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Old 3rd May 2003, 03:26 PM   #1
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Question Negative Feedback

When changing a negative feedback connection from 16 ohms to 8 ohms, how does one calculate the change in feedback resistor value?
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Old 3rd May 2003, 04:27 PM   #2
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0.707 times the 16 ohm value.
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Old 5th May 2003, 12:43 AM   #3
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If it has a capacitor across it, then you need to scale that by 1.414 (sqrt of 16/8) to maintain the correct time constant.
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Old 5th May 2003, 01:03 AM   #4
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Hi,

Usually I just leave it as is....
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Old 5th May 2003, 01:21 AM   #5
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Default The radio boys will hate me for this, but...

If you scrap an old MW/LW valve radio for its (large) variable air-spaced capacitor, you are likely to have a 2 x 365pF or 2 x 500pF capacitor that is ideal for quickly determining optimum feedback capacitor value.
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Old 5th May 2003, 01:23 AM   #6
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Default THE VARICAP?

Hi,

Yup...good lateral thinking.

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Old 5th May 2003, 01:38 AM   #7
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Default Empericism rules

Rotary, rotary...

Bung a square wave up it, and two seconds find the answer. I was quite surprised. You can put a variable resistor in series with the capacitor, if you like, and waggle both.
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