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Old 17th March 2013, 07:16 PM   #1
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Default LM3886 unity gain

Hi,

I have found application note AN-257 from Analog Devices describing how to make almost any opamp amplifier unity gain stable. Article starts on the page 6 and it is named "Trick those op amps".

According the information here is my take on calculating the needed values, which are Rc and Cc. I am not sure if my calculations are right, thats why I post them.

I choose minimum stable gain for LM3886 to be 11 (according to the datasheet it is 10).
For this gain I have Rf=10k Ohm and Ri=1k Ohm. (But I think Ri doesn´t matter and Rf I can choose whatever I can? - If I understand the article correctly.)

Rc= Rf/10, so Rc=1k Ohm

Cc= 1/(2*Pi*Rc*(fc/10)), so I need fc now.

fc for gain 11 is = open loop BW/closed gain, so fc=8 MHz/11, fc=727,2 kHz

Cc= 2,2nF, Cc should be made 1000 times bigger to lower the overshoot,
so Cc= 2,2uF

Calculated results for unity gain stable LM3886 should be:
Rf= 10k Ohm
Rc= 1k Ohm
Cc= 2,2uF

Are these calculations right?

Thanks.
Attached Files
File Type: pdf AN-257.pdf (611.4 KB, 158 views)
File Type: pdf lm3886t.pdf (745.3 KB, 146 views)
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Old 17th March 2013, 07:27 PM   #2
AndrewT is offline AndrewT  Scotland
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from my reading of the 3886 info it appears the gain of 10 phase margin is only ~30degrees.
For a more useful phase margin of 60degrees the gain should be set to ~20 to 24times.
For 80degrees set the gain to ~30times.
If these higher gains were substituted for the 10times, where would your network values end up?
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Old 18th March 2013, 10:40 AM   #3
jcx is offline jcx  United States
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the article is a start

I don't think the 1000x factor on the C is really "right" - more goes into the calulation

http://www.ti.com/lit/an/snoa486a/snoa486a.pdf

I don't like this next presentation as well but more different veiwpoints help learning for most people

http://www.analogzone.com/acqt0814.pdf

making the square wave overshoot ~ "the same" by eyeball on the 'scope isn't really a engineering critera - especially for a audio signal - especially with thier "single pole" recovery - at least look for ringing

for stability what you should want is a factor of 3-10 for the shelving noise gain to reach the min stable before the gain intercept - this makes it a function of the noise gain - not a constant 1000x

and you shouldn't calculate with 8 Mhz for LM3886 - completely "safe" would use the min GBW, not the typical spec

and I wouldn't use noise gain in a positive gain configuration without knowing that the source Z is low at frequencies near the gain intercept - not the case if you just hook up to a audio volume pot wiper

although if you can rely on CMRR not messing up apparantly a case can be made for the postive input R

http://www.ti.com/lit/an/snoa861/snoa861.pdf

Last edited by jcx; 18th March 2013 at 11:00 AM.
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Old 26th March 2013, 08:13 PM   #4
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Why do you want Unity gain on a 3886, just out of interest?
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Old 26th March 2013, 09:07 PM   #5
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I want to have simple unity-gain power buffer, which I want to drive with tube preamp. I have done it with OPA548/9 and it sounds very good to me, but looking for cheaper and more powerfull chip, which LM3886 fulfils perfectly except unity-gain stability. But it can be done unity-gain stable, just need some help with it.
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Old 26th March 2013, 10:07 PM   #6
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Is there anything wrong with a potential divider to reduce level, and then have the gain set to 11 or 12? At first glance I can't see much of a problem with noise.

Are you implying that your pre-amp outputs 35V peak?
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Old 26th March 2013, 10:11 PM   #7
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I want tube preamp to the all the voltage gain (aikido preamp) and LM3886 just current gain. Look here: Simple hybrid amplifier (35W/8Ohm)
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Old 26th March 2013, 10:40 PM   #8
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Quote:
Originally Posted by kacernator View Post
I want tube preamp to the all the voltage gain (aikido preamp) and LM3886 just current gain. Look here: Simple hybrid amplifier (35W/8Ohm)
OK. Interesting.

I wonder if all the extra loop gain you get has to be thrown away in getting it stabilised. I haven't read the article but my guess (and it is a guess) is probably not. So that's good

Actually it comes to mind that there is likely a limit on the input voltage of the 3886 which is likely to be less than full supply. Have you checked that?

Very interesting.

C
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Old 27th March 2013, 01:00 AM   #9
JMFahey is offline JMFahey  Argentina
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Quote:
Originally Posted by ChristianThomas View Post
Is there anything wrong with a potential divider to reduce level, and then have the gain set to 11 or 12? At first glance I can't see much of a problem with noise.

Are you implying that your pre-amp outputs 35V peak?
That's the way to do it.
The only way in fact.
Make it , say, 12X gain and apply a 12X voltage divider.
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Old 27th March 2013, 08:46 AM   #10
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No, I want current buffer, voltage follower. Tube preamp will make all voltage gain. Please read above posted link to the thread "Simple hybrid amplifier".
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