No miller compensation - 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 30th November 2004, 04:34 PM   #1
diyAudio Member
 
Join Date: Jun 2004
Location: -
Default No miller compensation

In my last prototype (very much like the classic design by D.Self but added cascode for long tail pair and hybrid CFP in power stage) did not use miller compensation at all but 50pF from VAS (collector) point directly to long tail pair negative input (like the basic NFB).

Now I wonder what disadvanteges this design may have?
My own first assumption was that there must exist TIM distortion in the VAS stage input. The simulation of the input and VAS stages together shows that without NFB nor compensation the open loop bandwith is about 20-30kHz (and amplification >100db). So may be TIM distortion is not probable, or is it?
  Reply With Quote
Old 30th November 2004, 06:12 PM   #2
Eva is offline Eva  Spain
diyAudio Member
 
Eva's Avatar
 
Join Date: Oct 2003
Location: Near the sea
Send a message via MSN to Eva
You should try the real circuit to know if it's stable or not. Simulation of the behavior of bipolar transistors at high frequencies is not accurate

Placing a miller capacitor across too many transistors causes unstability
  Reply With Quote
Old 30th November 2004, 06:29 PM   #3
The one and only
 
Nelson Pass's Avatar
 
Join Date: Mar 2001
Default Re: No miller compensation

Quote:
Originally posted by Electron
In my last prototype (very much like the classic design by D.Self but added cascode for long tail pair and hybrid CFP in power stage) did not use miller compensation at all but 50pF from VAS (collector) point directly to long tail pair negative input (like the basic NFB).

Now I wonder what disadvanteges this design may have?
My own first assumption was that there must exist TIM distortion in the VAS stage input. The simulation of the input and VAS stages together shows that without NFB nor compensation the open loop bandwith is about 20-30kHz (and amplification >100db). So may be TIM distortion is not probable, or is it?
This form of compensation forms a local high frequency loop
rather than lag compensation. If often works very well, and I've
used it many times in the "good old days" when my circuits were
more complicated.
  Reply With Quote
Old 30th November 2004, 07:22 PM   #4
diyAudio Member
 
Join Date: Jun 2004
Location: -
Eva, my stability observations are indeed from the real amp not from simulation, however I have not tested it with complex load.

Nelson Pass, how should I understand your comment "good old days" :-)
You are right it is not lag compensation but I don't figure out if the miller capacitor is needed at all. The role of that compensation is to shape the phase and amplitude behaviour so that when the real NFB is added the amp is still stable.

The differential stage and VAS stage together closed (by capacitor) is actually very much like closing the pnp driver and npn power device into CFP kind of loop. Only difference is that the both tails of pair are used to drive second stage (VAS).
Without the miller capacitance the VAS act more like voltage driven current source even on higher fregs. but when miller is added VAS acts like current drive current source. I beleave this is good but it leads to the higher impedance in the VAS output which is not good when driving nonlinear driver BJTs of the output stage. On the other hand there is much amplification to be used. This sounds like the old BJT amps with TIM and other problems but with exception that the diff stage and VAS stage together seems to be fast.

A bit similar approach is also used by A. Holton in his amp. The difference is that he used also miller compensation.

So, what is wrong in my thinking? There must be a reason why this way is not used.
  Reply With Quote
Old 30th November 2004, 10:42 PM   #5
The one and only
 
Nelson Pass's Avatar
 
Join Date: Mar 2001
Most of the time if you need lag compensation you will
need it just driving an 8 ohm resistor, or even no load at all,
and the symptom will be oscillation, which is easy to see if you
have a scope.

You can also look for ringing on square waves.

Still, feeding the high frequency signal back to the
(-) input is usually a better approach - assuming it works.
  Reply With Quote
Old 30th November 2004, 11:25 PM   #6
johnnyx is offline johnnyx  United Kingdom
diyAudio Member
 
Join Date: Jan 2004
Location: manchester
The late J.L.L. Hood used that form of compensation in his mosfet amp. He says (in his book, "valve and transistor audio amplifiers"), that it is better than miller comp at avoiding TIM, and is better with reactive loads. Square waves are reproduced better too. He had a resistor in series with the cap though, and chose the values appropriately for the phase shifts of the amp.
  Reply With Quote
Old 1st December 2004, 01:44 AM   #7
thanh is offline thanh  Viet Nam
diyAudio Member
 
thanh's Avatar
 
Join Date: Jan 2004
Location: ho chi minh city
Send a message via Yahoo to thanh
hi john! can you send to me a copy of his schematic?
__________________
Justice for Victims of Agent Orange
http://www.petitiononline.com/AOVN/Thank all of you!
  Reply With Quote
Old 1st December 2004, 03:22 AM   #8
diyAudio Member
 
wrenchone's Avatar
 
Join Date: Feb 2004
Location: Silicon Valley
I've also used AC feedback from the VAS stage several times with good results. As a starter, I set the capacitor to break against the global feedback resistor connected at the output at about 100kHz or so, with a resistor to introduce a zero at 10x this frequency. I then verify performance with square wave response (keeping the amp out of saturation) and/or doing a gain/phase plot with the analyzer at my work place. I haven't had an amp oscillate in a long time, and even then it was inner loop oscillation in the ouput compound pairs (don't use them any more).
  Reply With Quote
Old 1st December 2004, 05:38 AM   #9
jcarr is offline jcarr  United States
diyAudio Member
 
Join Date: Jul 2002
Location: Tokyo, Japan
I am running a power amp prototype which likewise uses no miller caps.

The general topology is a cascoded JFET input differential into a summing current mirror, a cascoded VAS with current-sink load, and a compound output stage (with an additional distortion-cancelling mechanism built-in), biased to about 200mA per device. Current limiting and shut-down safety circuitry are also included.

The power supply is based on a new low-noise switching circuit (patent applied for). Size (for a 500W power supply) is perhaps 8x16cmx8cm.

The compensation consists of an RC network at the input of the VAS, a shunt cap from the VAS output to the negative input node, and another RC network at the input of the Sziklai output stage (negative side only). There is an output RC (Zobel) network, but no output series inductor/resistor network.

It is completely stable, and despite that this is a Class AB design and runs on a switching power supply, it sounds very good. We've compared this against various power amps (including a Halcro and Accuphase A-60), and the sound was judged to be at least comparable overall, and superior in areas such as smoothness and low-level timbral and dynamic resolution. OTOH, when it comes to a total sense of ease and unlimited headroom into demanding speaker loads, the Halcro still has the edge.

hth, jonathan carr
__________________
http://www.lyraconnoisseur.com/, http://www.lyraaudio.com
  Reply With Quote
Old 1st December 2004, 11:54 AM   #10
Bonsai is offline Bonsai  Taiwan
diyAudio Member
 
Bonsai's Avatar
 
Join Date: Jul 2003
Location: Taipei, Taiwan
This type of compensation does work well, but you are taking feedback around the amp before the output stages - so distortion at high frequencies goes up because of crossover. Agree that simulation at high frequencies is probably not accurate with the generic models available for most spice work, but it still provides an indication. You will hardly notice the difference at 1KHz, but at 20KHz there is a significant increase in distortion (cross over). I only use VAS to -ve input feedback if I need to gain a little phase margin - but not as a main compensation scheme.

rgds
__________________
bonsai
Amplifier Design and Construction for MUSIC! http://hifisonix.com/
  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
Miller compensation with a series RC network? Bricolo Parts 5 27th February 2010 01:19 PM
Cascode VAS miller compensation Rafael L Solid State 9 5th March 2009 01:06 AM
Overcoming Miller dsavitsk Tubes / Valves 3 8th June 2007 10:40 PM
Miller compensation - pros/cons? Joel Tubes / Valves 25 14th January 2006 09:20 PM


New To Site? Need Help?

All times are GMT. The time now is 03:14 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