Lecture 28- Power Amplifiers II - Electronics- II (EEE 232)electronics-2.weebly.com/.../13056901/lecture_28-_power_amplifiers_ii.pdf · Lecture 28 Power Amplifiers II Class A, Class

1/13/2015

1

Electronics IILecture 28

Power Amplifiers II

Class A, Class B & Class AB Amplifiers

Muhammad TilalDepartment of Electrical Engineering

CIIT Attock Campus

COMSATS Fall 2014(Rev. 3.0)

The theme of this presentation is an inspiration from the one used in S2 Department of Chalmers University of Technology, Gothenburg, Sweden.

The COMSATS logo and “COMSATS” is the property of CIIT, Pakistan and subject to the copyrights and ownership of COMSATS.

Duplication & distribution of this work for Non Academic or Commercial use without prior permission is prohibited.

Previous Lecture

• Power Amplifiers

– Basics of Power Amplifiers.

– Classes of Power Amplifiers.

– Class A Power Amplifier.

1/13/2015 © Muhammad Tilal 2

COMSATS Electronics II

1/13/2015

2

Session Overview



Topic Power Amplifiers

ConceptsClass A Power Amplifier, Class B Amplifier.

Push Pull Circuits, Class AB Amplifiers.

Recommended

ReadingSections 15.2, 15.4 and 15.5(Partial), 15.6 of [1].

KeywordsPower Amplifier, Class A Class B, Class AB, Push-Pull, Push

Pull.

• Low efficiency but better signalreproduction and linearity.

• In class A amplifier operation, thecomplete input waveform isreproduced at the output.

• This exact amplified reproductionat the output is possible becausethe Class A amplifier is perfectlybiased within its active region.

• Due the this biasing it never entersthe cut-off or saturation region.

Class A Power Amplifier



• Class A amplifier uses a singletransistor for both input cycles.Because of its biasingarrangements, this amplifier alwayshas the current flowing at theoutput.

• This causes the poor efficiency asthe actual power delivered to theload is quite less than the actualpower converted.

• Most the power converted iswasted as heat thereby risking thedevice burn out if proper sinking isnot provided. Additionally sinksalso increase the cost.

1/13/2015

3

Class A Power Amplifier



http://www.electronics-tutorials.ws/amplifier/amp_1.html

Class A Power Amplifier- AC Operation



Robert L. Boylestad, Electronic Devices and Circuit Theory, 8th Edition, Pearson Education Inc, ISBN: 81-7808-590-9.

1/13/2015

4

• Input power is given as

– Pi(dc)= VCC*IcQ

• Output power

– RMS: Po(ac)= Vc2(rms)/ Rc

– Peak: Po(ac)= VCE2(p)/ 2Rc

– Peak-Peak: Po(ac)= VCE2(p-p)/ 8Rc

• Efficiency

– %� = [ Po(ac)/Pi(dc) ] x 100%

Power Considerations- Class A power

Amplifier



• Maximum Power Efficiency

• Example 15.1 (Boylestad): Calculate input power, output power

and power efficiency of the given power amplifier for input voltage

that causes the base current of 10mA peak.

Class A Power Amplifier- Example




1/13/2015

5

• Class B amplifier works only for1800 of output cycle. The DC biasingleaves the transistor turned offwhen there is not input ac signal.

• To obtain the output for full 3600

cycle of operation, two transistorwith output at the opposite cyclesare coupled together.

• This is termed as the push-pullconfiguration. During one halfcycle, one transistor causes theoutput to be high and during otherhalf cycle, the second transistorcauses the output to go low.

Class B Power Amplifier



Robert L. Boylestad, Electronic Devices and Circuit Theory,

8th Edition, Pearson Education Inc, ISBN: 81-7808-590-9.

• The efficiency of class B amplifier is

better than the class A amplifier.

Why ?




Better Efficiency, Poor signal reproduction at the output, Distorted output


1/13/2015

6

• Input DC power– Pi(dc)= Vcc*Idc.

– Idc= average or DC current.

– Idc= (2/π)*Ip.

– Pi(dc)= Vcc* (2/π)*Ip.

• Output AC Power– Po(ac)=VL2(rms)/RL.

– Po(ac)=VL2(p-p)/8RL.

– Po(ac)=VL2(p)/2RL.

• Efficiency

.

Power Considerations- Class B Power

Amplifier





Power Considerations- Class B Power

Amplifier





• Example 15.7(Boylestad):

1/13/2015

7

• Maximum output power isdelivered when VL(p)=Vcc.– Maximum Po(ac)=Vcc

2/2RL

• Corresponding maximum Peak accurrent– I(p)= Vcc/RL

• Maximum average current frompower supply

– Max Idc= (2/π) I(p)=2Vcc2/πRL

• Maximum input power– Max Pi(dc)= Vcc*(max Idc)

– Max Pi(dc)= 2Vcc2/πRL

Maximum Power Considerations




• Maximum % Efficiency, �– �% = [ Po(ac)/ Pi(dc) ]* 100

– �% =

– �% =

• Check the formulae for Maximumpower dissipated.

• Example 15.8 (Boylestad):







1/13/2015

8

• Example 15.9 (Boylestad):







• There are multiple circuits toobtain a class B power amplifieroperation.

• For a push-pull configuration, onepossible method is to split asingle input signal into twoopposite polarity signals.

• These opposite polarity signalsare fed into two similar inputcircuits (with each operating foralternate cycle) to get a full 360degrees of operation.

Class B Power Amplifier Circuit



• There are many arrangements tosplit a single input into twoopposite polarity signals.

• These arrangements include

– Transformer Coupled Method.

– BJT Method.

– Op-amp Method.

• All these arrangements areknown as phase splitter circuits.

1/13/2015

9

Phase Splitter Circuit for Class B PA





• A transformer splits the input signals into opposite polarity signals

which are connected to the input of two transistors.

Transformer Coupled Push Pull Circuits




Output transformer

drives the load in push

pull fashion.

Q1 operates during 1st

half cycle and Q2 in 2nd

half cycle.

1/13/2015

10

Complementary Symmetry Circuit





• Class AB operation is a mode ofoperation in between Class A andClass B.

• Class AB operation twocomplimentary transistors biasedwith very small voltage at thebase of the transistors.

• This biasing configuration keepthe amplifier close to its cut offstate.

• An input AC signal will cause thetransistor to operate in its activeregion.

Class AB Power Amplifiers



• In the absence of the input signal,a very small amount of collectorcurrent flows thereby preventingany crossover distortion.

• This arrangement causes theefficiency of Class AB better thanClass A and removes the crossover distortion present in Class Bamplifiers.

1/13/2015

11

Class AB Power Amplifiers




• Power Amplifiers

– Amplifier Distortion.

– Class C Amplifier.

– Class D Amplifier.

Next Lecture



1/13/2015

12

References

[1] Robert L. Boylestad, Electronic Devices and Circuit Theory, 8th Edition,

Pearson Education Inc, ISBN: 81-7808-590-9.

[2] Theodore F. Bogart, Jeffery S. Beasley, Guilermo Rico, Electronics Devices

and Circuits, 6th Edition, Pearson Education Inc, ISBN: 978-81-775-8887-3



Documents

Lecture 28- Power Amplifiers II - Electronics- II (EEE 232)electronics-2.weebly.com/.../13056901/lecture_28-_power_amplifiers_ii.pdf · Lecture 28 Power Amplifiers II Class A, Class