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Old 30th August 2004, 06:21 AM   #1
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Default How to determine optimal bias class A current?

I've been using resisitor to connect opamp's output stage to -V to achieve Class A operations. Yet, I have the following questions: -

1. How to determine the optimal bias current for different opamps?
2. What will happen if the current is too high?
3. What will happen if the current is too low?

Thanks

Sunny
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Old 30th August 2004, 08:22 AM   #2
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too high bias will generate heat ,or the chip die

too low bias=no use
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Old 30th August 2004, 09:49 AM   #3
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The best way to determine it is to look at the expected output current. You want a class A current just above the max expected peak output current, so that the output stage will not cut-off.

Example: Suppose your max output level is 5VRMS which is about 7V peak. Your load impedance is 1K. That means the max output current will be about 7mA. So you should bias the amp in class A say 8mA. With a supply of 15VDC that will need a resistor of just below 500 Ohms.

The dissipation of your opamp now becomes 8mA from the POS supply, say of +15VDC is about 60mW (plus the 'normal' quiescent dissipation, of course, see the data sheet), which should pose no dissipation problems.
Your values may be different, but that's the principle.

Jan Didden
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