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Old 27th May 2007, 10:00 PM   #1
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Default Class "A" biasing

I hope this is a simple question. If I want an amplifier to operate push pull in class A to produce power level "x" what standing bias level sould the output devices be set to? i.e. y(x). or another way to express what I want to know, if I need to deliver 100mA to the load, should I set the standing bias at 400mA or what for low distortion full class A operation.
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Old 27th May 2007, 10:19 PM   #2
bigwill is offline bigwill  United Kingdom
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I'm probably wrong, but with push pull you set the idle current to half your wanted peak current
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Old 27th May 2007, 11:47 PM   #3
fab is offline fab  Canada
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Hi Tubesman

see http://www.borbelyaudio.com/adobe/classa.pdf
you will find all you need to perform your calculation of bias current for class A in push-pull

Good luck
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Old 28th May 2007, 12:31 AM   #4
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thanks for the link I will read it now
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Old 28th May 2007, 01:14 AM   #5
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If you are after the bias that gives minimum distortion then biasing it slightly above the current needed. If you hook up an IV analyzer to a bjt you will see that as the input voltage nears cuttoff there is a nonlinear bend in the transistor's current curve. Biasing above this point so that the transistor doesnt reach this bend in the curve will keep the transistor in it most linear area and give minimum distortion.
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Old 28th May 2007, 01:45 AM   #6
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Quote:
Originally posted by bigwill
I'm probably wrong, but with push pull you set the idle current to half your wanted peak current
No, you're probably right.

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Old 28th May 2007, 01:56 AM   #7
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basicly start with the calculated current and go up from there with a THD meter.
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Old 28th May 2007, 02:00 AM   #8
lineup is offline lineup  Sweden
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Quote:
Originally posted by Nelson Pass


No, you're probably right.

yes, right & correct
from the matematik point of view


but as jerluwoo says:
we should add like +10 to 20% to this figure, for good output at wanted Max
sometimes, whether Push-Pull or Single End, I have added as much as 30%
... it depends on what amplifier circuit I use, from case to case different
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