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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.

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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.

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Class B Output Stage Q 1 and Q 2 form two unbiased emitter followers Q 1 only conducts when the input is positive Q 2 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

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Class B Current Waveforms I out I C1 I C2 time

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Class B Efficiency Average power drawn from the positive supply: I C1 Phase, A/R L 0 A sin( )

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By symmetry, power drawn from +ve and –ve supplies will be the same. Total power, therefore: Load power: Efficiency:

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Power Dissipation To select appropriate output transistors, the maximum power dissipation must be calculated. Just need to find the maximum value of P D to select transistors/heatsinks

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051015 0 0.5 1 1.5 Peak Output Amplitude, A [V] Power [W] PLPL PSPS PDPD E.g. V S = 15 V, R L = 100

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P D is a quadratic function of A, Maximum Power Dissipation maximum when:

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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.

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Design Example Design a class B amplifier which will deliver up to 25 W into a 4 load. Supply voltages must be larger than A max so choose V s = 15V.

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Each of the two output transistors must be able to safely dissipate up to 5.7 Watts. Using a TIP120 & TIP 125: But, with JC = 1.92 °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.

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Suggested heatsink Dimensions, 50mm x 50mm x 9.5mm Accommodates two devices Rating 6.5°C/W Cost 60p inc VAT

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Cross-Over Distortion A small base-emitter voltage is needed to turn on a transistor Q 1 actually only conducts when v in > 0.7 V Q 2 actually only conducts when v in < -0.7 V When 0.7 > v in > -0.7, nothing conducts and the output is zero. i.e. the input-output relationship is not at all linear.

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Actual Input-Output Curve v in v out +V BE -V BE

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Effect of Cross-Over Distortion

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Audio Demo Undistorted original Class B amplifier output

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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.

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