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Old 05-05-2009, 02:48 PM - Thread Starter
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I have a question on op amp gain. For a non inverting circuit where R2 is the resistor connected to ground, and R1 is the resistor between the output and positive input, the gain is roughly 1 + (R1 / R2) when open loop gain is large.

What if there's also a capacitor in parallel with R1? What's it's purpose, and does it effect gain?

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Old 05-05-2009, 03:40 PM
 
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What if there's also a capacitor in parallel with R1? What's it's purpose, and does it effect gain?

It forms a low pass filter. The capacitive reactance decreases as the frequency increases resulting in low gain at high frequencies, high gain at low frequencies.
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Old 05-05-2009, 04:18 PM - Thread Starter
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Ok, that makes sense. It's a very small cap I think? 100pf?

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Old 05-05-2009, 04:22 PM
 
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Xc = 1/(2*Pi*F*C)

You can figure out the gain at a certain frequency.
If this is on the output of a DAC, it could be part of the reconstruction filter, or it's rolling off the very high frequencies in order to be FCC compliant.
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Old 05-05-2009, 05:13 PM - Thread Starter
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Originally Posted by duvetyne View Post

Xc = 1/(2*Pi*F*C)

You can figure out the gain at a certain frequency.
If this is on the output of a DAC, it could be part of the reconstruction filter, or it's rolling off the very high frequencies in order to be FCC compliant.

Thanks a bunch. I will take a look.

I am looking for a book on electronics to learn some of this stuff, but I am on a budget. I might check the library tomorrow.

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Old 05-05-2009, 06:53 PM
 
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Check out used textbook stores, if there's any nearby.
What is this circuit from?
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Old 05-05-2009, 08:10 PM - Thread Starter
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It's from a receiver service manual. It's the op amp after the 8-channel DAC in the subwoofer stage.

I was curious as to whether the subwoofer signal path was different. The op amp used on other channels are setup for no gain. The subwoofer op amp has gain, and I was curious as to how much.

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Old 05-06-2009, 03:21 PM
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Small caps are often added parallel to the feedback resistor in audio circuits to make sure the circuit doesn't oscillate at very high frequencies due to stray capacitance on the circuit board. The gain drops to unity up high where the cap shorts the signal around the resistor so I guess you'd call it a shelving lowpass filter with a corner frequency well above the audio band.

Check out Siegfried Linkwitz's active crossover pages. He shows the theoretical circuits he's using with no parallel cap but his actual circuit board uses a 220pF cap in parallel with the feedback resistor.

http://www.linkwitzlab.com/filters.htm
http://www.linkwitzlab.com/xo_eq.htm

Edit: okay, here's a pic where SL shows the math for a shelving lowpass. f1 = 1/(2pi*R2*C) for a non-inverting circuit.


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