Power Amplifiers Chap22

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Power Electronics

Introduction

Bipolar Transistor Power Amplifiers

Classes of Amplifier

Four-layer Devices

Power Supplies and Voltage Regulators

Chapter 22




Introduction

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

22.1




Bipolar Transistor Power Amplifiers

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 isnot simply resistive but also has an inductive or

capacitive element

22.2




Current sources and loads

– when driving a reactive load we need to supply currentat some times (the output acts as a current source)

– at other times we need to absorb current (the outputacts as a current sink)




– the circuit above is a good current source but a poor

current sink (stored charge must be removed by R E )

– an alternative circuit using pnp transistors (below) is a

good current sink but a poor current source




Push-pull amplifiers

– combining these

circuits can produce

an arrangement thatis both a good current

source and a good

current sink

– this is termed apush-pull amplifier




Driving a push-pull stage




Distortion in push-pull amplifiers




Improved push-pull output stage arrangements




Amplifier efficiency

– an important consideration in the design of power

amplifiers is efficiency

– 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.

supplythefromabsorbedpower

loadtheindissipatedpower Efficiency




Classes of Amplifier

Class A

– active device conducts for complete cycle of input signal

– example shown here

– poor efficiency

(normally less

than 25%)

– low distortion

22.3




Class B

– active devices conducts

for half of the complete

cycle of input signal


– good efficiency

(up to 78%) – considerable distortion




Class AB


for more than half but

less than the complete

cycle of input signal


(with appropriate R bias)

– efficiency depends on bias

– distortion depends on bias




Class C


for less than half the

complete cycle of

input signal


–high efficiency(approaching 100%)

– gross distortion




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




Four-layer Devices

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 incommon with bipolar transistors

22.4




The thyristor

– a four-layer

device with a

pnpn structure – three terminals:

anode, cathode

and gate

– gate is thecontrol input




Thyristor operation

– construction

resembles two

interconnectedbipolar transistors

– turning on T2

holds on T1

– device thenconducts until

the current goes

to zero




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




Full-wave power control using thyristors

– full-wave control

required two devices – allows control from

0-100% of full power

– requires two gatedrive circuits

– opto-isolation oftenused to insulatecircuits from AC supply




The triac

– resembles a bidirectionalthyristor

–allows full-wave controlusing a single device

– often used with abidirectional trigger diode (a diac) to produce

the necessary drive pulses – this breaks down at a

particular voltage and fires the triac




A simple lamp-dimmer using a triac




Power Supplies and Voltage Regulators

Unregulated DC power supplies

22.5




Regulated DC power supplies




Voltage regulators




Switch-modepower supplies

– uses a switching

regulator – output voltage is

controlled by theduty-cycle of theswitch

– uses an averagingcircuit to ‘smooth’ output




An LC averaging circuit




Using feedback in a switching regulator




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 asthyristors or triacs

A transformer, a rectifier and a capacitor can be used toform a simple unregulated supply

A more constant output voltage can be produced by addinga regulator. This can use linear or switching techniques

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Power Amplifiers Chap22