N-Channel mosfet amplifier with servo bias - Page 2 - 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 16th September 2004, 07:49 PM   #11
f4bok is offline f4bok  France
diyAudio Member
 
Join Date: Aug 2004
Location: lyon
Hi cunningham
I will try many ampop at the input, i have some ad846 and i dont know what to do with them, they have a 450V/s slew rate but i don't think i'll need that.
I thing the 2 mosfets are working the same way : Vgs are both created by a current passing through a resistance connected to the source. The top res is not connected to the output. I will connect the top res under the 0.22R of the source and add one in the source of the bottom one, then current gain of the (output devices+R18,R19) will be approx 230 . That is not more than a darlington in bipolar configuration.
Bias (without compensation) is only made by the voltages at Q1 and Q3 bases so the output swing of the ampop must stay low to keep the same bias in the top mosfet then the bottom one is driven and viceversa (it's chance that id=f(Vgs) is flat for the bias zone and vertical after). Thath's why it needs a lot of open loop gain after the ampop. I want to keep an AB class and 2 N-Channel because i have a full gerkin pot of them !!
Bias current will be about 100mA.
What do you think of the fact that, the bias current decreases in the one device then the current increases in the oposite one ?
Rgds
  Reply With Quote
Old 16th September 2004, 08:25 PM   #12
f4bok is offline f4bok  France
diyAudio Member
 
Join Date: Aug 2004
Location: lyon
Hi Alaskanaudio
Tks for the information about LT1166, it seems to be a very easy way to make a bias control and currents limits. What i found about it is that it has been made for a complementary output. I did not find any on websites of french electronic parts sellers.
I put a irfp460 on a heatsink, tuned Vgs to get Id=100ma (Vrail 36V and a lamp in the drain) and had 3.5V at Vgs. I have increased current to 3A to warm it to 80C and returned to 100mA, Vgs was then 200mA. (at 20C it needs 3.6V to have 200mA). I think 0.1V is a "big" values and easy to compensate.
I'm right with you, compensating can't generate unstability as servo coulld do, but, what i want to try, is a very slow corrective closed loop because temperature does not move fast with a big inertia heatsink.
I have not understand yet how works the controller with dual opto isolator you are describing but i'm interrested in it. Could you post a link vhere i could found a schematic ?
Tks for your reply
Rgds
  Reply With Quote
Old 18th September 2004, 04:32 AM   #13
diyAudio Member
 
Join Date: Sep 2001
Location: Fairbanks, Alaska
Default Opto isolation bias control

f4bok,

In regards to using dual opto isolators for controlling bias in conjunction with a opamp. I have searched all my old computer files and have not been able to find the circuit diagram. I will continue to look for it. But I may have to draw it into a simulator from memory. My memory however does not recall all details.

Johannes
  Reply With Quote
Old 19th September 2004, 05:25 AM   #14
diyAudio Member
 
Join Date: Sep 2001
Location: Fairbanks, Alaska
Default The OPTO isolation circuit

I have tried to remember the isolation circuit I mentioned before in this tread. I believe that the link to the circuit I have drawn below is very close to being correct. It offers a voltage or current input and can convert that into an isolated current output. The circuit shown used a dual OPTO but a three or four section OPTO can be used if more isolated outputs are required. The circuit is not a complete bias system; it only covers the isolation section. Make component or circuit changes as required.

The PDF file is here. Some older versions of acrobat reader will not be able to read the file. http://www.audioamps.com/Misc/OPTO%20Bias.pdf

This circuit idea was originally published in Electronic Design Magazine in the early 1990s. The circuit listed here was drawn strictly from memory so it may not be exactly alike. Component values listed were tested in Multisim and may need to be optimized for best input to output current tracking. The tracking range is roughly 0 to 20ma. A larger range is possible. OPTO devices with MOSFET outputs may also be used.

Accuracy is enhanced by the use of multiple OPTO devices in one package which also provides good thermal tracking between the feedback and isolated output section.

Johannes
  Reply With Quote
Old 22nd September 2004, 06:32 PM   #15
f4bok is offline f4bok  France
diyAudio Member
 
Join Date: Aug 2004
Location: lyon
Hi Alaskanaudio
Tks for schematic. Its a good way to have linear output , the 2 optos, in the dual opto must have exactly the same current transfert ratio. I made a prototype of the amplifier, changed many values, it works all right now without the auto bias control, i'll do it in october and will post a new schematic. I have a little non-linearity of voltage output but i'm not sure beacause the output load is made of 2 // 15 Ohm big wired res. slew rate is 30V/s with an opa27, 33pf in direct feed back of the ampop, 5.6pf in the output feedback, and 5.6pf between BandC of Q 2and Q6 (tip42c today). I did not measure distorsion yet.
  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
Bias control problem with n-channel amplifier (Anthony E. Holton) Hielke02 Solid State 4 1st May 2011 06:54 PM
needed n channel mosfet amplifier zerohead_ak47 Solid State 8 6th June 2007 06:17 PM


New To Site? Need Help?

All times are GMT. The time now is 03:15 AM.


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