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Old 17th May 2006, 05:53 AM   #1
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Question Feedback resistor when using bridge mode

Hi everyone !

I read all the BPA200 datasheet, because I'm building it, and of course I know the manufacturer of the chip is right, BUT I DON'T understand why the feedback resistor (in red) is placed there !

The amp is used in inverse mode, so input signal enters in the inverse input, to have a inverse output to be connected to the -V of the speaker, and wow, you have the bridge!

The normal implementation (non inverted) use the feedback resistor to compensate the signal in, with the inverted signal, so, more feedback in the inverted input, less loud the sound will be...

So, WHY the inverted implementation don't use the feedback resistor connected to the non-inverted signal in? It don't make any sense to me...

Anybody ?!

Thanks !!
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Old 17th May 2006, 06:29 AM   #2
rpapps is offline rpapps  Antarctica
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The purpose of Rf & Ri in all four stages is to set the gain in accordance with the formula:
Vout/Vin = 1 + Rf/Ri
In this case, voltage gain is 22 or approximately 27 dB.
(Besides, if you connected the feedback to the non-inverting input you would have positive feedback, not negative. Oscillator anyone?)
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Old 17th May 2006, 06:59 AM   #3
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Sorry, I didn't read the post carefully and posted an inappropriate answer.
Greg Erskine
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Old 17th May 2006, 07:22 AM   #4
rpapps is offline rpapps  Antarctica
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Hi Greg, no need to apologise.
You were absolutely correct in what you said. I just used one formula to keep things simple. However, in practical terms, it makes less than a 1/2 dB difference which can be safely overlooked.
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