Very simple class B amplifier - Page 7 - 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 13th December 2012, 05:49 PM   #61
Struth is offline Struth  Canada
diyAudio Member
 
Join Date: Aug 2012
Location: books at londonpower.com
Hi Guys

Op-amp internal circuits and modern power amp schematics are pretty much identical. The difference is in the die area for each BJT.

Similarly, an op-amp driving an EF stage is commonly used as a line driver or headphone output, with TO-92 devices for the output. Change those to TO-220s, TO-3Ps, etc and you have current capability to drive a speaker.

In the buffered opamp circuit, there is typoically just an R from one side of the bias circuit to the supply. It is best to split this and bootstrap it for maximum head room. The current from this R goes through the opamp output.

A common error in such circuits is leaving out a small compensation cap around the opamp - typically 10s of pF - and a small series R between the opamp output and buffer - 10-100R.

Also, why do you want to amplify DC voltage? Best to incorporate a cap in the shunt portion of the feedback loop - should be 470uF or higher for low low-f THD.

Another also, why do you want to have scratchy pot noise? best to capacitively couple from the input level control to the opamp so the bias current of the latter does not flow through the pot wiper.

In the zeal to reduce parts count, most people over-simplify the circuit to the point of serious performance degradation. In a compound amplifier as you are discussing, there are certain parts that are necessary to assure stability unless you are going to extensively test components. Very good performance is possible with this arrangement.

Fitting in with the cheapo goal, a dual opamp would cover two channels, since presumably you want stereo.

The bootstrapped opamp supply trick is much older than LT, as national Semiconductor showed it in their 1980 Linear app book - likely predates even that.

Have fun
Kevin O'Connor
__________________
London Power / Power Press Publishing
www.londonpower.com
  Reply With Quote
Old 3rd April 2013, 09:10 PM   #62
diyAudio Member
 
tiefbassuebertr's Avatar
 
Join Date: Jul 2005
Location: D-55629 Schwarzerden
Quote:
Originally Posted by tiefbassuebertr View Post
Thank you for this information.
A similar topology I have found in the magazine "Audio Amateur" 3/96 (THREE: 1996), start from page 22:
"10 Watt Class-A" by Dick Crawford.
Equipped with a dual OP-Amp "LT1124", one pair MOSFET IRFP150/RFH25P08 and an active bias part. Actually not exact matched for this topic, but through the great similarity in the circuit topology I mentioned here.
Perhaps one of the member can scan and upload this article.
Who can upload this article?
  Reply With Quote
Old 3rd April 2013, 09:43 PM   #63
diyAudio Member
 
nigelwright7557's Avatar
 
Join Date: Apr 2008
Location: Carlisle, England
The difference in component count between class b and ab is minimal.

For the sake of a Vbe multiplier might as well go the whole hog and design a class AB.
  Reply With Quote
Old 4th April 2013, 10:19 AM   #64
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
Some ClassAB ensure more correct output biasing without the need for a Vbe multiplier.

Diode biasing and resistor biasing are both valid forms of output bias control.
__________________
regards Andrew T.
  Reply With Quote
Old 8th April 2013, 09:08 AM   #65
diyAudio Member
 
tiefbassuebertr's Avatar
 
Join Date: Jul 2005
Location: D-55629 Schwarzerden
Quote:
Originally Posted by AndrewT View Post
Some ClassAB ensure more correct output biasing without the need for a Vbe multiplier.

Diode biasing and resistor biasing are both valid forms of output bias control.
In older push pull class AB power amplifiers there are to find in most cases diodes like that one from

http://www.ic72.com/pdf_file/z/179610.pdf
together with a variable resistor instead a Vbe multiplier.

But I don't understand why this is an advantage. Which benefit concerning the stability of the output biasing is here to expect? Are there existing investigations ?
  Reply With Quote
Old 9th April 2013, 02:57 PM   #66
seanvn is offline seanvn  Viet Nam
diyAudio Member
 
Join Date: Sep 2010
Maybe I posted a message about my pure Class B amplifier circuit on this forum before:
A Pure Class B Audio Amplifier
You can easily alter it for much higher output using Darlington output transistor pairs and 3 diodes in series rather than one. You may need to experiment with the value of R3 to get the voltage at the emitters of Q3 and Q4 approximately to half the supply voltage.
  Reply With Quote
Old 10th August 2013, 05:27 PM   #67
diyAudio Member
 
Join Date: Apr 2011
Location: Pretoria
Here is the latest version of the circuit. A couple of resistors has been added around the Vbe pot to make the range of adjustment smaller and setting up easier. There is now a resistor on the output of the opamp, as suggested by Struth.

To take the concept from idea to reality, I have laid out a PC board. It is single-sided to simplify construction. There is still one zero ohm resistor where I could not quite figure things out.

A new version with a diode string replacing theVbe multiplier is on the drawing board. I am also thinking of adding short circuit protection in the form of a VI limiter. Putting it in detracts from the design goal of having a low component count, but is it worth the risk? The circuit is good enough for something like an active speaker, but in the real world bad stuff happens.
Attached Images
File Type: jpg simple1.jpg (179.4 KB, 275 views)
File Type: jpg simple2.jpg (133.8 KB, 272 views)
  Reply With Quote
Old 10th August 2013, 07:41 PM   #68
Bigun is offline Bigun  Canada
diyAudio Member
 
Bigun's Avatar
 
Join Date: Jan 2009
Blog Entries: 2
It's usual to include a capacitor in series with R1, something around 47uF should be fine. The idea is that the feedback network will divide the output signal for music signals so that feedback is less than 100%, but for d.c. it will ensure 100% feedback. This ensures you have gain at a.c. but no gain at d.c. so that the d.c. offset at the output is more stable. It's usually necessary to employ a dc-servo circuit or a series output capacitor if you don't do this.

Another extra you could add, is a RC filter at the input, a 1k resistor after C4 and a 200p to 300p cap to signal ground - this will remove r.f. trying to sneak in.

R7 needs to be rated at 1W to 2W so you'll want to size this part on the pcb appropriately.

Perhaps you could make better use of the board space by increasing the footprint of the power supply rail capacitors. Heck, you might have space to include the power supply altogether, add the rectifier diodes. Then all you need to do is wire up the transformer !
__________________
"The test of the machine is the satisfaction it gives you. There isn't any other test. If the machine produces tranquility it's right. If it disturbs you it's wrong until either the machine or your mind is changed." Robert M Pirsig.

Last edited by Bigun; 10th August 2013 at 07:54 PM.
  Reply With Quote
Old 10th August 2013, 08:02 PM   #69
diyAudio Member
 
Join Date: Apr 2011
Location: Pretoria
Thanks for pointing out my mistake with the size of R7, Bigun. I missed that one.

The other suggestions look good as well, but it seems that the original idea of a low component count has been lost by the realities of real life.
  Reply With Quote
Old 11th August 2013, 03:07 PM   #70
diyAudio Member
 
Join Date: Apr 2011
Location: Pretoria
Here is an improved version of the PCB. No more links.
Attached Images
File Type: jpg simple2.jpg (134.3 KB, 201 views)
  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
Class D Vs Class I (Balanced Current Amplifier) rmsaudio Class D 29 26th January 2014 11:25 PM
simple class a amplifier renjiish Solid State 11 26th June 2007 06:09 AM
More simple class A jerluwoo Solid State 7 24th October 2006 05:19 PM
How to build a very simple class D amplifier!!!For a newbie. thomgun_lc Class D 8 11th September 2005 06:13 AM


New To Site? Need Help?

All times are GMT. The time now is 06:13 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