diyAudio (http://www.diyaudio.com/forums/index.php)
-   Chip Amps (http://www.diyaudio.com/forums/chip-amps/)
-   -   LM3886 unity gain (http://www.diyaudio.com/forums/chip-amps/232233-lm3886-unity-gain.html)

 kacernator 17th March 2013 07:16 PM

LM3886 unity gain

2 Attachment(s)
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.

 AndrewT 17th March 2013 07:27 PM

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?

 jcx 18th March 2013 10:40 AM

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

 ChristianThomas 26th March 2013 08:13 PM

Why do you want Unity gain on a 3886, just out of interest?

 kacernator 26th March 2013 09:07 PM

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.

 ChristianThomas 26th March 2013 10:07 PM

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?

 kacernator 26th March 2013 10:11 PM

I want tube preamp to the all the voltage gain (aikido preamp) and LM3886 just current gain. Look here: http://www.diyaudio.com/forums/tubes...-35w-8ohm.html

 ChristianThomas 26th March 2013 10:40 PM

Quote:
 Originally Posted by kacernator (http://www.diyaudio.com/forums/chip-amps/232233-lm3886-unity-gain-post3428887.html#post3428887) I want tube preamp to the all the voltage gain (aikido preamp) and LM3886 just current gain. Look here: http://www.diyaudio.com/forums/tubes...-35w-8ohm.html
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

 JMFahey 27th March 2013 01:00 AM

Quote:
 Originally Posted by ChristianThomas (http://www.diyaudio.com/forums/chip-amps/232233-lm3886-unity-gain-post3428877.html#post3428877) 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.

 kacernator 27th March 2013 08:46 AM

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".

All times are GMT. The time now is 05:40 AM.

Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.