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Cross-Over Distortion The non-zero “turn-on” voltage of a transistor causes cross-over distortion in a class B output stage. Approximate transistor response. v in v out V BE 0 Ideal response

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Eliminating Cross-Over Distortion v in v out NPN response NPN response for v B = v IN +0.7 PNP response PNP response for v B = v IN -0.7

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Class AB Output Stage Eg. Positive half cycle:

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Practical Class AB Stages In practice, there isn’t an exact “turn-on” voltage (V BE ). V bias is set slightly high so that there is a non- zero quiescent collector current. Each transistor will now conduct for slightly more than 180° - i.e. Class AB operation.

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Class AB Efficiency Slightly more power is dissipated using a class AB stage compared with a class B due to the non-zero quiescent collector current. In a well designed circuit, this extra power should be insignificant so the class B efficiency calculations are still valid. I.e. maximum efficiency = 78 %.

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Thermal Effects The quiescent collector current depends on V BE and also on the junction temperature. So, in designing the biasing network, thermal effects must be considered. Net result is that if V BE is fixed, I C rises exponentially with temperature.

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Thermal Effects 2030405060 0 0.4 0.8 1.2 Temperature [°C] Collector Current [mA] (V BE =0.5 V)

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Thermal Runaway Collector Current Flows, so power is dissipated Temperature risesCollector current rises Power dissipation increases

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Suppressing Thermal Runaway Fit a bigger heatsink. Use series emitter-resistors. Use a temperature dependent bias voltage. The latter two are preferred methods. Both introduce negative feedback.

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Emitter Resistors So, if I C rises, V BE falls and I C is reduced. Note R E should be small compared with R L to minimise power wasted. By symmetry:

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Bias Voltage – The V BE Multiplier Base current is negligible, so: V BE

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V BE Multiplier – Temperature Effects If junction temperature rises but I C stays the same, V BE must fall causing V bias to fall also. Negative thermal feedback achieved if the transistor is in close contact with the output devices. Especially suitable for integrated circuits where close thermal contact is guaranteed.

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Design Example – (i) R E Let R L = 16 and A max = 12 V. (Also assume V out = 0 through d.c. feedback).

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Design Example – (ii) I bias NB. I bias is set well above minimum to ensure that a significant current flows through the V BE multiplier.

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Design Example – (iii) V bias Peak output current = 0.75 A, choose quiescent collector current to be small by comparison, e.g.

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Design Example – (iii cont) V bias For constant bias voltage,

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Class AB – Summary Class AB achieves the efficiency of a class B output stage but without cross-over distortion. Biasing arrangements are more complex, however, as the threat of thermal runaway must be eliminated.

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