Class-A Opamp with output stage - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State

Solid State Talk all about solid state amplification.

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
Reply
 
Thread Tools Search this Thread
Old 23rd July 2011, 01:00 AM   #1
diyAudio Member
 
Join Date: Nov 2004
Location: Florida
Cool Class-A Opamp with output stage

Here's a schematic I made and I'm about to construct the actual circuit.

It will be used as a part of a preamp for a diy sound system. I added an output stage to drive low impedance loads without bogging down the OpAmp. It will be driving a 560 ohm load (driving inverting input on power amp)

To preserve clarity, for my stereo L & R channels, and have less %THD, I want to do class-A preamp, with a good op-amp that has low THD as well.

Any opinions or tips, or questions about the circuit, feel free to post.

-Eworkshop1708
Attached Images
File Type: gif Class-A-Opamp.gif (6.0 KB, 763 views)
__________________
You can call me Mad Professor, building crazy experiments in my Electronics Workshop
  Reply With Quote
Old 23rd July 2011, 01:58 AM   #2
jaycee is offline jaycee  United Kingdom
diyAudio Member
 
Join Date: Apr 2004
Location: Norwich, UK
Should work quite well. Also works well as a headphone amp.
  Reply With Quote
Old 23rd July 2011, 02:08 AM   #3
jcx is offline jcx  United States
diyAudio Member
 
Join Date: Feb 2003
Location: ..
you'd have less AC load on the op amp output if you connected the bias R to the output Q emitter - the Vbe will be relatively constant and "bootstrap" the R to appear as a much higher impedance load to the op amp output - even after lowering the bias R value to give the desired current with the Vbe drop instead of the negative supply V
  Reply With Quote
Old 23rd July 2011, 02:40 AM   #4
diyAudio Member
 
Join Date: Nov 2004
Location: Florida
Quote:
Originally Posted by jcx View Post
you'd have less AC load on the op amp output if you connected the bias R to the output Q emitter - the Vbe will be relatively constant and "bootstrap" the R to appear as a much higher impedance load to the op amp output - even after lowering the bias R value to give the desired current with the Vbe drop instead of the negative supply V
Interesting.........Thanks for the tip, I've made the change to the circuit.

I lowered the Class-A bias some on the OpAmp, but it should still remain in Class-A still because the 470 should still bypass enough current to keep the OpAmp biased on, even if the transistor is not loaded, because of the constant DC load from the CCS Class A output.
Attached Images
File Type: gif Class-A-Op-Amp.gif (5.7 KB, 691 views)
__________________
You can call me Mad Professor, building crazy experiments in my Electronics Workshop
  Reply With Quote
Old 23rd July 2011, 10:35 AM   #5
Bonsai is offline Bonsai  Taiwan
diyAudio Member
 
Bonsai's Avatar
 
Join Date: Jul 2003
Location: Taipei, Taiwan
Nice! I did somee work on a class A opamp based GP output stage a few months ago, and managed to get some measurements done on an AP. The results were really good, so I can vouch for this approach. You can read about my results here:-

Audio Amplifer Designhifisonix.com | hifisonix.com

see 'A Universal Small Signal Class A Buffer'

Good luck with your project :-)
__________________
bonsai
Amplifier Design and Construction for MUSIC! http://hifisonix.com/
  Reply With Quote
Old 23rd July 2011, 02:18 PM   #6
diyAudio Member
 
Join Date: Sep 2010
Quote:
Originally Posted by EWorkshop1708 View Post
.
I lowered the Class-A bias some on the OpAmp, but it should still remain in Class-A still because the 470 should still bypass enough current to keep the OpAmp biased on, even if the transistor is not loaded, because of the constant DC load from the CCS Class A output.
Why not use a dedicated (and adjustable) CSS for the op-amp like in Bonsai's circuit, or this oldie from Walt Jung (Qn/D2/R?)?

http://waltjung.org/PDFs/WTnT_Op_Amp_Audio_2.pdf

(That is also a well performing op-amp buffer, able to run in class A. The four articles of the series are here:

Services )
  Reply With Quote
Old 23rd July 2011, 02:36 PM   #7
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
I like the bootstrapped resistor (ClassA single ended opamp). I had not thought of that as a bootstrap, but seems so obvious when one is told.

The 470r draws ~1.2 to 1.4mA through the opamp. Will all opamps, those with high output bias and those with low output bias, perform similarly with that amount of single ended output current?
__________________
regards Andrew T.
  Reply With Quote
Old 23rd July 2011, 03:04 PM   #8
Dr_EM is offline Dr_EM  United Kingdom
diyAudio Member
 
Dr_EM's Avatar
 
Join Date: Oct 2006
Location: Swindon
Looks nice in a quick Spice sim with 3904s and 32Ohm load, 2Vpp. How can I perform a distortion analysis? I think this will be useful as a headphone amplifier.

Click the image to open in full size.
Attached Images
File Type: jpg Op amp buffer.jpg (107.9 KB, 626 views)
  Reply With Quote
Old 23rd July 2011, 03:53 PM   #9
jcx is offline jcx  United States
diyAudio Member
 
Join Date: Feb 2003
Location: ..
some "audio" op amps appear to have ~ 100 uA output bias

some early generation DSL driver op amps may have a few mA - they were competing on MHz distortion specs - Scott Wurcer claims newer DSL chips are accepting a higher distortion number as adequate and trying to cut power dissipation

how "good" the Class A performance for a given bias can be easily calculated assuming op amp internal emitter follower and added buffer Q base current demand
  Reply With Quote
Old 23rd July 2011, 04:07 PM   #10
diyAudio Member
 
Join Date: Mar 2007
Location: Canandaigua, NY USA
I've used a very similar circuit for a headphone amp with great results. If you use a MOSFET for the pass element, the offset voltage on the gate means that the opamp output doesn't pass through zero until you get to larger signal levels, IMO a good feature if you can stand the slight loss of headroom. Heatsink the circuit well, bias it high enough, and you can even drive small speakers.
__________________
May the root sum of the squares of the Forces be with you.
  Reply With Quote

Reply


Hide this!Advertise here!
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
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Smooth OpAmp output stage. wandelblu Chip Amps 1 2nd February 2008 10:25 PM
Aikido + class D in the output stage jmartins Tubes / Valves 2 6th June 2006 08:51 PM
Class A with AB Output-stage ManUtd Solid State 14 1st December 2003 05:49 PM
Class h or g output stage? Opie Solid State 1 24th October 2001 09:30 PM


New To Site? Need Help?

All times are GMT. The time now is 12:59 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright ©1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2