Anybody played with Self Class A bias regulator? - 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 3rd April 2001, 12:09 AM   #1
mlloyd1 is offline mlloyd1  United States
diyAudio Member
 
mlloyd1's Avatar
 
Join Date: Feb 2001
Location: illinois
I've seen the thread on Self's Trimodal amp. That kind of changed into discussions on the merits of regulating rails. Looking back at the Self schematic (more class a than trimodal, I suppose), has anybody tried his class a bias regulator in any projects? Just curious. I thought it was kinda' clever.

Is anybody building/simulating/testing/listening etc. to BJT output class a here? Seems mostly MOSFET that since I've been hanging around. No problem, again, just curious?

Michael
  Reply With Quote
Old 3rd April 2001, 09:27 AM   #2
ergo is offline ergo  Estonia
diyAudio Member
 
Join Date: Mar 2001
Location: Tallinn, Estonia
I'm designing a class A amp based throughout by bjt transistors. This project and the idea to use only bjt-s is also kind of an experiment. I will try some other less explored ideas there, but I'm not willing to discuss them jet. I hope it will get into some material form in about 3 months and then I'm also going to put a web page up....

__________________
Ergo
  Reply With Quote
Old 4th April 2001, 07:34 AM   #3
djk is offline djk
diyAudio Member
 
djk's Avatar
 
Join Date: Feb 2001
Location: USA
The Linear Technology LT1166 power output stage automatic bias system is what is needed.Do not use the pin 2 mode of operation.Also supports short circuit protection if desired.While designed for FETs it is also good for triple darlington BJT output stages.
  Reply With Quote
Old 5th April 2001, 02:37 PM   #4
Geoff is offline Geoff  United Kingdom
diyAudio Member
 
Join Date: Apr 2001
Location: UK
This is my first posting. I have only registered with this forum in order to correct an apparent misunderstanding.

The bias regulator refered to in the original query is that controlling the output bias current when the amp is operating in Class-A mode. It is a feedback regulator which compares the voltage across the two output emitter resistors with a fixed reference and maintains the Class-A Iq at the appropriate level. If the load current exceeds the Class-A current limit, the Amp will operate in Class-AB mode and the Iq is controlled by a conventional Vbe multiplier. Similarly, if the amp is switched to Class-B mode the conventional Vbe multiplier is used to control Iq.

The LT1166 is designed to control the Iq in a Class-B or Class-AB amp only. It does this by forcing the voltage across the sense (emitter) resistors to +/-20mV. The output stage Iq is therefore equal to 20mV divided by Rsense. For a Class-A Iq of 1.5A (the design value for the Trimodal amp) the sense resistor would require a value of 0.0133 ohms. Not a practical proposition! The LT1166 can only be used for output stage Iqs up to 200mA (with 0.1ohm sense resistors).

I hope this provides some clarification.

Geoff

  Reply With Quote
Old 6th April 2001, 08:22 AM   #5
djk is offline djk
diyAudio Member
 
djk's Avatar
 
Join Date: Feb 2001
Location: USA
>>The LT1166 is designed to control the Iq in a Class-B or Class-AB amp only. It does this by forcing the voltage across the sense (emitter) resistors to +/-20mV. The output stage Iq is therefore equal to 20mV divided by Rsense. For a Class-A Iq of 1.5A (the design value for the Trimodal amp) the sense resistor would require a value of 0.0133 ohms. Not a practical proposition! The LT1166 can only be used for output stage Iqs up to 200mA (with 0.1ohm sense resistors).<< A simple resistor voltage divider will set the bias current to any value you want with any value emitter resistors you want.You can set it to 1.5A class A and it will go to class B above this point.A switch may be added to go to full time class B.The LT1166 data sheet shows how to do this.Packaged in an SO8 chip and not needing to be connected to the heatsink in any way allows for easy retrofits on existing equipment.The Self design is ok but the LT1166 is much more versitile.

  Reply With Quote
Old 6th April 2001, 01:49 PM   #6
Geoff is offline Geoff  United Kingdom
diyAudio Member
 
Join Date: Apr 2001
Location: UK
djk

I'm sorry if I seem pedantic, but the current data sheet for the LT1166 does not show (or refer to) the arrangement you are suggesting. However, that is not to say that it will not work.

The LT1166 may be versatile, but that is of little use here in the UK since none of the usual suppliers stock the device. A discrete component alternative (eg the Doug Self arrangement) can at least be used wherever common type transistors are available.

Geoff
  Reply With Quote
Old 8th April 2001, 08:36 AM   #7
djk is offline djk
diyAudio Member
 
djk's Avatar
 
Join Date: Feb 2001
Location: USA
>>Geoff: djk,I'm sorry if I seem pedantic, but the current data sheet for the LT1166 does not show (or refer to) the arrangement you are suggesting. However, that is not to say that it will not work.<< Figure 4 on page 7 shows how to use tapped resistors to change the limit and sense points. This is from the 16 page data and application sheet.It is also on the LinearView CD rom.>> The LT1166 may be versatile, but that is of little use here in the UK since none of the usual suppliers stock the device. A discrete component alternative (eg the Doug Self arrangement) can at least be used wherever common type transistors are available.<< I don't know about UK distribution of LT parts.In the USA one can request samples for free.



  Reply With Quote
Old 8th April 2001, 10:43 AM   #8
Geoff is offline Geoff  United Kingdom
diyAudio Member
 
Join Date: Apr 2001
Location: UK
djk

Thanks for your reply. Figure 4 does indeed show the use of a tapped sense resistor but this is solely to adjust the limit current. The quiescent current is still controlled directly by the sense resistor (ie the resistance between pins 3 & 8 and pins 3 & 5); in the case of the example in Figure 4 the sense resistance is 1ohm giving an Iq of 20mA. As the lowest practical sense resisor is 0.1ohm, the maximum Iq given by the circuit in Figure 4 is 200mA.

It should be possible to provide a second resistor chain (using higher value resistors) in parallel with the output stage emitter resistor and to tap this second chain at the appropriate point to give an Iq above 200mA. This is the arrangement I thought you were referring to in your previous posting, but this arrangement is not shown in the LT1166 data sheet.

I am not sure how the use of a second resistor chain will affect the accuracy of Iq control since the LT1166 is no longer directly sensing and monitoring the output BJT emitter (or MOSFET drain) current. Unfortunately, I do not have a sample of the device in order to be able to test this arrangement. If anyone has successfully used this device to control a Class-A output stage, perhaps he/she could post the details.

Returning to Michael's original posting and query, yes there are people who have built and are listening to BJT Class-A amps. I'm one of them. My next project will most probably be a Doug Self Trimodal amp so time will tell whether the novel Class-A biasing arrangement is effective, though I've no doubt that it will be.

Geoff
  Reply With Quote
Old 8th April 2001, 02:32 PM   #9
djk is offline djk
diyAudio Member
 
djk's Avatar
 
Join Date: Feb 2001
Location: USA
>>This is the arrangement I thought you were referring to in your previous posting, but this arrangement is not shown in the LT1166 data sheet.<< You are absolutely correct. While the exact arrangement I suggested is not shown in figure 4 the idea for tapping either of the sense or limit voltages is raised.While Bob Carver used emitter resistors as low as .05 ohms (M1.5T) values in the range of .1~.5 ohms are more common for BJTs.For the IRFP240/9240 I think .5 ohms would be the minimum I would use.I am building a variation of this patent for biasing an IRFP240/9240 pair in a common source set up. http://www.delphion.com/cgi-bin/view...d/US04570129__
  Reply With Quote
Old 8th April 2001, 03:45 PM   #10
Geoff is offline Geoff  United Kingdom
diyAudio Member
 
Join Date: Apr 2001
Location: UK
djk

Thanks. I'm glad we're in agreement at last! A diagram could have saved us a lot of time and typing but I've not yet found a way of pasting one into a message.

I hope you're successful with your project.

Geoff


  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
Grid bias with Class A SE ? ThSpeakerDude88 Tubes / Valves 11 20th April 2008 06:01 PM
Class A speakers, as in BIAS Pan Multi-Way 3 10th February 2005 02:28 AM
Class G Bias Questions Randy Knutson Solid State 7 18th December 2004 03:28 PM
lm6172 class a bias? crippledchicken Parts 0 3rd September 2004 05:33 AM


New To Site? Need Help?

All times are GMT. The time now is 11:26 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright Đ1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2