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Old 11th December 2009, 01:50 AM   #1
okapi is offline okapi  United States
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Default Throwing away open loop gain with an opamp near or at unity gain

In this link (below) Nelson Pass suggests a method to throw away some gain when operating an op amp at low and unity gains.

opamp inverting input sounds better?

I understand the principle but can't come up with a schematic for the method he suggests:

"If the circuit has less than 20 dB gain, I toss some away.
With an inverting amp, a resistor from the - input to ground
will do it. If not, then resistance from both + and - to
ground is used."

My best shot at the inverting configuration is shown below.

For the non-inverting config i can't come with anything that isn't simply throwing away input or changing the gain in the feedback loop.

I am interested in trying this out with an amp i am building that is based on Juma's schematic which is a version of the GC SuperSymmetry also proposed by Nelson. Juma's schematic can be seen here with opamp input buffer:

GC SuperSymmetry

and here with B1 style input buffer:

GC SuperSymmetry

I plan on using a LME49710 which has 140dB of open loop gain as the input buffer and am preparing for the possibility that it may be near oscillation at unity.

The alternative approach i am considering is having less gain around the opa1632 and adding gain to the LME49710. Even if i go this way i would still like to better understand Nelson's idea.

Thanks for helping.
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File Type: jpg inverting gain-tossing.jpg (68.5 KB, 205 views)
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Old 22nd December 2009, 01:37 AM   #2
okapi is offline okapi  United States
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Old 22nd December 2009, 01:39 AM   #3
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Yeah, that's the ticket!

Sorry I missed you...

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Old 25th December 2009, 01:23 PM   #4
okapi is offline okapi  United States
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Nelson, thanks for the confirmation on the inverting configuration as well as increasing this threads view count by almost 3X.

Does anyone want to take a shot at the non-inverting configuration. I can't seem to do it without changing the gain or throwing away input.

Merry Christmas
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