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Chip Amps Amplifiers based on integrated circuits 

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16th February 2004, 05:44 PM  #21 
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Join Date: Oct 2003
Location: Toronto

I just finish playing with the SPICE simulation and it looks like the DC FB res has to be around 470k1Meg and the cap around 2uF to get 3Hz @ 3dB. I'm not sure yet what influances the 3dB freq. Help would be apreciated.
/Greg 
16th February 2004, 07:30 PM  #22 
diyAudio Member

What about the noise when you have 1 M ohms source resistance?
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16th February 2004, 07:33 PM  #23  
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Quote:
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16th February 2004, 08:01 PM  #24  
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Quote:
Yes it is, so not much worrying about that. I'd really like to find out a formula for the value of the cap. It's not just the DCFB res. that it depends on. /Greg 

16th February 2004, 08:15 PM  #25 
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Join Date: Oct 2002
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The devil is in the details.
"Do you ever bother to do orderofmagnitude calculations"
Yes, when appropriate. ESR is non linear. Go read a little about what a capacitor's ESR is caused by before you throw stuff out there with seemingly little concern for credibility. I guess you don't believe in wine tasting but rely strictly on chemical analysis to differentiate wine. Who could trust anything as subjective as taste to compare wines........ It just flies in the face of common sense. 
16th February 2004, 08:27 PM  #26  
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Re: The devil is in the details.
Quote:
Fred (or anybody else), Can you tell me how to calculate the value of the FB cap in the sch. above, please? Thanks upfront. /Greg 

16th February 2004, 08:55 PM  #27 
diyAudio Moderator

Fred, you still haven't considered orderofmagnitude. Why not pick a good quality cap (not designer stuff, just something like a cap one might find in an Adcom or Rotel) and show how much the variation in ESR compares to the series resistance? I can run through it if you don't want to be bothered with such trivia.
Reality check: what's the distortion of an Adcom or Rotel? How much is from the electrolytic in the feedback loop? Disclaimer: I use servoes, so this is an academic exercise for me.
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Remember: life is ten per cent what happens to you, ten per cent how you respond to it, and eighty per cent how good your reflexes are when the Tall Ones come at your throat with their pincers. 
16th February 2004, 10:05 PM  #28  
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Re: Re: The devil is in the details.
Quote:
I guess I'll answer my own question. I just found out that the equivalen resistance that determines the 3dB freq. is =Rdc/gain of the amp. In the case above: Rdc=1meg The gain is 22k/1k=22, so the equiv. res (IF the AC NF res are significantly smaller than the Rdc) is 1meg/22=45k, so the f=1/(6.3x45kx1uF)=3.5Hz. 

16th February 2004, 10:45 PM  #29 
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Join Date: Feb 2004
Location: Silicon Valley

This is in reply to Per's post. A resistor is a noise source only when it has current flowing through it. In the case of the simulation using a LM324, you are right  the 1M feedback value wasn't appropriate, not only considering noise. The 324 is a bipolar input amp with 45250 nA bias current. The bias current and resistance result in about 45250 mV extra offset. The offset could be cancelled out with a 1M resistor on the + input as well, but then you have 45250 mV of steady common mode input. Not nice, either way. For a FET op amp, however, the 1M feedback resistor would be more appropriate, as the bias current is orders of magnitude smaller. A person wanting to eke out the very best noise performance could always use a somewhat smaller feedback resistor and larger coupling capacitor. I would do the calculation of noise current X resistance first, though.
I did the analysis of the network a few years back, and found to my surprise that the capacitor value was not simply a function of the RC time constant of the coupling capacitor and the unity feedback resistor. If I have time to work it out again, I'll post either the analysis equations or a spreadsheet . 
18th February 2004, 01:51 AM  #30 
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Join Date: Apr 2003
Location: New York

About capacitor
I did some measurements on this subject today.
I did measurements on a NIGC with a LM3875. I added the input capacitor. If I remove the resistor from + input to ground the voltage over the capacitor is almost equal to the rail voltage. I did not connect the + input to anything. The offset I measured on the output was very high 21 V. I tried different caps in this position. A 30 uF film and a 47 uF NP electrolytic. I could not distinguish between either and a straight wire. Now I have a question about the National bpa200. They do not have an input + connection to ground (Except before the buffer stage). How does this work? Am I missing something? Also in a parallel amp could the capacitor be shared by the parallelled chips of is this not a good idea? Thanks Harry 
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