Slide 1

Chapter 7

Power Amplifiers

Spring 20144th Semester MechatronicsSZABIST, KarachiCH 7Power Amp17 141Course Support

Nasreen.bano@szabist.edu.pkOffice: 100 Campus (304)

Official: ZABdesk217 14CH 7Power Amp2Chapter ContentsIntroductionClass A amplifiersClass AB AmplifiersClass B AmplifiersClass C and D Amplifiers317 14CH 7Power Amp34Introduction 17 14CH 7Power Amp4Amplifiers:In small-signal amplifiers the main factors are:Amplification Linearity Gain

Since large-signal, or power, amplifiers handle relatively large voltage signals and current levels, the main factors are:EfficiencyMaximum power handling capabilityImpedance matching to the output deviceIntroduction517 14CH 7Power Amp5Amplifiers Performance Factors:Amplifier Efficiency:The ratio of output to input power

Measure of effectiveness of an amplifier to convert dc power into ac

Gets higher from A to D

Introduction617 14CH 7Power Amp% efficiency of the amplifierthe ac load powerthe dc input power6Amplifiers Performance Factors:Amplifier Distortion:Any undesired change in the shape of a signal

Introduction717 14CH 7Power Amp

Appears as flat lines b/w the output signal alterationsCaused by both transistors being in cutoff for a short duration of time b/w +ve and ve alteration of the input signal

7

Amplifiers Performance Factors:Harmonic Distortion:

Introduction817 14CH 7Power AmpIf the output of an amplifier is not a complete AC sine wave, then it is distorting the output. The amplifier is non-linear.

This distortion can be analyzed using Fourier analysis. In Fourier analysis, any distorted periodic waveform can be broken down into frequency components. These components are harmonics of the fundamental frequency.

According to Fourier analysis, if a signal is not purely sinusoidal, then it contains harmonics. 8Introduction917 14CH 7Power AmpHarmonic Distortion:

Total Harmonic Distortion:9Introduction1017 14CH 7Power AmpExample 12.13:Example 12.14:Calculate the harmonic distortion components for an output signal having fundamental amplitude of 2.5 V, second harmonic amplitude of 0.25 V, third harmonic amplitude of 0.1 V, and fourth harmonic amplitude of 0.05 V.Calculate the total harmonic distortion for the amplitude components given in Example 12.13.10Introduction1117 14CH 7Power AmpSingle transistor amplifier Conducts the entire input cyclelow distortion, high loss = 25 %Small signal amplifiers (drive high power stages)Class ATwo transistor amplifierEach conducts for of the input cyclesome distortion, lower loss = 78.5 %High power amplifiersClass BSingle transistor amplifier Conducts for less than 180o of the input cyclehigh distortion, lowest loss = 99%Nearly ideal power amplifierClass C11Amplifier Types:Class AThe amplifier conducts through the full 360 of the inputThe Q-point is set near the middle of the load line

Class BThe amplifier conducts through 180 of the inputThe Q-point is set at the cutoff point

Class ABA compromise between the class A and B amplifiersThe amplifier conducts somewhere between 180 and 360 The Q-point is located between the mid-point and cutoff.

Class CThe amplifier conducts less than 180 of the inputThe Q-point is located below the cutoff level Class DAn amplifier biased especially for digital signals

Introduction17 14CH 7Power Amp1212Introduction17 14CH 7Power AmpPower Amplifiers13

13Introduction17 14CH 7Power AmpClass A AmplifierThe output of a class A amplifier conducts for the full 360 of the cycle.

The Q-point is set at the middle of the load line so that the AC signal can swing a full cycle. Remember that the DC load line indicates the maximum and minimum limits set by the DC power supply.

1414Introduction17 14CH 7Power AmpClass B AmplifierA class B amplifier output only conducts for 180 or one-half of the AC input signal.The Q-point is at 0V on the load line, so that the AC signal can only swing for one-half cycle.

1515Introduction1617 14CH 7Power AmpClass AB AmplifierThis amplifier is a compromise between the class A and class B amplifierthe Q-point is above that of the Class B but below the class A.

The output conducts between 180 and 360 of the AC input signal.

16Introduction1717 14CH 7Power AmpThe output of the class C conducts for less than 180 of the AC cycle. The Q-point is below cutoff.Class C

17Introduction1817 14CH 7Power AmpEfficiency Comparison

Efficiency refers to the ratio of output to input powerThe lower the amount of conduction of the amplifier the higher the efficiency1819Class A Power Amplifiers17 14CH 7Power Amp19Class A2017 14CH 7Power AmpSeries-Fed Class A AmplifierThis is similar to the small-signal amplifier except that it will handle higher voltagesThe transistor used is a high-power transistor

A small input signal causes the output voltage to swing to a maximum of Vcc and a minimum of 0VThe current can also swing from 0mA to ICSAT (VCC/RC)20Class A2117 14CH 7Power AmpSeries-Fed Class A AmplifierThe power into the amplifier is from the DC supply. With no input signal, the DC current drawn is the collector bias current, ICQ.

Output Power

Input Poweror

Efficiency21Class A2217 14CH 7Power AmpExample 12.1:

Calculate the efficiency:

2223Class B Power Amplifiers17 14CH 7Power Amp23Class B2417 14CH 7Power AmpClass B AmplifierIn class B, the transistor is biased just off. The AC signal turns the transistor onThe transistor only conducts when it is turned on by one-half of the AC cycleIn order to get a full AC cycle out of a class B amplifier, you need two transistors:

An npn transistor that provides the negative half of the AC cycle A pnp transistor that provides the positive half.24Class B2517 14CH 7Power AmpClass B Amplifier

25Class B2617 14CH 7Power AmpClass B Amplifier-EfficiencyFor maximum power, VL=VCC

The maximum efficiency of a class B is 78.5%..

26Class B2717 14CH 7Power AmpExample 12.7:For a class B amplifier providing a 20-V peak signal to a 16- load (speaker) and a power supply of VCC = 30 V, determine the input power, output power, and circuit efficiency2728Class C & D Power Amplifiers17 14CH 7Power Amp28Class C and D2917 14CH 7Power AmpClass C Amplifier

A class C amplifier conducts for less than 180.

In order to produce a full sine wave output, the class C uses a tuned circuit (LC tank) to provide the full AC sine wave.

Class C amplifiers are used extensively in radio communications circuits.29Class C and D3017 14CH 7Power AmpClass D AmplifierA class D amplifier amplifies pulses, and requires a pulsed input.

This circuit has applications in digital circuitry.

30Reading:

Summary Equations

Problems:

Relevant

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