Download Chapter 22

Power Electronics
Chapter 22
 Introduction
 Bipolar Transistor Power Amplifiers
 Classes of Amplifier
 Four-layer Devices
 Power Supplies and Voltage Regulators
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Introduction
22.1
 Amplifiers that produce voltage amplification or
current amplification also produce power
amplification
 However, the term power amplifier is normally
reserved for circuits whose main function is to deliver
large amounts of power
 These can be produced using FETs or bipolar
transistors, or using special purpose devices such as
thyristors and triacs
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Bipolar Transistor Power Amplifiers
22.2
 When designing a power amplifier we normally
require a low output resistance so that the circuit can
deliver a high output current
– we often use an emitter-follower
– this does not produce voltage gain but has a low
output resistance
– in many cases the load applied to a power amplifier is
not simply resistive but also has an inductive or
capacitive element
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 Current sources and loads
– when driving a reactive load we need to supply current
at some times (the output acts as a current source)
– at other times we need to absorb current (the output
acts as a current sink)
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– the circuit above is a good current source but a poor
current sink (stored charge must be removed by RE)
– an alternative circuit using pnp transistors (below) is a
good current sink but a poor current source
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 Push-pull amplifiers
– combining these
circuits can produce
an arrangement that
is both a good current
source and a good
current sink
– this is termed a
push-pull amplifier
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 Driving a push-pull stage
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 Distortion in push-pull amplifiers
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 Improved push-pull output stage arrangements
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 Amplifier efficiency
– an important consideration in the design of power
amplifiers is efficiency
Efficiency 
power dissipated in the load
power absorbed from the supply
– efficiency determines the power dissipated in the
amplifier itself
– power dissipation is important because it determines
the amount of waste heat produced
 excess heat may require heat sinks, cooling fans, etc.
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Classes of Amplifier
22.3
 Class A
– active device conducts for complete cycle of input signal
– example shown here
– poor efficiency
(normally less
than 25%)
– low distortion
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 Class B
– active devices conducts
for half of the complete
cycle of input signal
– example shown here
– good efficiency
(up to 78%)
– considerable distortion
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 Class AB
– active devices conducts
for more than half but
less than the complete
cycle of input signal
– example shown here
(with appropriate Rbias)
– efficiency depends on bias
– distortion depends on bias
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 Class C
– active devices conducts
for less than half the
complete cycle of
input signal
– example shown here
– high efficiency
(approaching 100%)
– gross distortion
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 Class D
– in class D amplifiers the active devices are switches
and are either ON or OFF
– an ideal switch would dissipate no power
 since either the current or the voltage is zero
– even real devices make good switches
– amplifiers of this type are called switching amplifiers
or switch-mode amplifiers
– efficiency is very high
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Four-layer Devices
22.4
 Although transistors make excellent switches, they
have limitations when it comes to switching high
currents at high voltages
 In such situations we often use devices that are
specifically designed for such applications
 These are four-layer devices
– these are not transistors, but have a great deal in
common with bipolar transistors
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 The thyristor
– a four-layer
device with a
pnpn structure
– three terminals:
anode, cathode
and gate
– gate is the
control input
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 Thyristor operation
– construction
resembles two
interconnected
bipolar transistors
– turning on T2
holds on T1
– device then
conducts until
the current goes
to zero
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 Use of a thyristor in
AC power control
– once triggered the device
conducts for the remainder
of the half cycle
– varying firing time
determines output power
– allows control from 0-50%
of full power
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 Full-wave power
control using thyristors
– full-wave control
required two devices
– allows control from
0-100% of full power
– requires two gate
drive circuits
– opto-isolation often
used to insulate
circuits from AC supply
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 The triac
– resembles a bidirectional
thyristor
– allows full-wave control
using a single device
– often used with a
bidirectional trigger
diode (a diac) to produce
the necessary drive pulses
– this breaks down at a
particular voltage and fires the triac
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 A simple lamp-dimmer using a triac
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Power Supplies and Voltage Regulators
22.5
 Unregulated DC power supplies
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 Regulated DC power supplies
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 Voltage regulators
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 Switch-mode
power supplies
– uses a switching
regulator
– output voltage is
controlled by the
duty-cycle of the
switch
– uses an averaging
circuit to ‘smooth’
output
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 An LC averaging circuit
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 Using feedback in a switching regulator
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Key Points
 Power amplifiers are designed to deliver large amounts of
power to their load
 Bipolar circuits often use an emitter follower circuit
 Many power amplifiers use a push-pull arrangement
 The efficiency of an amplifier is greatly affected by its class
 While transistors make excellent switches, in high power
applications we often use special-purpose devices such as
thyristors or triacs
 A transformer, a rectifier and a capacitor can be used to
form a simple unregulated supply
 A more constant output voltage can be produced by adding
a regulator. This can use linear or switching techniques
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