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Old 6th February 2012, 06:59 AM   #11
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Thanks everyone for the replies. I intended this scheme only for use with JFET-input amplifiers, so I don't think there would be issues with voltage offset even with 1M-ohm resistor - correct?

Quote:
Originally Posted by janneman View Post
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.
This is for the inverting case, but is that any different to the non-inverting case from my first post? The source DC current will be its offset divided by R4. This is a pretty common case I think.

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Originally Posted by Bonsai View Post
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.
Is it amplified by the loop gain, or by the closed loop gain of (R1+R2)/R1? I had almost convinced myself of the latter qualitatively, then I ran the attached sims which I hope are OK to model what you're referring to. If so, no difference between the two cases.
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File Type: gif alt-antenna.gif (48.3 KB, 199 views)
File Type: gif typ-antenna.gif (44.3 KB, 192 views)
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Old 6th February 2012, 09:36 AM   #12
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This is exactly the same circuit as shown in Linear Audio Vol 1 by Kendall Castor-Perry. He also makes the point that it can only be successfully be used with very low leakage inputs.

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Old 6th February 2012, 12:58 PM   #13
Bonsai is offline Bonsai  Taiwan
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You know what steveh49, you are correct. In the non inverting mode, the noise is amplified by the closed loop gain and not the loop gain. Easy to see why. The opamp simply adjusts the output so that Vinv=Vninv. So the noise at the opamp output must thus be the closed loop gain x the noise.

However , I would still not put the cap there. You are basing the op amp with a high value resistor (1M). This limits you to a fet input device, or some offset adjustment.

Any reason why you wanto AC couple te signal like this? Any event opamp will give you negligible offset, even with the 27dB you show. Easier in my view to simply couple te input signal through a cap and be done with it.
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Old 6th February 2012, 03:20 PM   #14
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Bonsai, it allows you to use a 50x or so smaller cap with the same LF rolloff. You are effectively bootstrapping the cap.
See attached

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File Type: jpg cap bootstrap.jpg (29.6 KB, 146 views)
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Old 6th February 2012, 08:01 PM   #15
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Quote:
Originally Posted by janneman View Post
This is exactly the same circuit as shown in Linear Audio Vol 1 by Kendall Castor-Perry. He also makes the point that it can only be successfully be used with very low leakage inputs.

jan didden
I might have to look at getting this article...


Quote:
Originally Posted by Bonsai View Post
Any reason why you wanto AC couple te signal like this? Any event opamp will give you negligible offset, even with the 27dB you show. Easier in my view to simply couple te input signal through a cap and be done with it.
I wasn't necessarily looking at chip opamps, I am also considering power amps. A lot of power amps include an electrolytic capacitor in series with R1 and I started by trying to include this capacitor in the feedback loop (sounds like that was what Kendall Castor-Perry did). It doesn't seem to work as intended though, as the distortion is amplified which is how I knew to question the amplification of the antenna pickup.

As Jan says, the idea now is simply to use a much smaller capacitor. Because it can be reduced by orders of magnitude, this can make the difference between electrolytic and plastic film (distortion). Now that you've raised antenna pickup, I assume that a 10uF electrolytic is better than a 470uF electrolytic in this regard, so there is that potential improvement also. Keen to hear your thoughts.
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Old 6th February 2012, 08:15 PM   #16
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Self shows meaurements of distortion from Al electrolytic feedback caps at the gnd of the gain network - make it really big and there's little audio frequency Vdrop to distort
it really is a non-problem by measured distortion

and by being big and connected to gnd a big electro shunts interference to gnd up to frequencies where it becomes inductive

you should take other measures to avoid hi frequency high amplitude interferance any where near your amp feedback inputs anyway

the high impedance of the small cap and MegOhm feedback R at the op amp input node makes a very sensitive electrostatic antenna - likely more than making up for smaller "collecting" area


fet input op amps have very high ib tempco - get them even a little hot and they have noticable input current - certainly when magnified by 1 MegOhm

Last edited by jcx; 6th February 2012 at 08:23 PM.
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Old 14th February 2012, 12:19 AM   #17
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Quote:
Originally Posted by jcx View Post
and by being big and connected to gnd a big electro shunts interference to gnd up to frequencies where it becomes inductive
Does this mean it is better to have the feedback capacitor connected between ground and R1, than between the feedback node and R1 (R1 connected to ground)?


Quote:
Originally Posted by jcx View Post
the high impedance of the small cap and MegOhm feedback R at the op amp input node makes a very sensitive electrostatic antenna - likely more than making up for smaller "collecting" area
Does this apply equally if a small capacitor is used at the output as shown in the attachment? Is it better to use an oversized capacitor here also?
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Old 23rd February 2012, 06:34 AM   #18
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Originally Posted by steveh49 View Post
Does this apply equally if a small capacitor is used at the output as shown in the attachment? Is it better to use an oversized capacitor here also?
Still interested in including output capacitors in the feedback loop, so bringing this back to the top of the pile.
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Old 5th June 2012, 03:43 PM   #19
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I wonder if it is possible and how should it be implemented this alternative feedback arrangement in a MyRefC.

That was a design proposed by Mauro Penasa in 2005 for the LM3886 chip.

The task would be to be able to use a smaller cap value for C4/C29.
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Old 5th June 2012, 04:27 PM   #20
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That would rather be C9 and C24, I guess? I think the easiest way would be using some bipolars instead. 47..100 F would do then.
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