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Transcript
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. Class B Output Stage Q1 and Q2 form two unbiased emitter followers Q1 only conducts when the input is positive Q2 only conducts when the input is negative Conduction angle is, therefore, 180° When the input is zero, neither conducts i.e. the quiescent power dissipation is zero Class B Current Waveforms Iout time IC1 time IC2 time Class B Efficiency Average power drawn from the positive supply: P ve VS I C1 IC1 A/RL A sin() 0 1 I C1 2 2 2 Phase, 1 A A 0 I C1 d 2 0 RL sin d RL P ve VS A RL By symmetry, power drawn from +ve and –ve supplies will be the same. Total power, therefore: 2VS A PS P ve P ve 2 P ve RL Load power: 2 vout A2 sin 2 t A2 PL RL RL 2 RL Efficiency: PL A2 RL A PS 2 RL 2VS A 4VS NB. A VS / 4 78.5% Power Dissipation To select appropriate output transistors, the maximum power dissipation must be calculated. 2VS A A2 PD PS PL RL 2 RL Just need to find the maximum value of PD to select transistors/heatsinks E.g. VS = 15 V, RL = 100 W 1.5 PL PS PD Power [W] 1 2VS A PS RL A2 PL 2 RL 0.5 0 0 5 10 Peak Output Amplitude, A [V] 15 Maximum Power Dissipation PD is a quadratic function of A, 2VS A A2 PD RL 2 RL dPD 0 maximum when: dA 2VS A 0 RL RL PD (max) A 2VS 4VS2 2VS2 2VS2 2 2 2 RL RL RL 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. Design Example Design a class B amplifier which will deliver up to 25 W into a 4 W load. A2 PL 25 25 A 14.1 V 2 RL Supply voltages must be larger than Amax so choose Vs = 15V. 2VS2 PD max 2 11.4 W 2 5.7 W RL Each of the two output transistors must be able to safely dissipate up to 5.7 Watts. Using a TIP120 & TIP 125: TJ TA JA PD TJ JA PD TA JA PD max TA TJ max 5.7 JA 25 150 JA 22 C/W But, with JC = 1.92 °C/W JA JC CA 1.92 CA CA 20 C/W i.e. Either two heatsinks rated at less than 20°C/W are required or a single heatsink rated at less than 10°C/W. Suggested heatsink Dimensions, 50mm x 50mm x 9.5mm Accommodates two devices Rating 6.5°C/W Cost 60p inc VAT Cross-Over Distortion A small base-emitter voltage is needed to turn on a transistor Q1 actually only conducts when vin > 0.7 V Q2 actually only conducts when vin < -0.7 V When 0.7 > vin > -0.7, nothing conducts and the output is zero. i.e. the input-output relationship is not at all linear. Actual Input-Output Curve vout vout vin VBE -VBE +VBE vout vin VBE vin Effect of Cross-Over Distortion Audio Demo Undistorted original Class B amplifier output Class B Summary A 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.