Power Electronics Introduction Bipolar Transistor Power Amplifiers 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 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

Bipolar Transistor Power Amplifiers 22.2 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 is not simply resistive but also has an inductive or capacitive element

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)

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

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

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.

Classes of Amplifier Class A 22.3 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

Class B active devices conducts for half of the complete cycle of input signal example shown here good efficiency (up to 78%) considerable distortion

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

Class C active devices conducts for less than half the complete cycle of input signal example shown here 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

22.4 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

The thyristor a four-layer device with a pnpn structure three terminals: anode, cathode and gate gate is the control input

Thyristor operation construction resembles two interconnected bipolar transistors turning on T2 holds on T1 device then conducts 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 gate drive circuits opto-isolation often used to insulate circuits from AC supply

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

A simple lamp-dimmer using a triac

Power Supplies and Voltage Regulators 22.5 Power Supplies and Voltage Regulators Unregulated DC power supplies

Regulated DC power supplies

Voltage regulators

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

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 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