Class An Output Stage - Recap

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Why is class A so wasteful ?. Single transistor can just lead in one direction.D.C. predisposition current is expected to adapt to negative going signals.75 % (or more) of the supplied force is scattered by d.c.Solution : dispose of the inclination current.. Class B Output Stage. Q1 and Q2 structure two fair emitter followersQ1 just leads when the information is positiveQ2 just directs when the data is negative

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Slide 1

Class An Output Stage - Recap Class A yield stage is a basic straight current enhancer. It is additionally extremely wasteful, run of the mill greatest productivity in the vicinity of 10 and 20 %. Suitable for low power applications. High power requires much better productivity.

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Why is class A so wasteful ? Single transistor can just lead in one heading. D.C. inclination current is expected to adapt to negative going signs. 75 % (or more) of the provided power is disseminated by d.c. Arrangement : dispose of the predisposition current.

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Class B Output Stage Q 1 and Q 2 frame two fair emitter adherents Q 1 just leads when the info is certain Q 2 just directs when the information is negative Conduction edge is, in this way, 180° When the info is zero, neither behaviors i.e. the quiet power dissemination is zero

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Class B Current Waveforms I out time I C1 time I C2 time

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I C1 A/R L 0 p 2p Phase, q Class B Efficiency Average power drawn from the positive supply: A wrongdoing( q )

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Load control: Efficiency: By symmetry, control drawn from +ve and –ve supplies will be the same. Add up to control, in this way:

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Power Dissipation To choose proper yield transistors, the most extreme power dissemination must be ascertained. Simply need to locate the greatest estimation of P D to choose transistors/heatsinks

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E.g. V S = 15 V, R L = 100 W 1.5 P L P S P D 1 Power [W] 0.5 0 5 10 15 Peak Output Amplitude, A [V]

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greatest when: Maximum Power Dissipation P D is a quadratic capacity of A,

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Efficiency/Power Dissipation Peak effectiveness of the class B yield stage is 78.5 %, considerably higher than class A. Not at all like class A, control dissemination shifts with yield plentifulness. Keep in mind, there are two yield gadgets so the power scattering is shared between them.

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Design Example Design a class B enhancer which will convey up to 25 W into a 4 W stack. Supply voltages must be bigger than A maximum so pick V s = 15V.

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But, with q JC = 1.92 °C/W Each of the two yield transistors must have the capacity to securely disseminate up to 5.7 Watts. Utilizing a TIP120 & TIP 125: i.e. Either two heatsinks evaluated at under 20°C/W are required or a solitary heatsink appraised at under 10°C/W.

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

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Cross-Over Distortion A little base-emitter voltage is expected to turn on a transistor Q 1 very directs when v in > 0.7 V Q 2 quite leads when v in < - 0.7 V When 0.7 > v in > - 0.7, nothing conducts and the yield is zero. i.e. the info yield relationship is not in the least direct.

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

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Effect of Cross-Over Distortion

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Audio Demo Undistorted unique Class B speaker yield

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Class B Summary A class B yield stage can be much more productive than a class A phase (78.5 % greatest proficiency contrasted and 25 %). It additionally requires twice the same number of yield transistors… … and it isn't exceptionally straight; traverse mutilation can be critical.