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demons_wing 6th October 2004 02:55 AM

class B and AB idle current
would anyone be able to explain to me what the idle current for class b and ab are??...i understand that for class a it is 1/2 of do you calculate for b and ab??...any help would be appreciated

wolver 6th October 2004 04:17 AM

Class AB is 5~20mA(Ie) from experience......

Enzo 6th October 2004 04:37 AM

The simple answer is that you want to bias the output devices on just enough to eliminate the crossover distortion.

sam9 6th October 2004 04:53 AM

D. Self has done some work which seems to say that the voltage across the Vbe multiplier is what should be measured for least crossover distortion rather than current. ~2.89V for a EF output stage and 1.29V for aCFB configuration.

Of course the best solution, not available to many DIYers is to adjust it for minimal distortion.

Mr Evil 6th October 2004 04:58 AM

For class B, quiescent current is ideally 0, but in reality will be bsomething very small. Class AB is anything greater than class , but less than the maximum peak load current. More than that is class A.

You should note that the optimum bias current for minimum distortion is in class B, just below class AB. i.e. you should set the bias as high as possible without the devices conducting significantly. The reason for this is well described here.

john curl 6th October 2004 05:32 AM

It has been found by myself, that running as much current as possible without overheating is usually the best. If you want to get the best transisiton between class A and class B, which almost always is necessary, then with output bipolar transistors, drop 15mV-22mV across any emitter resistors. This is a good first approximation.

demons_wing 6th October 2004 06:14 AM

thanx for your help guys

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