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.
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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
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
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.
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 %.
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.
Thermal Effects 2030405060 0 0.4 0.8 1.2 Temperature [°C] Collector Current [mA] (V BE =0.5 V)
Thermal Runaway Collector Current Flows, so power is dissipated Temperature risesCollector current rises Power dissipation increases
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.
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:
Bias Voltage – The V BE Multiplier Base current is negligible, so: V BE
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.
Design Example – (i) R E Let R L = 16 and A max = 12 V. (Also assume V out = 0 through d.c. feedback).
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.
Design Example – (iii) V bias Peak output current = 0.75 A, choose quiescent collector current to be small by comparison, e.g.
Design Example – (iii cont) V bias For constant bias voltage,
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.