Class i and siblings - Page 12 - 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 2nd January 2012, 10:48 PM   #111
ontoaba is offline ontoaba  Indonesia
diyAudio Member
 
ontoaba's Avatar
 
Join Date: Nov 2009
Location: Kudus, Malang, Dieng
Quote:
Originally Posted by Edmond Stuart View Post
I don't get spikes at all, only a tiny wobble .....
If you get spikes then something is fundamentally wrong, either your schematic or your simulator.
.....
Sorry I am not explain before, that I am using less than 50mA (just30or20) bias and lower the loop gain at HF for stability, so the spike normally appeared. There also no significant spike at 100mA or higher bias.

Looks like that hitachi patent used for HAxxxxx IC, and sadly they sound not good enough, in other side LA4xxx from sanyo using common simpler technique with much better to hear. Wrong technique that been patented, just a waste. I agree that the patent isn't worth.

I see that pioneer IC similar with kendall classI, don't know for sure.
  Reply With Quote
Old 3rd January 2012, 04:44 PM   #112
diyAudio Member
 
Edmond Stuart's Avatar
 
Join Date: Nov 2003
Location: Amsterdam
Default Spikes

>I am using less than 50mA (just 30 or 20) bias
Hey man, that's Class-B. No wonder you get spikes.
For two reasons that doesn't work, or at least far from optimal:

1. It's generally agreed that MOSFET OPSes should be operated in class-AB. That means a quiescent current of at least 100mA (150mA is a good compromise between low distortion and excessive heat production).

2. Auto bias circuits (Class-I, AB2, LT1166 etc) are based on the so called geometric mean concept. IOW, they try to keep the product of the P and N channel FET currents at a constant level. You can't use this technique for class-B output stages; only for class-AB. Why?

Suppose Iq = 30mA and the maximum current is 3A (100 times more). That means -for a constant product- that the trannie(s) of the other gender should draw 30mA/100 = 0.3mA. This tiny current times a typical sense resistor of say 0.33 Ohm gives 0.1mV.
A simple circuit built around a few discrete poorly matched trannies can never handle such small a voltage with sufficient accuracy. An ideal precision multiplier might handle it, but only at low frequencies. Above 100Hz or so, capacitive effects (gain roll-off etc) affect the accuracy too much and the circuit goes berserk, i.e. spikes.

Now, suppose Iq = 150mA and the maximum current is (again) 3A, i.e.20 times more. In this case the current through the other tranny is 150mA/20 = 7.5mA, being 25 more than in the previous case and it gives 2.5mV across the sense resistors. With reasonable matched trannies this voltage is manageable. But to be on the safe side (the circuit should work under a wide range of temperatures and components tolerances), I even prefer a much higher minimum current in the order of 50mA. As a result, no spikes at all and a much lower distortion. Happily, you already figured that out by yourself.

Cheers,
E.

edit: NB, in Kedall's class-I BJT OPS the bias current is 135mA.
__________________
Een volk dat voor tirannen zwicht, zal meer dan lijf en
goed verliezen dan dooft het licht…(H.M. van Randwijk)

Last edited by Edmond Stuart; 3rd January 2012 at 04:52 PM.
  Reply With Quote
Old 3rd January 2012, 08:59 PM   #113
miralin is offline miralin  Russian Federation
diyAudio Member
 
Join Date: Dec 2010
Default Product

Hi Edmond
Some ten years ago I’ve analyzed a circuit that is now called “Class-I” (don’t know why – maybe ask Mr.Nakayama?). This cirquit (unlike the most of other nonlinear ones) yields to precise mathematical analysis without simulations or approximations. The result shows that the product (Ip*In) is not constant. The relationship is slightly more complex.
Can’t say anything about other circuits. May be your simulation software can plot the product against output current like the currents themselves in Fig.8 of your basic “AutoBias_II.html” for those and new variants. I’m especially interested of your ingenious crossconnection solution.
Cheers Mir
PS. I think that either precondition on or relationship between Ip&In will disturb the main condition:
Ip-In=G*Vin, where resulting transconductance G is constant (except those which are the consequences of this condition).
  Reply With Quote
Old 3rd January 2012, 09:06 PM   #114
ontoaba is offline ontoaba  Indonesia
diyAudio Member
 
ontoaba's Avatar
 
Join Date: Nov 2009
Location: Kudus, Malang, Dieng
Thanks for useful explanations, I am, just showing there, it fit with XM better than my OPS. In this case, current sense isn't just resistor, nor just diode/schottky, but it has special arrangement of some components.

Hi, Edmond, can't it modified to something that not need transistor matching? Because pioneer that uses special IC also sometimes one channel perform right, and other channel just worse.
Pioneer has 204mA bias.
  Reply With Quote
Old 4th January 2012, 05:28 PM   #115
diyAudio Member
 
Edmond Stuart's Avatar
 
Join Date: Nov 2003
Location: Amsterdam
Quote:
Originally Posted by miralin View Post
Hi Edmond
Some ten years ago I’ve analyzed a circuit that is now called “Class-I” (don’t know why – maybe ask Mr.Nakayama?).
Hi Mir,

Kendall Castor-Perry was the man who dubbed it Class-I. I'm not sure who's the first inventor, he or Nakayama, though I think it was Kendall.

Quote:
This circuit (unlike the most of other nonlinear ones) yields to precise mathematical analysis without simulations or approximations. The result shows that the product (Ip*In) is not constant. The relationship is slightly more complex.
Can’t say anything about other circuits. May be your simulation software can plot the product against output current like the currents themselves in Fig.8 of your basic “AutoBias_II.html” for those and new variants.
You're right. The relation is more complex and the product isn't really constant. That's why I've said that the circuit only approximates a constant product or tries to keep it constant. Regarding the AB2 circuit as posted on this forum, this one behaves the same as depicted by fig.8 on my website. The pictures below, black curve, shows the product. As you see, far from constant. The main reason is that the minimum Ie has been considerably elevated to 40mA on purpose (for safety reason).

Quote:
I’m especially interested of your ingenious cross-connection solution.
Cheers Mir
The cross-connection reduces the feed-through of switching currents (of the sense trannies) into the main signal path. As a result, less induced distortion from these trannies. However, it has hardy any effect on the Ip*In product.

Quote:
PS. I think that either precondition on or relationship between Ip&In will disturb the main condition:
Ip-In=G*Vin, where resulting transconductance G is constant (except those which are the consequences of this condition).
That's a very good point. Also Damir put a (related) question about this subject. For a BJT OPS there exist the so called Barny Oliver optimum point (26mV across both emitter resistors), which provide a more-or-less constant gm over the whole range of output currents.
The schematic on page 41 however, seems to violate this rule, as the sum of voltages across R35 & R36 is much higher: 66mV. That means gm doubling, isn't it? Nope! Apparently, it's all a matter of definition. In this case we should define Vin as the mean voltage of the top and bottom driver input. Thus Vin = 0.5 * ( V(B1) + V(B2) ) and the ouput current is I(R37).
So the incremental transconductance becomes: gm = del( I(R37) / del (0.5 * ( V(B1) + V(B2) ) - V(out) )
To my own surprise, gm is almost perfectly constant. At Vout =0 there is a tiny dip of about 2.7%. No gm doubling at all. See the green curve.

In case of a MOSFET output stage, the picture is quite different. There is no optimal bias point, with or without auto bias. Gm gets reasonably flat at only a very impractical Rs of 1 Ohm or more and a bias current of at least 1A. With Rs=0.22 Ohm and Iq = 150mA my simulation shows a gm variation of about 1:3. Except class-heat, I'm afraid there's nothing we can do about it.

Cheers,
E.
Attached Images
File Type: png AB2-PP-GM.png (23.7 KB, 245 views)
__________________
Een volk dat voor tirannen zwicht, zal meer dan lijf en
goed verliezen dan dooft het licht…(H.M. van Randwijk)
  Reply With Quote
Old 5th January 2012, 11:06 AM   #116
dadod is offline dadod  Croatia
diyAudio Member
 
dadod's Avatar
 
Join Date: Apr 2006
Location: Zagreb
Quote:
Originally Posted by Edmond Stuart View Post
That's a very good point. Also Damir put a (related) question about this subject. For a BJT OPS there exist the so called Barny Oliver optimum point (26mV across both emitter resistors), which provide a more-or-less constant gm over the whole range of output currents.
The schematic on page 41 however, seems to violate this rule, as the sum of voltages across R35 & R36 is much higher: 66mV. That means gm doubling, isn't it? Nope! Apparently, it's all a matter of definition. In this case we should define Vin as the mean voltage of the top and bottom driver input. Thus Vin = 0.5 * ( V(B1) + V(B2) ) and the ouput current is I(R37).
So the incremental transconductance becomes: gm = del( I(R37) / del (0.5 * ( V(B1) + V(B2) ) - V(out) )
To my own surprise, gm is almost perfectly constant. At Vout =0 there is a tiny dip of about 2.7%. No gm doubling at all. See the green curve.

Cheers,
E.
Hi Edmond,
I stated that Barny Oliver optimum point is 26mV across one emittor resistor(plus re of the transistor) not both emitter resistors, so you are not so far of it.
Damir
  Reply With Quote
Old 5th January 2012, 12:24 PM   #117
diyAudio Member
 
Edmond Stuart's Avatar
 
Join Date: Nov 2003
Location: Amsterdam
Default VT

Hi Damir,

You are right: 26mV across one emitter resistor. That was a stupid mistake, sorry (I was misled by D. Self who defined Vq as the sum of both voltages).
With 26mV across each of the emitter resistors (and keeping Iq equal), distortion drops by 10-20%.

Cheers,
E.
__________________
Een volk dat voor tirannen zwicht, zal meer dan lijf en
goed verliezen dan dooft het licht…(H.M. van Randwijk)

Last edited by Edmond Stuart; 5th January 2012 at 12:31 PM. Reason: deleted 'IMHO, not worth the extra trouble.'
  Reply With Quote
Old 5th January 2012, 12:58 PM   #118
diyAudio Member
 
Edmond Stuart's Avatar
 
Join Date: Nov 2003
Location: Amsterdam
Default VT

Hi Damir,

It appears to be quite simple to lower the voltage across the emitter resistors to just 26mV. I was afraid that it should compromise the temperature stability, but that wasn't the case. It's even slightly better, see table.
Simply insert two 3.9R resistors (R38 & R39) as shown below and decrease RE from 0.33 to 0.25R.
Code:
Ta (C)  Iq (mA)
0       105.4
25      105.4
50      106.4
Cheers,
E.
Attached Images
File Type: png AB2-VT.png (7.2 KB, 218 views)
__________________
Een volk dat voor tirannen zwicht, zal meer dan lijf en
goed verliezen dan dooft het licht…(H.M. van Randwijk)
  Reply With Quote
Old 5th January 2012, 03:26 PM   #119
diyAudio Member
 
Edmond Stuart's Avatar
 
Join Date: Nov 2003
Location: Amsterdam
Default The Barny Oliver optimum point

For those who prefer an optimal bias current over a larger temperature range, make R8=R9=R11=R12=1Ohm and adjust the current sources to get an Iq of 100mA at room temperature again (page 41, R7=R10=107 Ohms). Now Iq is proportional to the abs. temp. so that the voltage drop across the emitter resistors equals the thermal voltage VT=kT/q.

Cheers,
E.
__________________
Een volk dat voor tirannen zwicht, zal meer dan lijf en
goed verliezen dan dooft het licht…(H.M. van Randwijk)
  Reply With Quote
Old 5th January 2012, 04:37 PM   #120
dadod is offline dadod  Croatia
diyAudio Member
 
dadod's Avatar
 
Join Date: Apr 2006
Location: Zagreb
Quote:
Originally Posted by Edmond Stuart View Post
For those who prefer an optimal bias current over a larger temperature range, make R8=R9=R11=R12=1Ohm and adjust the current sources to get an Iq of 100mA at room temperature again (page 41, R7=R10=107 Ohms). Now Iq is proportional to the abs. temp. so that the voltage drop across the emitter resistors equals the thermal voltage VT=kT/q.

Cheers,
E.
What about R38 and R39, not needed??
Damir
  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-AB meets Class-D: Yamaha's EEEngine Topology - where are this Diy Projects? tiefbassuebertr Solid State 29 14th April 2014 04:12 PM
Collection of Class B topologies <100mA Idle and Sound closest by Class A tiefbassuebertr Solid State 37 27th July 2012 08:04 AM
Can a Class AB PP amp be said to be operating in Class A at low signal levels? ray_moth Tubes / Valves 19 23rd January 2009 07:52 PM
How about a round-up of Class A kit power amps, or collectable vintage class A? Brisso57 Solid State 4 14th February 2007 10:30 AM


New To Site? Need Help?

All times are GMT. The time now is 08:58 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