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Chapter 22. Power Electronics. Introduction Bipolar Transistor Power Amplifiers Classes of Amplifier Four-layer Devices Power Supplies and Voltage Regulators. 22.1. Introduction. Amplifiers that produce voltage amplification or current amplification also produce power amplification

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Power electronics l.jpg

Chapter 22

Power Electronics

  • Introduction

  • Bipolar Transistor Power Amplifiers

  • Classes of Amplifier

  • Four-layer Devices

  • Power Supplies and Voltage Regulators

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  • 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

  • When designing a power amplifier we normallyrequire 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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  • 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.

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Classes of Amplifier

  • Class A

    • active device conducts for complete cycle of input signal

    • example shown here

    • poor efficiency(normally lessthan 25%)

    • low distortion

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  • Class B

    • active devices conductsfor half of the completecycle of input signal

    • example shown here

    • good efficiency(up to 78%)

    • considerable distortion

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  • Class AB

    • active devices conductsfor more than half butless than the completecycle 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 conductsfor less than half thecomplete cycle ofinput 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

  • 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-layerdevice with apnpn structure

    • three terminals:anode, cathodeand gate

    • gate is thecontrol input

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  • Thyristor operation

    • constructionresembles twointerconnectedbipolar transistors

    • turning on T2 holds on T1

    • device thenconducts untilthe current goesto zero

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  • Use of a thyristor inAC power control

    • once triggered the deviceconducts for the remainderof the half cycle

    • varying firing timedetermines output power

    • allows control from 0-50%of full power

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  • Full-wave powercontrol using thyristors

    • full-wave controlrequired two devices

    • allows control from0-100% of full power

    • requires two gatedrive circuits

    • opto-isolation oftenused to insulatecircuits from AC supply

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  • The triac

    • resembles a bidirectionalthyristor

    • allows full-wave controlusing a single device

    • often used with abidirectional triggerdiode (a diac) to producethe necessary drive pulses

    • this breaks down at aparticular voltage and fires the triac

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Power Supplies and Voltage Regulators

  • Unregulated DC power supplies

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  • Switch-modepower supplies

    • uses a switchingregulator

    • output voltage iscontrolled by theduty-cycle of theswitch

    • uses an averagingcircuit to ‘smooth’output

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  • An LC averaging circuit

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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