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Old 3rd December 2008, 06:11 PM   #81
lineup is offline lineup  Sweden
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Default Re: Current-feedback inverting amp

Quote:
Originally posted by gpapag
I would like to see a simple drawing of an inverting op.amp. but with current feedback. I want to build one and compare how it sounds against a voltage feedback.
Regards
George
Hmmm.
Inverting op-amp with current feedback.
How can I do this
I'd like to see one discrete such op. Thanks!
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Old 3rd December 2008, 07:30 PM   #82
wg_ski is offline wg_ski  United States
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Quote:
Originally posted by Pjotr
Maybe it is already said and have I missed it in this long discussion:

If you use the inverting topology for audio the op-amp MUST be unity gain stable!

You can get rid of this by placing a sufficient low resistor at the input, according to the attached schematic, to keep closed loop gain above 20 dB under all input conditions.

The 'proper' way to do this is to hang that extra 10k resistor directly between the + and - input terminals of the op-amp to set the noise gain. Doesn't anyone read the app notes for basic op-amps anymore? You can get a LOT of design ideas studying those old data books.

Overall, the inverting configuration is considered 'better' (from a theoretical standpoint) because the common-mode voltage is zero regardless of closed loop gain and input voltage. If the input stage LTP uses just a resistor for bias instead of a CCS this advantage is not insignificant - it will result in better linearity. Some op amps are also better behaved with zero CM voltage.
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Old 13th September 2011, 07:50 AM   #83
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by Nelson Pass View Post
Every time you mention inverting phase or phase
splitting to the semi technical audiophile, they go into
shock.



I have another hypothesis which occurs when the stage
is not being used for much gain:

It is my experience that at low and unity gains, a lot of
op amps are on the edge of oscillation. Over the years
I have made a number of bad sounding circuits (no, not
ones you've seen) and many of them were bad sounding
not because they were oscillating, but because they were
near the edge. If I opened up the feedback gain, they
got better as they moved farther from the edge.

As a result, whenever I use op amps in an audio circuit, which
is not often, I make sure that I either use up or throw away
about 20 dB of open loop gain. If the circuit has a gain of
10 or greater, this is automatically achieved.

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.

Works like glue....

I know this thread is 10yrs old now but this comment (and others) got me thinking.

How valid a technique is this as a method of compensating an opamp ?

I have posted some real scope shots and measurements here,
A Different Opamp Compensation Technique.

Thanks...
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Old 23rd November 2011, 06:13 PM   #84
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Quote:
Originally Posted by Mooly View Post
I know this thread is 10yrs old now but this comment (and others) got me thinking.

How valid a technique is this as a method of compensating an opamp ?

I have posted some real scope shots and measurements here,
A Different Opamp Compensation Technique.

Thanks...
More useful than the cap method. Your resistor example forces 0v DC offset.
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Old 23rd November 2011, 06:24 PM   #85
Mooly is offline Mooly  United Kingdom
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Hi Daniel,
it's certainly a technique I would pursue further. Maybe a compromise of reducing open loop gain a little and still keeping a (smaller) comp cap.
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Old 23rd November 2011, 06:59 PM   #86
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I see the corners getting sharper on your scope, so it seems that you're repairing the inverting amp dullness problem? Oh, good job!


Could you do me a huge favor while you've got that scope and a handy power op amp?
It would be:
Put two of 220uF (one on each rail--low impedance, low ESR types) and one of 2uF (solo cap across rails--250v ecap or 60v~100v polyester dip) and two of 3.3nF (one on each rail) for power smoothing, all at very close proximity to the amp. It may be necessary to either move any ringingly large caps away from the amp or select low impedance types. That amp could be non-inverting STK, ST or Inverting NatSemi. The scope may show something very interesting if there was before and after photos. Disclaimer: The little power circuit example is great for the amp that needs it but terrible for the amp that doesn't.

Last edited by danielwritesbac; 23rd November 2011 at 07:22 PM.
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Old 24th November 2011, 04:42 AM   #87
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With a non-inverting amplifier, a frequently ignored topic is the current in the feedback resistor and the feedback shunt resistor.
When observed at exactly the same gain setting. . .
It seems that when the feedback is a mirror image or "sample" of the output, you can simulate overrun or underrun conditions via changing the current in the feedback.
The sound effect differences? Maybe that is from changing the current.

P.S.
Overrun non-inverting amplifier, even slightly, even just overruning the pre, not the whole thing, will make the support circuit look really crazy or normal values not working as expected. For example, the op-amp seeming to need really really big caps right at it or else the sound is bad? Well, that thing is suffering overrun. But, that's a typical gainclone.

Last edited by danielwritesbac; 24th November 2011 at 04:47 AM.
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Old 24th November 2011, 07:28 AM   #88
Mooly is offline Mooly  United Kingdom
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Hi Daniel... power op-amps aren't really my thing tbh but don't let that stop anyone repeating the tests
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Old 24th November 2011, 12:13 PM   #89
AndrewT is offline AndrewT  Scotland
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I see post85 repeats what I just posted on the linked thread. (can it be a repeat if the post preceded the second?)

Mooly,
did you try that halfway house of using a bit of both compensation methods?
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Old 24th November 2011, 12:53 PM   #90
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by AndrewT View Post
I see post85 repeats what I just posted on the linked thread. (can it be a repeat if the post preceded the second?)

Mooly,
did you try that halfway house of using a bit of both compensation methods?
I haven't although on future projects I would into this more.
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