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 18th May 2007, 02:22 PM   #1
diyAudio Member
 
Join Date: Sep 2005
Default Fully differential and pseudo-differential stages

I was looking over a project inspired by one of Anthony Holton's designs (aussieamplifiers.com), and I had a question about the input differential stage and the VAS stage that follows.

Mr. Holton's topology has a truly differential input stage (with tail supply), followed by another truly differential VAS stage (another differential pair with another tail supply). I would think that this would improve CMRR because of the two differential pairs.

==> Does this topology have other advantages, as compared to

Fully differential input followed by a pseudo differential VAS (which would be two single ended VAS stages following the differential input stage)

Conventional Fully differential input stage, output taken single-ended into a regular VAS stage

I was just curious...
  Reply With Quote
Old 18th May 2007, 03:11 PM   #2
ilimzn is offline ilimzn  Croatia
diyAudio Member
 
Join Date: Feb 2005
Location: Zagreb
One advantage would be that the diff input into diff VAS topology has strictly defined stage currents at all times, their maximum being the tail current of each stage. If dimensioned correctly and perhaps augumented with some diodes, it is quite easy to insure no part of the amp goes into heavy saturation when overdriven, which will give you very good clipping performance.

On the oposite end of the scle you have a single ended VAS, which in it's simplest incarnation has no limit on the VAS transistor current save for beta * tail current of the input differential. This is often a rather high current, and often completely overlooked, to the point where there are amps that will smoke their VAS transistor when the output protection engages. Whereas the differential VAS swings equal current both ways, a single edned VAS rarely does unless measures are taken to limit the VAS transistor current - which in turn implies some sort of emitter resistor for it, and consequently degeneration, which we don't necesairly want.

Pseudo-differential VAS stages would not work well without some means of local feedback or modification to the VAS stages as one would normally view them. This is because with no defined tail current as in the differential VAS, the two oposing VAS stages have no DC current defined, not even with feedback. So, again, we either have to add emitter degeneration in order to define DC operating points, or there has to be another means of setting it (usually they are all forms of feedback). The disadvantage is that you have lost another stage capable of differential amplification, which is what you really want.
  Reply With Quote
Old 18th May 2007, 10:57 PM   #3
CBS240 is offline CBS240  United States
diyAudio Member
 
CBS240's Avatar
 
Join Date: Aug 2005
Location: K-town
Hi

I built(building) an amp such as described, but using a common mode feedback. IOW, the tail current CCS of the first diff stage is dependent on the bias current of the second diff stage's CCS via this feedback. This establishes stable DC bias since the second stage current is already dependent on the leg currents of the input stage. It makes a bias loop that is all dependent on the second stage CCS operating point, set by a single resistor. Also bias is not affected as much by temperature since it is dynamic. I will have to work out of town for a few weeks, as DIYaudio is a 'hobby', but I should have some new PCB's to work with by then.... As for the last proto, it works better than I expected.
__________________
All the trouble I've ever been in started out as fun......
  Reply With Quote
Old 18th May 2007, 11:05 PM   #4
forr is offline forr  France
diyAudio Member
 
forr's Avatar
 
Join Date: Dec 2004
Location: Next door
Hi Rtarbell,
As this terminology is not very clear, could you provide some schematics to help the discussion ?
Thanks
  Reply With Quote
Old 20th May 2007, 04:41 AM   #5
diyAudio Member
 
Join Date: Sep 2005
==> As this terminology is not very clear, could you provide some schematics to help the discussion ?


Certainly! See attached...
The top circuit is what I'm referring to as a fully differential input stage followed by a fully differential VAS stage, while the bottom circuit is what I'm calling a "pseudo differential" VAS stage, because the two VAS transistors (Q9 and Q7) do not share a common tail supply current source.


Also, to CBS240:
==> I built(building) an amp such as described, but using a common mode feedback.
Yes! This is the main point of what I'm trying to experiment with in my simulation - a method of common mode feedback so that the DC bias points of all points in the circuit are well defined. I can find many references on CMFB for IC opamp designs (opamps that are powered by less than 5V), but I can't find anything on CMFB methods for high voltage, high power fully differential amplifiers (such as an FD audio amplifier). May I ask, how did you implement your CMFB?

Ultimately, I would like to successfully simulate this kind of circuit (FD means fully differential for my shorthand notation):
An FD input stage with a current mirror tail source (for high CMRR) and an active load for each leg of the diff pair (this active load would also be made with a current mirror)
This would be followed by an FD VAS stage that also has a tail current source and an active load for each VAS transistor. To date, as I simulate the circuits in TopSpice with my methods of CMFB, my DC bias points are terrible....



Attached Images
File Type: jpg bbb.jpg (38.0 KB, 1024 views)
  Reply With Quote
Old 20th May 2007, 08:21 AM   #6
forr is offline forr  France
diyAudio Member
 
forr's Avatar
 
Join Date: Dec 2004
Location: Next door
Thanks, Rtarbell, for your schematics. This is a very interesting discussion for me. I've sometimes seen a curious pseudo-differential stage loaded by a current mirror. The first fully differential stage I saw was the voltage stage of the JBL SA660 amplifier (1968, I think).


Hi CBS240,
I have been thinking for long of the same idea as yours It has been used in some Kaneda regulated power supplies.
The attached image is the basic schematics in an all NPN configuration. It can't be used with a split power supply (+/gnd/-) for an outpout voltage at 0V. To achieve this, it needs additionnal circuit to refer the input voltages of the second stage to the negative power supply. An intermediate common base PNP stage with base voltage refered to the positive power supply could do it. Maybe you have some other ideas to implement a common mode loop, so my question is the same as Rtarbell : May I ask, how did you implement your CMFB?
Attached Images
File Type: jpg dualdiffsamepolarity.jpg (10.6 KB, 814 views)
  Reply With Quote
Old 20th May 2007, 05:27 PM   #7
CBS240 is offline CBS240  United States
diyAudio Member
 
CBS240's Avatar
 
Join Date: Aug 2005
Location: K-town
Quote:
Originally posted by forr
May I ask, how did you implement your CMFB?

Certainly

Here is a simplified drawing of what Iíve done. By simplified I mean my circuit uses cascode input stage and VAS, along with a more complex mirror for Qís 11 & 12. I prefer J-fet inputs but I guess BJT would work also. Iím using very low current J-fets in an attempt to increase or eliminate R2 and R5. This means the input stage will be biased with just the base currents of the next stage, something around 10uA. In the last proto, the input stage bias is around 25uA.
You might be able to use a 2 transistor BJT CCS for Q1, but self bias J-fet here is simpler. R3 is chosen based upon the Pch J-fetís Vgs @ Id for the correct current through R1. I used http://www.fairchildsemi.com/ds/MM/MMBF5460.pdf for Q6, but I suppose it doesnít matter. Since a J-fet has no gate current, the gate can be used to sense the voltage on the CCS, Q4 & Q5 without affecting the CCS. When too much current tries to bias Q7, Q8 too heavily, the J-fet increases in conductance increasing voltage on R1 so current in Q1, and Q2/Q3 are reduced and bias less to Q7 & Q8, CMFB. The cascode Q9 is because most small J-fets donít like lots of volts. VAS bias is set by 0.6V/R4. The active bias makes the circuit temperature independent because we all know that when electrons flow through a transistor it creates heat and the temperature rise increases gain and reduces Vbe, and affects bias. Iím sure there are other ways to implement some type of CMFB, but this seems to work for me. Simulation results might be interesting, I havenít a good simulator so I just made a real circuit.


PS: I remembered that I made a thread about it a while ago. Not much discussion on CMFB though. The compensation isn't quite correct here but is too much hassle to change since there is a new circuit in the works, with some minor circuit changes, but more towards transistor arrays instead of completly descrete. Less component packages and better transistor matching with an actual PCB instead of veroboard. That POS still works and sounds great though, believe it or not.:xeye

New amp results
Attached Images
File Type: jpg simple.jpg (80.0 KB, 910 views)
__________________
All the trouble I've ever been in started out as fun......
  Reply With Quote
Old 25th May 2007, 11:21 PM   #8
forr is offline forr  France
diyAudio Member
 
forr's Avatar
 
Join Date: Dec 2004
Location: Next door
Thanks, CBS240, for publishing your very original schematics. CMFB is not a concept often seen implemented in power amplifiers, I like this way of stabilising the operating points.
  Reply With Quote
Old 26th May 2007, 02:55 AM   #9
CBS240 is offline CBS240  United States
diyAudio Member
 
CBS240's Avatar
 
Join Date: Aug 2005
Location: K-town
Hi

One good thing about this circuit is DC offset is low, at least in the circuit I built. It depends on how well you match the devices, but IMO not needing a DC offset pot is beneficial. The proto-topology check circuit has about 15 mVDC with an Av of 65. In the new PCB(still only in existence on my computer ) there is a place for the R2 & R5, but I would like to not use them and bias just base current through the input LTP legs. I left options like this in the PCB so as different compensation techniques can be experimented with. The circuit in the link uses both Miller compensation and Miller feedback. Would like to try a 2 pole scheme. I am still doing some final clean-up work on the PCB though. I always hate the anxiety when ordering boards.




__________________
All the trouble I've ever been in started out as fun......
  Reply With Quote
Old 26th May 2007, 09:59 PM   #10
forr is offline forr  France
diyAudio Member
 
forr's Avatar
 
Join Date: Dec 2004
Location: Next door
Hi CBS240,
CMFB applied to the differential input of an audio amp is not a subject much covered. I wonder if there were other advantages than stable DC operating points. For example, cascoded input differential stages with upper pair bias referenced to the emitters of the input pair are known to have a good common mode rejection. I wonder if CMFB does the same job, or even better.

I have once seen the circuit shown here
http://www.diyaudio.com/forums/showt...96#post1214396
with some explanations in an american book about analog circuits , but I have never been able to find it again. Has anyone an idea of what book it could be ?
  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



New To Site? Need Help?

All times are GMT. The time now is 10:10 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