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Файл:Basics of electronics. Study aid
.pdf
Reverse
breakdown
voltage
V
BR
Latching
current
Holding
current
I
T
Forward voltage drop
V
(conducting)
TM
Gate
triggered
I
L
I
H
Forward
breakover
voltage
V
T
V
RRM
Reverse
leakage
current I
Specified minimum
reverse
blocking
voltage
RRM
Forward
leakage
current I
Specified minimum
DRM
off-state
blocking
voltage
V
DRM
V
BO
Fig. 7.3. Static V-I characteristics of SCR device
After a thyristor has been switched off by forced commutation, a finite
time delay must have elapsed before the anode can be positively biased in the
off-state. This minimum delay is called the
time
(tQ). Attempting to positively bias the anode within this time causes the
circuit commutated turn-off
thyristor to be self-triggered by the remaining charge carriers (holes and
electrons) that have not yet recombined.
For applications with frequencies higher than the domestic a.c. mains
supply (e. g. 50 Hz or 60 Hz), thyristors with lower values of t
Such
fast thyristors are made by diffusing into the silicon heavy metals ions
are required.
Q
such as gold or platinum which act as charge combination centers.
Alternatively, fast thyristors may be made by neutron irradiation of the silicon.
Applications
Thyristors are mainly used where high currents and voltages are
involved. They are often used to control alternating currents, where the
change of polarity of the current causes the device to automatically switch
off. This is referred to as Zero Cross operation. It can be said that the device
operates synchronously as, once the device is open, it conducts current in
phase with the voltage applied over its cathode to anode junction, and no
further gate modulation is required to replicate; the device is biased fully on.
Thyristors can be used as the control elements for phase angle triggered
controllers. They are also known as phase fired controllers.
81
G
VS
AK
V
m
v
GK
0
t
v
A
0
v
a.c.
R
v
90
out
o
180
o
Delay
v
out
Vm-V
F
0
(b)(a)
t
t
Fig. 7.4. Load voltage regulated by thyristor phase control:
(a) SCR supplying a load; (b) voltage waveforms
Thyristors can also be found in power supplies for digital circuits, where
they can be used as a sort of ‘circuit breaker’ to prevent a failure in the power
supply from damaging downstream components. The thyristor is used in
conjunction with a Zener diode attached to its gate, and when the output
voltage of the supply rises above the zener voltage, the thyristor conducts,
shorting the power supply output to ground (and in general blowing an
upstream fuse).
The first large scale application of thyristors, with associated triggering
diac, was related to stabilized power supplies within color television
receivers in the early 1970s. The stabilized high voltage d.c. supply for the
receiver was obtained by moving the switching point of the thyristor device
up and down the falling slope of the positive going half of the a.c. supply
input (if the rising slope was used the output voltage would always rise
towards the peak input voltage when the device was triggered and thus defeat
the aim of regulation). The precise switching point was determined by the
load on the output d.c. supply as well as fluctuations on the input a.c. supply.
They proved to be unpopular with the a.c. grid power supplier companies
because the simultaneous switching of many television receivers, all of them
at approximately the same time, introduced asymmetry into the supply
waveform and, as a consequence injected d.c. back into the grid with a
tendency towards saturation of transformer cores and overheating. Thyristors
were largely phased out in this kind of application.
82
Thyristors have been used for decades as lighting dimmers in television,
motion pictures, and theater, where they replaced inferior technologies such
as autotransformers and rheostats. They have also been used in photography
as a critical part of flashes (strobes).
Failure Modes
Thyristors have usual failure modes due to exceeding voltage, current or
power ratings. Moreover, they have their own particular modes of failure,
including:
Turn on di/dt – in which the rate of rise of on-state current after
triggering is higher than the rate that can be supported by the spreading speed
of the active conduction area (SCRs and triacs).
Forced Commutation – in which the transient peak reverse
recovery current causes such a high voltage drop in the sub-cathode region
that it exceeds the reverse breakdown voltage of the gate cathode diode
junction (SCRs only).
Switch on dv/dt – the thyristor can be spuriously fired without trigger
from the gate if the rate of rise of voltage anode to cathode is too great.
Snubber Circuits. Because thyristors can be triggered on by a high rate
of rise of off-state voltage, in many applications this is prevented by
connecting a resistor-capacitor (RC) snubber circuit between the anode and
cathode terminals in order to limit the dv/dt (i. e., rate of change of voltage
versus time).
7.2. Types of Thyristor
An early gas filled tube device called a Thyratron provided the
electronic switching capability, where a small control voltage could switch a
large current. The term ‘thyristor’ is derived from a combination of
‘thyratron’ and ‘transistor’.
Modern thyristors can switch large amounts of power (up to megawatts).
In the realm of very high power applications, they are still the primary
choice. However, in low and medium power (from few tens of watts to few
tens of kilowatts) they have almost been replaced by other devices with
superior switching characteristics like MOSFETs or IGBTs. One major
problem associated with SCRs is that they are not fully controllable switches.
The GTO
(Gate Turn-off Thyristor) and IGCT are two related devices
which solve this problem. In high-frequency applications, thyristors are not
convenient due to large switching times arising from
bipolar conduction.
MOSFETs, on the other hand, have much faster switching capability because
of their
unipolar conduction (only majority carriers carry the current).
83
Comparisons to Other Devices
The functional drawback of a thyristor is that, like a diode, it conducts in
one direction only. A similar self-latching 5-layer device, called a TRIAC,
can work in both directions. This capability also can become a shortfall.
Because the TRIAC can conduct in both directions, reactive loads can cause
it to fail to turn off during the zero-voltage instants of the a.c. power cycle.
Because of this, the use of TRIACs with (for example) heavily-inductive
motor loads usually requires the use of a ‘snubber’ circuit around the TRIAC
to assure that it will turn off with each half-cycle of mains power. Inverse
parallel SCRs can be also used in place of the triac; because each SCR in the
pair has an entire half-cycle of reverse polarity applied to it, the SCRs, unlike
TRIACs, are sure to turn off. However, in this case two separate but
essentially identical gating circuits must be used.
Types of Thyristor
AGT – Anode Gate Thyristor – A thyristor with a gate on the n-type
layer near the anode
ASCR – Asymmetrical SCR
BCT – Bidirectional Control Thyristor – A bidirectional switching
device containing two thyristor structures with separate gate contacts
BOD – Breakover Diode – A gateless thyristor triggered by the
avalanche current
DIAC – Bidirectional trigger device
Dynistor – Unidirectional switching device
Shockley diode – Unidirectional trigger and switching device
SIDAC – Bidirectional switching device
Trisil, SIDACtor – Bidirectional protection devices
GTO – Gate Turn-Off thyristor
DB-GTO – Distributed Buffer Gate Turn-Off thyristor
MA-GTO – Modified Anode Gate Turn-Off thyristor
IGCT – Integrated Gate Commutated Thyristor
LASCR – Light Activated SCR, or LTT – Light triggered thyristor
LASS – Light Activated Semiconducting Switch
MCT – MOSFET Controlled Thyristor – It contains two additional
FET structures for on/off control.
BRT – Base Resistance Controlled Thyristor
RCT – Reverse Conducting Thyristor – It has an integrated reverse
diode, so it is not capable of reverse blocking. These devices are
advantageous where a reverse or freewheel diode must be used.
84
PUT or PUJT – Programmable Unijunction Transistor – A thyristor
with a gate on the n-type layer near to the anode used as a functional
replacement for the unijunction transistor.
SCS – Silicon Controlled Switch or Thyristor Tetrode – A thyristor
with both cathode and anode gates.
SCR – Silicon Controlled Rectifier
SITh – Static Induction Thyristor, or FCTh – Field Controlled
Thyristor – containing a gate structure that can shut down the anode
current flow.
TRIAC – Triode for Alternating Current – A bidirectional switching
device containing two thyristor structures with a common gate contact.
Quadrac – Special type of thyristor which combines a DIAC and a
TRIAC into a single package.
Silicon Carbide Thyristors. In recent years, some manufacturers have
developed thyristors using Silicon carbide (SiC) as the semiconductor
material. They are applied in high temperature environments, being capable
of operating at temperatures up to 350 °C.
Self-Assessment Questions
1. Draw the symbol which represents a SCR. What are the names of
the terminals?
Draw and explain static V-I characteristics of SCR.
2.
What are the conditions for a thyristor to conduct?
3.
Describe the SCR turn off methods.
4.
What are the failure modes of SCRs?
5.
6.
What are the applications of SCR?
How can we use a SCR to control the speed of an electric motor?
7.
What types of thyristors do you know?
8.
85
CHAPTER 8. AMPLIFIERS
8.1. Classification of Amplifiers
The purpose of an amplifier is to increase the power in a signal without
distorting it. This process is called
are used to increase the power in the signal from a microphone so that it can
drive a loudspeaker, or to amplify control signals produced by the pilot of an
aircraft so that they move the flaps on the wings and other control devices.
The extra power must come from a power supply. In the case of an electronic
amplifier, the power supply is usually a d.c. voltage power supply (which
gets its power from the a.c. supply mains) or a battery.
The input signal makes the amplifier control the flow of the current
from this voltage supply to the load. Thus, more power may be delivered to
the load than is taken from the input signal source. In practice, amplification
usually means the increase of the voltage amplitude of the signal at a given
load. The opposite of amplification is called
to a decrease in the signal voltage.
An electronic system, which is designed primarily to give an output
voltage proportional to the input signal voltage, without taking a significant
amount of the signal current, is called a
specified, but its current gain is not, so it may give an increase in the signal
current if the load impedance is low enough.
An example of this is the unity-gain buffer. It is designed so that its
output voltage is almost equal to its input voltage, but the output current may
be much larger than the input current. So the voltage gain can be specified as
nearly unity, but the current gain cannot be specified.
An amplifier designed primarily to give an output current proportional
to the input signal current, without requiring a significant input signal
voltage, is called a
current amplifier.
All these examples involve
In electronics a distinction is made between two types of the component:
those which can only absorb or transfer the signal power, such as resistors
and transformers, which are called passive components, and those, such as
transistors, which can accept power from an extra power source and amplify
the signal power. These are called active components, or
Circuits composed entirely of passive components, are known as passive
networks, and circuits containing active components are known as active
networks. An active network is not necessarily an amplifier. It may not be
intended as an amplifier. The converse, however, is certainly true. An
amplifier must be an active network.
amplification. For example, amplifiers
attenuation and it usually refers
voltage amplifier. Its voltage gain is
power amplification of course.
active devices.
86
8.2. Common-Emitter Amplifier
In this subsection the performance of a common-emitter circuit is
considered.
R
C
R
V
in
C
1
1
i
B
VT
R
2
R
E
C
2
V
R
out
L
V
CC
i
C
i
E
C
E
Fig. 8.1. A common-emitter amplifier
Figure 8.1 shows the usual way of reducing the variation of the
operating point, due to differences in the value of , in a discrete commonemitter amplifier.
Two resistors R
and R2 form a potential divider and set the voltage level of
1
the base terminal (the d.c. operating point also known as quiescent point or Q
point). The emitter current is then determined mainly by the emitter resistor R
.
E
For example, if you want to design a circuit in which the emitter
operating current is 3 mA, you can choose R
to be 330 to produce a
E
voltage drop of 1 V across it. This is much greater than the likely variations
in V
voltage. Since V
at a voltage of about 1.45 V. With the power supply voltage V
if I
, so it fixes the emitter current quite accurately for a given value of base
BE
0.45 V, it follows that the base terminal should be held
BE
= 12 V, and
CC
is small compared with the current flowing in R1 and R2 for all likely
B
values of , this can be achieved with the potential divider by making the
ratio R
and I
C
= 1.45/10.55 (e. g. R1 = 1.2 k and R2 = 10 k). Then currents IE
2/R1
(its d.c. component) do not deviate much from 3 mA, even for extreme
values of 100 or 300.
The inclusion of R
in the circuit (and omitting capacitor CE) reduces the
E
voltage gain considerably. But it also defines the gain quite accurately, which
may be a more useful factor than a high gain in many circumstances.
The emitter capacitor C
in this circuit is called a bypass capacitor. Its
E
purpose is to bypass, or short-circuit, the emitter resistor at signal
frequencies. If its reactance is much lower than R
, RE has little effect. But its
E
87
reactance must be lower than re too, at the lowest signal frequency. When this
R
is done, the emitter terminal is simply connected to the common rail in the
a.c. equivalent circuit.
Capacitors C
attenuating the signal. The selection of the value of C
and C2 provide d.c. blocking and must be prevented from
1
and C2 is based on the
1
requirement that it has negligible effect on the signal current at any frequency
at which the amplifier is operated.
The voltage gain of this circuit can be determined as
VIR RR
where
G
rr r
in b e
1
out L L C L
V
VIRrRR
in in in in C L
is the input resistance of the BJT in the common-
emitter amplifier.
Self-Assessment Questions
1. What is the amplification process?
What types of amplifiers do you know?
2.
What does the term ‘current gain’ mean?
3.
Draw the amplitude curve for an amplifier. How can we calculate
4.
the voltage gain?
An amplifier has an input signal voltage of 0.054 mV. The output
5.
voltage is 12.5 V. Find the voltage gain in dB.
You measure an output voltage at the lower cut-off frequency of
6.
3.25 V
. What will be the output voltage at the upper cut-off frequency?
p-p
Negative feedback (NFB) and positive feedback (PFB). What is the
7.
difference?
,
11
r
in
R
CL
88
CHAPTER 9. ANALOGUE INTEGRATED CIRCUITS
In 1963, Fairchild Semiconductor introduced the 702, which was the
first commercially available I operational amplifier, or op-amp. Realizing
the op-amp’s ability to adapt to a wide variety of applications, National
Semiconductor introduced the LM101 op-amp in 1965. In 1967, Fairchild
once again made history by presenting the very popular uA741 op-amp I,
which is still widely used today. From that point on, improvements in
technology have refined the op-amp and led to a variety of other spin-off ICs
that internally include several op-amps along with their other circuitry. These
ICs include voltage regulators, timers, function generators, phase-locked
loops, analogue-to-digital (A/D) converters, digital-to-analogue (D/A)
converters, sample-and-hold amplifiers, and a variety of other specialized
ICs. These ICs, along with the op-amp, are called ‘linear integrated circuits
(linear ICs)’ because they provide linear signal amplification, in contrast to
‘digital integrated circuits (digital ICs)’ that are primarily used for pulse
signal processing.
As you continue your course in electronics, you will see how these ICs
can be used in circuits. We will examine the most popular of the linear ICs,
the op-amp, and see how it can be used as a comparator, an inverting or
noninverting amplifier, a signal processor, a signal generator, and an active
filter. We will continue to discuss linear circuits by describing the operation
and application of the frequently used linear IC type: timers.
9.1. Operational Amplifiers
The operational amplifier was initially a vacuum tube circuit used in the
early 1940s in analogue computers. The name
‘operational amplifier’ or
‘op-amp’ was chosen because the circuit was used as a high-gain d.c.
‘amplifier’ performing mathematical ‘operations’. These early circuits were
expensive and bulky, and they found very little application until the
semiconductor integrated circuit was developed in 1958 by Jack Kilby at
Texas Instruments. Circuits that once needed hundreds of discrete or
individual components can now be integrated into a single IC, making
equipment smaller, more energy efficient, cheaper, and easier to design.
Today’s IC op-amp is a very high-gain d.c. amplifier that can have its
operating characteristics changed by connecting different external
components. This makes the op-amp very versatile, and it is this versatility
that has made the op-amp the most widely used linear IC.
89
Operational Amplifier Basics
To begin with, fig. 9.1 introduces the operational amplifier, or op-amp,
by showing its schematic symbol in fig. 9.1 and internal circuit in fig. 9.5.
The op-amp’s internal circuit is a combination of three amplifier circuits.
These three circuits are all interconnected and contained within a single IC,
and together they function as a high-gain, high input impedance, low output
impedance amplifier.
Op-Amp Symbol
Referring again to fig. 9.l(a), you can see that the triangle-shaped
amplifier symbol is used to represent the op-amp in an electronic schematic
diagram. Comparing the two symbols, you may have noticed that in some
cases the two power supply connections are not shown, even though power is
obviously applied.
Let us now examine the op-amp’s input and output terminals shown in
fig. 9.1. The two op-amp inputs are labeled ‘–’ and ‘+’. The ‘–’ or negative
input is called the inverting input because any signal applied to this input
will be amplified and inverted between input and output (output is 180° out
of phase with input). On the other hand, the ‘+’ or positive input is called the
noninverting input because any signal applied to this input will be amplified
but not inverted between input and output (output is in phase with input). An
input signal will normally be applied to only one of these inputs, while the
other input is used to control the op-amp’s operating characteristics.
+V
cc
V
V
1
2
–
+
-V
cc
(a) (b)
Fig. 9.1. Operational amplifier symbols
V
out
inputs
output
The two power supply connections to the op-amp are labeled ‘+V’ and
‘–V’. Figure 9.2(a) shows how power to the op-amp can be supplied by dual
supply voltages or by a single supply voltage. When two supply voltages are
used (dual supply voltages), the voltage values are of the same value but of
opposite polarity (for example, +12 V and –12 V). On the other hand, when
only one supply voltage is used (single supply voltage), a positive or negative
90
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