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Old 4th February 2012, 05:36 AM   #1
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Default Alternative feedback capacitor arrangement

Are there any flaws with the feedback capacitor arrangement shown in the attachments? It seems to allow a much smaller value capacitor to be used than a capacitor in series with R1, which is what I have seen used commonly. It sims OK in LTspice with the ideal opamp but, since it's so simple yet I haven't seen it used, I want to check.
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File Type: gif inv-capFB.gif (10.3 KB, 382 views)
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Old 4th February 2012, 07:16 AM   #2
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I can't see anything wrong with it. You separated out the DC and AC feedback, which allows you a much higher R for the DC feedback hence a lower C value. Creative circuit.
There are two issues I think you should verify:

* In practise, there is a limit to how high you can make the DC feedback R before you get in trouble with output offset due to bias currents. I expect your 1 Meg to be too high in most cases, so it will be less advantageous as it looks in your diagram;

* The source is now DC coupled to the output. Depending on resistance values and source DC offset, a DC current will flow through the source. Don't know what the effect of that on the source will be.

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Old 4th February 2012, 08:03 AM   #3
effebi is offline effebi  Italy
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Another problem that might arise is thermal noise. It depends only on value of a resistor , for a given T. I believe Self mentions this problem in the feedback in one of the earlier ( 2nd? ) editions of his book.
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Old 4th February 2012, 11:18 AM   #4
AndrewT is online now AndrewT  Scotland
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I have not got my head around the concept yet.
Would looking at "noise gain" of the circuits help?
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Old 4th February 2012, 08:16 PM   #5
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If I'm not mistaken, C1 would have to carry very little signal current here, which I'd see as a definite advantage for crummy nonlinear caps in particular. However, you still want to have C1's impedance to be reasonably small vs. R1||R2 in the relevant frequency ranges in order to keep noise down.

I could definitely see this variant being of use under certain circumstances (not like it would be all new or anything, definitely seen it in old power amp IC datasheets). When using film caps and low gains, I'd prefer the "standard" one though.
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Old 5th February 2012, 12:48 AM   #6
Bonsai is offline Bonsai  Taiwan
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You have placed a capacitor between the feedback resistor junction and the inv input of the opamp. Any noise picked up by the cap will be amplified by the full loop gain if the opamp. The capacitor is physically large and acts as an antenna compared to just putting the junction of the feedback resistors as close as possible to the inv input which is a must with opamps or any feedback amplifiers. My honest opinion: this is not a step forward and for the reasons cited, is why you do not see this approach used in general.
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Old 5th February 2012, 01:43 AM   #7
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Refer to Luxman's duo beta feedback arrangement
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Old 5th February 2012, 03:27 AM   #8
Bonsai is offline Bonsai  Taiwan
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Its still a sub optimal design. For straight signal amplifiers put nothing between the feedback resistor junction and the inverting input.
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Old 5th February 2012, 12:49 PM   #9
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The + input also wants to see 1M to GND at DC, else suffer an offset.
If you have to use an input cap to do that, perhaps conspire to have
this extra "antenna?" on the + input buck the - pickup problem...
Same orientation etc. etc...

Not sure I actually believe that, but assuming it was true...

Last edited by kenpeter; 5th February 2012 at 01:00 PM.
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Old 5th February 2012, 01:52 PM   #10
Bonsai is offline Bonsai  Taiwan
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I thought for a minute you were being serious there . . . ;-)

Its amazing how high end audio companies will wrap absolute marketing b.s. around something that is totally sub optimal and then market it as the best thing since sliced bread, to then be taken up by the high end community as gospel. As bad as all those make-up adverts . . .

. . . the latest being a print advert featuring Rachel Weitz which was banned by the Advertising Standards Authority in the UK for being too misleading - not Ms RW that is, but the makeup company's claims. It appears they were too heavy handed with the photoshopping, since RW is 41 years old they did not feel the photo represented reality.
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Last edited by Bonsai; 5th February 2012 at 02:00 PM.
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