PowerPoint PresentationSlide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Slide 29Slide 30Slide 31Slide 32Slide 33Slide 34Slide 35Slide 36Slide 37Chapter 9 Output Stages And Power AmplifiersLow Output Resistance – no loss of gainSmall-Signal Not applicableTotal-Harmonic Distortion (fraction of %)EfficiencyTemperature RequirementsCollector current waveforms for transistors operating in (a) class A, (b) class B, (c) class AB, and (d) class C amplifier stages.An emitter follower (Q1) biased with a constant current I supplied by transistor Q2.Class ATransfer CharacteristicsTransfer characteristic of the emitter follower. This linear characteristic is obtained by neglecting the change in vBE1 with iL. The maximum positive output is determined by the saturation of Q1. In the negative direction, the limit of the linear region is determined either by Q1 turning off or by Q2 saturating, depending on the values of I and RL.Crossover distortion can be eliminated by biasing the transistors at a small, non-zero current.A bias Voltage VBB is applied between Qn and Qp.For vi = 0, vo = 0, and a voltage VBB/2 appears across the base-emitter junctionof each transistor.iNiPIQISeVBB2 VTVBB is selected to result the required quiscent current IQvoviVBB2 vBENiNiPiLvBENvEBP VBBVTlniNIS VTlniPIS 2 VT lniQISiN2IQ2iN2iLiN IQ2 0Class ATransfer CharacteristicsClass ATransfer CharacteristicsFrom figure 9.3 we can see thatvomaxVCCVCE1satIn the negative direction, the limite of the linear region is determined either by Q1 turning offvOminI RLor by Q2 saturatingvOminVCC VCE2satDepending on the values of I and RL. The absolutely lowest output voltage is that given by the previous equation and is achieved provided that the bias current I is greater than the magnitude of the corresponding load currentIVCC VCE2satRLClass ATransfer CharacteristicsExercises D9.1 and D9.2Class ASignal Waveforms0 5 10101vo t( )t0 5 10012vcE1 t( )t0 5 10012ic1 t( )t0 5 1000.51pD1 t( )tClass APower DissipationP VCCILargest Power Dissipation When vo = 0Q1 must be able to withsatnd a continuous dissipation of VCC*IThe power dissipation of Q1 depends on the value of RL.If RL is infinite, iC1 = I and the dissipation in Q1 depends on vo.Maximum power dissipation will occur when vo = -VCC since vCE1 will be 2VCC.pD1 = 2VCC*I. This condition would not normally persist for a prolonged interval, sothe design need not be that conservative. The average pD1 = VCC*IWhen RL is zero a positive voltage would result in a theoretically infinite current (large practical value) would flow through Q1. Short-circuit protection is necessary.Class APower Conversion Efficiencyload_power PL supply_power PS Voaverage voltagePL12Vo2RLPS2 VCC I14Vo2I RL VCC14VoI RLVoVCCVoVCC VoI RLmaximum efficiency is obtained when VoVCCI RLClass AExercise 9.4Vopeak 8 I 100 103 RL100 VCC10PLVopeak22100 PL0.32Pplus VCCI Pplus 1Pminus VCCI Pminus 1 PSPplus PminusPLPS  0.16Biasing the Class B Output•No DC current is used to bias this configuration.•Activated when the input voltage is greater than the Vbe for the transistors.•npn Transistor operates when positive, pnp when negative.•At a zero input voltage, we get no output voltage.CLASS AMany class A amplifiers use the same transistor(s) for both halves of the audio waveform. In this configuration, the output transistor(s) always has current flowing through it, even if it has no audio signal (the output transistors never 'turn off'). The current flowing through it is D.C. A pure class 'A' amplifier is very inefficient and generally runs very hot even when there is no audio output. The current flowing through the output transistor(s) (with no audio signal) may be as much as the current which will be driven through the speaker load at FULL audio output power. Many people believe class 'A' amps to sound better than other configurations (and this may have been true at some point in time) but a well designed amplifier won't have any 'sound' and even the most critical 'ear' would be hard-pressed to tell one design from another. NOTE: Some class A amplifiers use complimentary (separate transistors for positive and negative halves of the waveform) transistors for their output stage. Class APower Conversion EfficiencyClass B output stage. Class BCircuit OperationCLASS 'B' A class 'B' amplifier uses complimentary transistors for each half of the waveform. A true class 'B' amplifier is NOT generally used for audio. In a class 'B' amplifier, there is a small part of the waveform which will be distorted. You should remember that it takes approximately .6 volts (measured from base to emitter) to get a bipolar transistor to start conducting. In a pure class 'B' amplifier, the output transistors are not "biased" to an 'on' state of operation. This means that the the part of the waveform which falls within this .6 volt window will not be reproduced accurately. The output transistors for each half of the waveform (positive and negative) will each have a .6 volt area in which they will not be conducting. The distorted part of the waveform is called 'crossover' or 'notch' distortion. Remember that distortion is any unwanted variation in a signal (compared to the original signal). The diagram below shows what crossover distortion looks like.Transfer characteristic for the class B output stage in Fig. 9.5.Class BCircuit OperationOperationWhen the input voltage rises to be large enough to overcome the Vbe, it will begin to cause an output voltage to appear. This occurs because Qn begins to act like an emitter follower and Qp shuts off. The input will be followed on the emitter until the transistor reaches saturation. The maximum input voltage is equal to the following:vimaxVCCVCENsatThe same thing will begin to happen if the input voltage is negative by more than the Veb of the transistor. This causes the Qp to act like an emitter follower and Qn turns off. This will continue to behave this way until saturation occurs at a minimum input voltage of:viminVcc