Low distortion current input VAS - 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 11th October 2011, 12:48 AM   #1
diyAudio Member
 
Join Date: Aug 2009
Location: Kaunas
Default Low distortion current input VAS

Hello everyone, I am trying to design a discrete class Class A amplifier for my university special course. I manage to get 0.006 % THD @ 20kHz @ 1V differential current flowing from differential stage, but I can't get my p-p VAS distort less than about 0,1% at open loop. I've made it on folded cascode basis as in Self's Handbook Figure 5.10. I added current sources instead of collector resistors. The mentioned distortion is at emitters of common-base transistors Q4 and Q6. At collector of Q4 I still have distortion of 0,01% and at the output of VAS I have 0,3% THD at open loop at 20V p-p swing. Open loop gain is 20dB That is 34dB increase in distortion. It distorts more than amplifies. Any suggestions? I have tried optimising bias current, cascode reference voltage, mirror emitter resistor values and the best I get is result mentioned above.

The Class A output stage I have designed has (simulated) THD of 0,002% at 2V p-p output to 3R load at 20kHz. Output of IPS-VAS shows 0,003% at same level. I would be glad to achieve IPS-VAS THD many (ten?) times less that that of output stage assuming I can't get any lower there without current increase. I would be glad to get away without NFB nor sophisticated pile of transistors just for simplicity and reliability of the design.

Link to mentioned schematic in google books.


Best regards,
Paulius.
  Reply With Quote
Old 11th October 2011, 01:14 AM   #2
diyAudio Member
 
nigelwright7557's Avatar
 
Join Date: Apr 2008
Location: Carlisle, England
NFB just needs two resistors and one capacitor and would help linearise your amplifier.
__________________
PCBCAD50 software. http://www.murtonpikesystems.co.uk
  Reply With Quote
Old 11th October 2011, 02:29 PM   #3
jcx is online now jcx  United States
diyAudio Member
 
Join Date: Feb 2003
Location: ..
Default read Cherry, Hawksford, the Groner thread...

Cherry’s “ESTIMATES OF NONLINEAR DISTORTION IN FEEDBACK AMPLIFIERS” JAES V48#4 2000 p299-313 provides a method of calculating individual component distortion mechanisms contribution to overall distortion of a amplifier, more importantly it gives a intellectual framework for reasoning about distortion and feedback in a feedback amplifier. The example calculations for a simple audio amplifier circuit in the article also would give you some perspective that will make much of this wandering about in simulation land more directed and informative.

Cherry's paper is the best I know of for understanding distortion in audio amps, but it isn't free online - you have to go to the Library - people do remember those??


On the cascode VAS results, Hawksford’s REDUCTION OF TRANSISTOR SLOPE DISTORTION IN LARGE SIGNAL AMPLIFIERS, M.O.J. Hawksford, JAES, vol.36, no.4, pp.213-222, April 1988 explains that the major distortion in the VAS isn’t a “simple” exponential series from gm/Vbe but the more difficult to model nonlinear CB and CE conductances. Hawksford goes on to give one of the few explanations of the “correct” version of the cascode connection I have seen (of course Pass gets this right too) and Hawksford shows the Baxendall “Super Pair” (without attribution, but no one else I’ve read seems aware of its origin either)
http://www.essex.ac.uk/csee/research...%20cascode.pdf

Comments on Audio Power Amplifier Design Handbook by Douglas Self


the Cherry paper in particular can help with understanding interaction - how do you characterize the distortion of the "VAS" (for University paper "brownie points" you should properly refer to it as a Transimpedance amplifier: TIS) when output stage current gain gives a nonlinear load on the "VAS"?

Last edited by jcx; 11th October 2011 at 02:37 PM.
  Reply With Quote
Old 11th October 2011, 11:25 PM   #4
diyAudio Member
 
Join Date: Aug 2009
Location: Kaunas
Thank you for the papers! The first one is in AES library to which my institution has access so no problem here.
  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
A superior VAS clamp and VAS current limit for the Blameless. GK Solid State 128 19th October 2011 04:03 PM
no current mirror, current mirror, cascoded current mirror, bjt or jfet VAS? maxpou Solid State 31 17th June 2008 05:31 PM
VAS thermal tracking and distortion suzyj Solid State 46 13th August 2007 04:55 AM
low VAS sealed better than high VAS? matos81 Subwoofers 3 7th April 2006 12:28 PM
Choke-Input: How do I calculate the right snubber for high-current-low-voltage Blitz Solid State 20 24th March 2004 09:37 PM


New To Site? Need Help?

All times are GMT. The time now is 09:37 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