LM3886 unity gain
 User Name Stay logged in? Password
 Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read Search

 Chip Amps Amplifiers based on integrated circuits

 Please consider donating to help us continue to serve you. Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
 Thread Tools Search this Thread
kacernator
diyAudio Member

Join Date: Jan 2008
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
 AN-257.pdf (611.4 KB, 217 views) lm3886t.pdf (745.3 KB, 181 views)

 17th March 2013, 07:27 PM #2 AndrewT   diyAudio Member   Join Date: Jul 2004 Location: Scottish Borders 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? __________________ regards Andrew T. Sent from my desktop computer using a keyboard
 18th March 2013, 10:40 AM #3 jcx   diyAudio Member   Join Date: Feb 2003 Location: .. 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.
 26th March 2013, 08:13 PM #4 ChristianThomas   diyAudio Member   Join Date: Feb 2006 Location: On a hill, in a wooden shack, next to the woods, in Somerset. Why do you want Unity gain on a 3886, just out of interest?
 26th March 2013, 09:07 PM #5 kacernator   diyAudio Member   Join Date: Jan 2008 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.
 26th March 2013, 10:07 PM #6 ChristianThomas   diyAudio Member   Join Date: Feb 2006 Location: On a hill, in a wooden shack, next to the woods, in Somerset. 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?
 26th March 2013, 10:11 PM #7 kacernator   diyAudio Member   Join Date: Jan 2008 I want tube preamp to the all the voltage gain (aikido preamp) and LM3886 just current gain. Look here: Simple hybrid amplifier (35W/8Ohm)
ChristianThomas
diyAudio Member

Join Date: Feb 2006
Location: On a hill, in a wooden shack, next to the woods, in Somerset.
Quote:
 Originally Posted by kacernator 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

JMFahey
diyAudio Member

Join Date: Mar 2009
Location: Buenos Aires - Argentina
Quote:
 Originally Posted by ChristianThomas 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.

 27th March 2013, 08:46 AM #10 kacernator   diyAudio Member   Join Date: Jan 2008 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".

 Thread Tools Search this Thread Search this Thread: Advanced Search

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is Off Forum Rules
 Forum Jump User Control Panel Private Messages Subscriptions Who's Online Search Forums Forums Home Site     Site Announcements     Forum Problems Amplifiers     Solid State     Pass Labs     Tubes / Valves     Chip Amps     Class D     Power Supplies     Headphone Systems Source & Line     Analogue Source     Analog Line Level     Digital Source     Digital Line Level     PC Based Loudspeakers     Multi-Way     Full Range     Subwoofers     Planars & Exotics Live Sound     PA Systems     Instruments and Amps Design & Build     Parts     Equipment & Tools     Construction Tips     Software Tools General Interest     Car Audio     diyAudio.com Articles     Music     Everything Else Member Areas     Introductions     The Lounge     Clubs & Events     In Memoriam The Moving Image Commercial Sector     Swap Meet     Group Buys     The diyAudio Store     Vendor Forums         Vendor's Bazaar         Sonic Craft         Apex Jr         Audio Sector         Acoustic Fun         Chipamp         DIY HiFi Supply         Elekit         Elektor         Mains Cables R Us         Parts Connexion         Planet 10 hifi         Quanghao Audio Design         Siliconray Online Electronics Store         Tubelab     Manufacturers         AKSA         Audio Poutine         Musicaltech         Aussie Amplifiers         CSS         exaDevices         Feastrex         GedLee         Head 'n' HiFi - Walter         Heatsink USA         miniDSP         SITO Audio         Twin Audio         Twisted Pear         Wild Burro Audio

 Similar Threads Thread Thread Starter Forum Replies Last Post tiefbassuebertr Solid State 9 9th June 2011 06:49 AM soundcheck Solid State 2 10th August 2010 02:44 PM okapi Pass Labs 3 25th December 2009 01:23 PM yagas Tubes / Valves 7 24th January 2006 01:30 PM Tomo Chip Amps 12 22nd January 2004 01:47 AM

 New To Site? Need Help?

All times are GMT. The time now is 12:13 PM.

 Home - Contact Us - Advertise - Rules - diyAudio Store - Sponsors - Privacy Statement - Terms of Service - Top - Opt-out policy

Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 15.00%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
Copyright ©1999-2017 diyAudio
Wiki