Presentation on theme: "Class A Output Stage - Recap"— Presentation transcript:
1 Class A Output Stage - Recap Class A output stage is a simple linear current amplifier.It is also very inefficient, typical maximum efficiency between 10 and 20 %.Only suitable for low power applications.High power requires much better efficiency.Why is class A so inefficient ?Single transistor can only conduct in one direction.D.C. bias current is needed to cope with negative going signals.75 % (or more) of the supplied power is dissipated by d.c.Solution : eliminate the bias current.
2 Class B Output Stage Q1 and Q2 form two unbiased emitter followers Q1 conducts only when the input is positiveQ2 conducts only when the input is negativeConduction angle is, therefore, 180°When the input is zero, neither conductsi.e. the quiescent power dissipation is zero(We are temporarily ignoring the need to FWDBias the BE junctions for conduction to occur, which causes “crossover distortion” in the output.)
3 Class B Current Waveforms IouttimeIC1Ideal waveforms shown.-ignoring“crossover distortion”IC2timetime
4 Class B Efficiency IC1 Average power drawn from the positive supply: ^Vo/RLp2pPhase, q = wtA sin(q)Waveform of + supplycurrent has avg. valuegiven by peak/piBy symmetry, power drawn from +ve and –ve supplies will be the same.Total suply power, therefore:
5 The average load power will be EfficiencyMax efficiency occurs for Vo = VCCand equals pi/4 = 0.75 =75%^(In actual practice max. value is limited toVcc – VCE sat VCC)Max. avg.load power is found by subst. Vo = VCCinto equation for PL aboveand equals (1/2 ) (VCC)2/RL
6 Power DissipationTo select appropriate output transistors, the maximum power dissipation must be calculated.Just need to find the maximum value of PD to select transistors/heatsinksFor Class B quiescent power dissipation = 0(it was max. under quiescent conditions for Class A)When an input signal is applied the avg. powerDissipated in the Class B stage isSubst. For Ps and PL from eqns. on previous page(eqn 9.19)To find maximum differentiateSubst this value in eqn. (9.19) yieldsThus(From symmetry half the total PD is dissipated in eachtransistor.)
7 We can find the efficiency at the point of max power dissipation by subst.into the eqn. for Class B efficiency to get h = 50%Plotting eqn. (9.19), which gives avg. powerdissipated vs. output signal amplitude, shows thatpower dissipation decreases after it it reachesa maximum while operating at a higher signalamplitude. However, at higher signal amplitudesthere is greater nonlinear distortion as a result ofapproaching saturation in the transistors.
8 Þ Example It is required to design a class B output stage to deliver an average power of 20W to an 8 ohm load.The power supply voltage VCC is to be 5 voltsgreater than the peak output voltage.Determine:the supply voltage required.the peak current drawn from each supply,the total supply power,the power conversion efficiency,the maximum power that each transistor can dissipatesafely.Solution:ÞThus=
9 Efficiency / Power Dissipation Example (Cont’d)Efficiency / Power DissipationPeak efficiency of the class B output stage is %, much higher than class A.Unlike class A, power dissipation varies with output amplitude.Remember, there are two output devices so the power dissipation is shared between them.
10 Cross-Over Distortion A small base-emitter voltage is needed to turn on a transistorQ1 actually only conducts when vin > 0.5 VQ2 actually only conducts when vin < -0.5 VWhen 0.5 > vin > -0.5, nothing conducts and the output is zero.i.e. the input-output relationship is not at all linear.
13 Efficiency / Power Dissipation Peak efficiency of the class B output stage is 78.5 %, much higher than class A.Unlike class A, power dissipation varies with output amplitude.Remember, there are two output devices so the power dissipation is shared between them.
14 Class B SummaryA class B output stage can be far more efficient than a class A stage (78.5 % maximum efficiency compared with 25 %).It also requires twice as many output transistors……and it isn’t very linear; cross-over distortion can be significant.