at –4V. This does not
GS
V
is low. However as V
DS
DS
increases, the voltage VDG across the gate p-n junction also increases, causing
the transition region of the gate p-n junction to widen at the drain end of the
channel. The gate-source voltage which completely closes the channel is
called the
pinch-off voltage V
value of the drain-source voltage
then
I
increases as VDS is increased, as shown by the
D
fig. 6.2. The region of operation up to pinch-off, where the
, at which no drain current flows whatever the
p
V
. But, if V
DS
is insufficient to cut-off I
DS
V
GS
4V
V
versus I
DS
line in
,
D
D
76
characteristics are curved, is called the ‘linear region’. Beyond this is the
‘saturation region’, in which the current increases much more slowly than in
the linear region. Here the channel gets shorter at the drain end as the gate
transition region becomes wider due to the increased reverse voltage across
it. The current is determined by the channel dimensions and the electric field.
i
(mA)
v
GS
(V)
D
I
DSS
V
p
ohmic
-8 -6 -4 -2
4
2
1
pinch-off region
12345678910
0
V
V
V
breakdownsaturation regionohmic
V
GS
GS
GS
GS
=0 V
=2 V
=4 V
=8 V
v
DS
(V)
Fig. 6.2. N-channel JFET characteristic curves
The voltage VGS applied to the gate controls the current flowing between
V
the drain and the source terminals. Voltage
between the gate and the Source while
refers to the voltage applied
GS
V
refers to the voltage applied
DS
between the drain and the source. Because a junction field effect transistor is
a voltage controlled device, ‘no current flows into the gate’, then the source
I
current
and therefore
flowing out of the device equals the drain current flowing into it
S
I
= IS.
D
Drain current in the saturation (or active) region can be found as
V
V
2
GS
.
p
II
DDSS
1
Note that the value of the drain current will be between zero (pinch-off)
and I
(maximum current).
DSS
6.2. Applications
Field-effect transistors, like bipolar transistors, are used for a wide
variety of analogue and digital circuits. FET analogue circuits have close
similarity with bipolar ones, differing mainly in their gate bias arrangements.
Their small-signal equivalent circuit is very similar to that of the bipolar
transistor, with the parameters associated with the bipolar’s emitter, base and
collector replaced by those for the FET’s source, gate and drain. One
simplification is that the FET’s input (gate-source) resistance is normally so
77
high that the input current can be ignored, although its input capacitance is
similar to that of the BJT, and is significant in high-frequency circuits.
The best way to look at an FET is as a voltage controlled variable
resistor. The resistor is between the source and drain pins. The value of the
resistor depends on the voltage between the gate and source V
voltage is zero volts, the resistance is very low (a few ohms or less). If V
. If the
GS
GS
is
above a certain level, the resistance is very high (several million ohms), and
is essentially an open circuit.
The main situation where FETs are superior is in high current circuits.
Suppose we want to switch a motor, electrical heater or other high current
load. FETs are produced with very low R
and source. The lower the R
, the more efficient the circuit is.
ds
, the resistance between the drain
ds
MOSFETS compared with Bipolar Transistors
The main electrical advantage of MOSFETs over bipolar transistors is
their zero low-frequency gate current, because of the insulating oxide layer
between gate electrode and substrate. Thus the low-frequency input power
can be very low. However, at higher frequencies considerable input current
flows into the gate-substrate capacitance and this current has to be supplied
from the previous stages, increasing the overall power dissipation.
The advantage of MOSFETs from a production point of view is that, in
general, they are smaller and cheaper to manufacture than BJTs.
Their main disadvantage is that the control of output current by the input
current is less effective, so that the mutual conductance and, hence, the
voltage gain available from MOSFET analogue amplifiers is less than that
from bipolar transistor amplifiers operating under similar conditions.
Self-Assessment Questions
1. What are the names of three terminals of the JFET?
2.
What is the level of gate current in a FET?
Draw the transfer curve of a FET and explain the operating
3.
principle.
Draw and explain static characteristics curves of an n-channel JFET.
4.
What are the advantages and disadvantages of a MOSFET?
5.
78
CHAPTER 7. THYRISTORS
The thyristor is a solid-state semiconductor device with four layers of
alternating n- and p-type material. They act as a switch, conducting when
their gate receives a current pulse, and continue to conduct as long as they are
forward biased (that is, as long as the voltage across the device has not
reversed).
Some sources define silicon controlled rectifiers and thyristors as
synonymous. Others define thyristors as a larger set of devices with at least
four layers of alternating n- and p-type material, including: gate turn-off
thyristor (GTO), triode a.c. switch (TRIAC), static induction transistor (SIT),
static induction thyristor (SITH) and MOS-controlled thyristor (MCT).
Non-SCR thyristors include devices with more than four layers, such as
triacs.
7.1. Silicon Controlled Rectifier
The thyristor is a four-layer semiconducting device, with each layer
consisting of alternately n-type or p-type material, for example p-n-p-n. The
main terminals, labeled anode and cathode, are across the full four layers, and
the control terminal, called the gate, is attached to p-type material near the
cathode. The operation of a thyristor can be understood in terms of a pair of
tightly coupled bipolar junction transistors, arranged to cause the selflatching action (see fig. 7.1)
Thyristors have three states:
Reverse blocking mode – Voltage is
1.
A
applied in the direction that would be
blocked by a diode
Forward blocking mode – Voltage is
2.
G
K
applied in the direction that would cause a
diode to conduct, but the thyristor has not yet
been triggered into conduction
Forward conducting mode – The
3.
Gate
thyristor has been triggered into conduction
and will remain conducting until the forward
current drops below a threshold value known
as the ‘holding current’.
Fig. 7.1. SCR symbol
and equivalent circuit
Function of the Gate Terminal
The thyristor has three p-n junctions (serially named J1, J2, J3 from the
anode).
Anode
VT1
VT2
Cathode
79
J
1
J
2
J
3
ANODE
I
A
ppnn
I
1
I
2
I
G
GATE
I
3
CATHODE
I
C
Fig. 7.2. Layer diagram of thyristor
When the anode is at a positive potential VAK with respect to the cathode
with no voltage applied at the gate, junctions J
while junction J
takes place (Off state). Now if V
voltage V
of the thyristor, avalanche breakdown of J2 takes place and the
BO
is reverse biased. As J2 is reverse biased, no conduction
2
is increased beyond the breakdown
AK
and J3 are forward biased,
1
thyristor starts conducting (On state).
If a positive potential V
the cathode, the breakdown of the junction J
By selecting an appropriate value of V
is applied at the gate terminal with respect to
G
occurs at a lower value of VAK.
2
, the thyristor can be switched into the
G
on state immediately.
It should be noted that as avalanche breakdown has occurred, the thyristor
continues to conduct, irrespective of the gate voltage, until either the potential
is removed or the current through the device (anode-cathode) is less than
V
G
holding current specified by the manufacturer. Hence V
the
can be a voltage
G
pulse, such as the voltage output from a UJT relaxation oscillator.
These gate pulses are characterized in terms of the
) and the gate trigger current (IGT). The gate trigger current varies
(V
GT
gate trigger voltage
inversely with the gate pulse width in such a way that it is evident that a
minimum gate charge is required to trigger the thyristor.
Switching Characteristics
In a conventional thyristor, once it has been switched on by the gate
terminal, the device remains latched in the on-state (i. e. it does not need a
continuous supply of gate current to conduct), providing the anode current
has exceeded the
latching current (I
). As long as the anode remains
L
positively biased, it cannot be switched off until the anode current falls
below the
holding current (I
A thyristor can be
to become
negatively biased. In some applications this is done by switching
switched off if the external circuit causes the anode
).
H
a second thyristor to discharge a capacitor into the cathode of the first
thyristor. This method is called
forced commutation.
80
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