1. 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

2. 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

3. 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

4. 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)

5. 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

6. 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

7. Driving a push-pull stage

8. Distortion in push-pull amplifiers

9. Improved push-pull output stage arrangements

10. 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.

11. 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

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

13. 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

14. Class C
active devices conducts for less than half the complete cycle of input signal
example shown here
high efficiency (approaching 100%)
gross distortion

15. 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

16. 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

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

18. Thyristor operation
construction resembles two interconnected bipolar transistors
turning on T2 holds on T1
device then conducts until the current goes to zero

19. 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

20. 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

21. 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

22. A simple lamp-dimmer using a triac

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

24. Regulated DC power supplies

25. Voltage regulators

26. 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

27. An LC averaging circuit

28. Using feedback in a switching regulator

29. 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