| 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? |
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| 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 |
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| Nelson Pass |
| 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. |
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| Electron |
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. :( |
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| Nelson Pass |
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. |
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| johnnyx |
| 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.:) |
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| thanh |
| hi john! can you send to me a copy of his schematic? |
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| wrenchone |
| 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). |
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| jcarr |
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 |
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| ACR |
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 |
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| Electron |
| quote: | Originally posted by ACR
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. |
With the compensation I explaned and without main NFB loop there is as much (or actually more) gain and phase margin left for NFB than there is with miller compensation.
The raw gain at 20kHz is >100db and after this compensation there is still 50db left. The other 50db what was used for compensation, linearises the output of VAS and lowers the output impedance of VAS too. Unfortunately the phase margin is coming a bit worse however the phase response if flat -90deg.This is similar to miller compensation except by miller the phase margin runs out too soon.
Please explain why there is a significant increase in distortion (cross over). |
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| PB2 |
I'd like to offer another question as a thought exercise:
Take an an amplifier such as the Citation or Tiger as a simple example and open the loop, no feedback. Front end DC offset will be amplified and not corrected by feedback so let's assume that there's no imbalance and the output has a 0V quiescient point.
Remove all feedback, compensation, all caps except for the power supply caps and assume they're ideal.
If we sweep frequency and measure distortion with an 8 ohm load, let's say at 10W avoiding the crossover issue, there's a significant increase at 20 kHz as compared to 1 kHz.
Why? What is the most significant cause?
What would be required but perhaps not physically realizable to have no rise in distortion from 1 kHz on up? |
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| dimitri |
> Remove all feedback
> there's a significant increase at 20 kHz as compared to 1 kHz
Just measure the frequency dependence of current consumption (from the power supply) without load. It will increase with the frequency – the answer is one step forward. :D |
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| PB2 |
That's a hint, sounds more like testing CMOS digital logic, but OK.
Anyone? |
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| AndrewT |
Hi,
JLH went further, stating in his various articles and book that the VAS to inverting input feedback loop would sound better even though he accepted that distortion was not as low as Miller Comp. Basically his major claim was improved sound quality.
He also showed the location of about 4 other feedback mechanisms that seemed to be required to optimise the stability and response characteristics. I think this indicates that it is more difficult to set up than Miller comp. I have seen it stated by a number of authors that Miller comp is a simple way to ensure stability and reading between the lines I think they are suggesting that for DIY this gives a satisfactory amp that swaps qualtiy for ease of reproducability.
Finally a large improvement in slew rate and avoidance of transient distortion results from avoiding Miller comp. The Miller method is limited by the current ability of the LTP to supply or source the VAS cap. Again this is supported by a number of authors.
regards Andrew T. |
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| ACR |
Electron, what I meant is that the crossover distortion in the output is not corrected as well as if you had NOT taken the feedback from the VAS collecter to the -ve input.
I have to add that in the designs I have been working on I only have 15 to 20dB feedback at 20Khz - so not the 50dB you are working with around the total loop at 20KHz.
rgds |
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| johnnyx |
| I think that this compensation scheme is better for the Hitachi/Maplin MOSFET amp discussed in this thread, than the Miller type. With a differential VAS, the two halves are loaded by different impedences, so Maplin's version doesn't seem right. Hitachi use one miller cap, but this method would be better, I think. :) |
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| PB2 |
Backing up with this question, how about we start with why the frequency response rolls off as we go up in frequency, what is the major cause of loss of voltage gain?
Same conditions as below:
I'd like to offer another question as a thought exercise:
Take an an amplifier such as the Citation or Tiger as a simple example and open the loop, no feedback. Front end DC offset will be amplified and not corrected by feedback so let's assume that there's no imbalance and the output has a 0V quiescient point.
Remove all feedback, compensation, all caps except for the power supply caps and assume they're ideal.
If we sweep frequency and measure distortion with an 8 ohm load, let's say at 10W avoiding the crossover issue, there's a significant increase at 20 kHz as compared to 1 kHz.
Why? What is the most significant cause?
What would be required but perhaps not physically realizable to have no rise in distortion from 1 kHz on up? |
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| thanh |
I have just tried to made "no miller compensation" but failed
I use 47p |
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| Electron |
| quote: | Originally posted by thanh
I have just tried to made "no miller compensation" but failed
I use 47p |
What do mean you failed? |
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| thanh |
| quote: | Originally posted by Electron
What do mean you failed? |
my amp has no stable . The volume is set at zero but i still hear some "boop ..boop" sound from speaker
oh ! oop ..oop ... i did it again ( a fun song
:( |
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| PB2 |
Thanh, sounds like you tried this in a real amp? I believe it was meant to be a thought exercise. At least one must consider another way to provide stability if the Cdom cap is removed.
Experimenting with compensation in real power amp circuits can often lead to smoke unless current limiting is used in the power supply.
Another thought question: where is the real Miller cap, and what is unusual/interesting about it? Why is it called Miller capacitance? |
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| thanh |
| quote: | Originally posted by PB2
Thanh, sounds like you tried this in a real amp? I believe it was meant to be a thought exercise. At least one must consider another way to provide stability if the Cdom cap is removed.
Experimenting with compensation in real power amp circuits can often lead to smoke unless current limiting is used in the power supply.
Another thought question: where is the real Miller cap, and what is unusual/interesting about it? Why is it called Miller capacitance? |
yes ! i tried to do that thing with real amp .The end, i rebuilt Cdom 100p and 1n |
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| sajti |
| quote: | Originally posted by thanh
yes ! i tried to do that thing with real amp .The end, i rebuilt Cdom 100p and 1n |
Thanh,
if You have one proper designed amplifier, with Miller type compensation, and You want to modify it, than You have to redesign the compensation. It's not as easy, to move the Miller capacitor to the other location.
The Hood type compensation use larger value capacitor, due the missing Miller effect...
sajti |
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| sajti |
| quote: | Originally posted by PB2
Thanh, sounds like you tried this in a real amp? I believe it was meant to be a thought exercise. At least one must consider another way to provide stability if the Cdom cap is removed.
Experimenting with compensation in real power amp circuits can often lead to smoke unless current limiting is used in the power supply.
Another thought question: where is the real Miller cap, and what is unusual/interesting about it? Why is it called Miller capacitance? |
Miller capacitance is between the collector, and the base of the transistor (same as the input, and output of inverting amplifier). The most important, if You want to transform it into the input of the stage, You have multiply the vale with (1+stage gain). So with 100pF and A=100 resuslts 10nF equal input capacitance
sajti |
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| PB2 |
Thank you Sajti,
Yes, the interesting point is that the effective input capacitance is multiplied by the stage gain.
For those who're new to this: The Miller effect was first documented by John Miller in the 1920s while investigating the characteristics of tube amplifiers. I believe that the main point was that the devices own internal parasitic capacitance provided a feedback path, and that while this parasitic capacitance was small it had much more influence than expected because it is multiplied by the voltage gain of the amplifier.
We see the same thing in transistors with the capacitance of the reverse biased BC junction.
Figure 7 on this page provides an overview:
http://carcino.gen.nz/tech/elec/millereffect.php
Here's an interesting link, BTW nice control theory page, about half way down he has a link to a pdf with the original writeup by John Miller:
http://64.233.161.104/search?q=cache:TrngNR6DzMAJ:web.mit.edu/klund/www/+%22miller+effect%22+history&hl=en
This answers part of the question that I posed, the open loop HF response rolls off because of junction capacitances with the miller capacitance in the stage with high voltage gain dominating because of the voltage gain multiplication factor. However, this does not answer why the distortion is frequency dependent. |
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| sajti |
I don't use Miller compensation. I use series RC network at the input of the VAS stage. I think it gives better compensation, and less risk for the oscillation.
sajti |
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| jhead |
| speaking of Miller cap I have a BK ST-140 amp that use' s a 320pf silver mica cap the reddish epoxy covered type I believe to be rather generic has anyone tryed useing polystrene or what might be considered higher quality silver mica's I.E. Soshin silver mica's as miller compensation caps ? |
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| jhead |
| The BK ST-140 use's a pair of Hitachi 25SK135/1SJ50 on it's output stage it also I find sort of high value 47uf 16Volt cap on it's input maybe a 4.7uf Blackgate N series would be better served here. |
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| janneman |
| quote: | Originally posted by PB2
[snip]This answers part of the question that I posed, the open loop HF response rolls off because of junction capacitances with the miller capacitance in the stage with high voltage gain dominating because of the voltage gain multiplication factor. However, this does not answer why the distortion is frequency dependent. |
Hi,
Would that not be because as frequency rises, the cap impedance goes down. That has two effects I think: first, it shunts the output Z of the a gain stage therefore giving less gain thus less for the feedback (assuming global fb) to work its magic. Second, the stage itself could have more distortion working in an increasingly non-linear load (the cap is also level dependent).
Dimitri?
Jan Didden |
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| dimitri |
Jan, if you discussed intermediate stage (VAS) I am doubtful that it distortion characteristics will be frequency dependent, while the gain is not frequency dependent. Thereby I could not find frequency dependent distortion mechanism in intermediate stage, although it sure exists it the output stage.
I sorted out my old piles and found 'Programmable audio attenuator' in WW, May 80. Are your hobbies, apart from audio, still include reading science fiction? ;) |
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| janneman |
| quote: | Originally posted by dimitri
Jan, if you discussed intermediate stage (VAS) I am doubtful that it distortion characteristics will be frequency dependent, while the gain is not frequency dependent. Thereby I could not find frequency dependent distortion mechanism in intermediate stage, although it sure exists it the output stage.
I sorted out my old piles and found 'Programmable audio attenuator' in WW, May 80. Are your hobbies, apart from audio, still include reading science fiction? ;) |
Well, I thought that the miller cap being nonlinear with level, AND needing more charge/discharge current with freq, would lead to higher HD with frequency. But I have no proof for it.
Om WW May80: I sold 500 sci-fi pockets last year to a 2nd hand bookstore (De Slegte for my Dutch friends). Moved on to other interests. BTW, do you know I have troubles following my own math in that article? I wonder how many really read it;)
Jan Didden |
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| dimitri |
| jan, you mean _external_ Miller cap or p-n junction? As the cap is low value, we can use extra high quality cap here (vacuum) :bigeyes: |
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| janneman |
| quote: | Originally posted by dimitri
jan, you mean _external_ Miller cap or p-n junction? As the cap is low value, we can use extra high quality cap here (vacuum) :bigeyes: |
Actually, I was thinking about the intrinsic p-n junction cap, but now realise you guys may have been talking about the external brute force:D comp cap. That one, of course, has no (almost none) freq dependent HD.
Jan Didden |
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| PB2 |
Jan,
You've hit all the points, lets ignore the external Cdom cap for the moment. Even if the CB junction capacitance was ideal it would have an impact on frequency response just as you state, and of course this leads to less feedback and higher distortion in the closed loop case. But let's talk open loop for now.
You also seem to be suggesting that the CB junction capacitance is nonlinear, yes this is very true and it is the answer I was looking for. A reverse biased junction makes a fairly good varactor diode, so the large signal swing at the collector of the VAS results in modulation of the Miller capacitance by the signal, it is a time varying capacitance. At low freq stage gain is set by resistive loads since the reactance of the nonlinear capacitance is high in comparision, this reactance becomes more significant as frequency rises and distortion increases as a result.
A linear external cap can reduce this effect since the capacitance variation is lower on a percentage basis, however we loose HF gain also. The distortion primarily results from the VAS nonlinear input capacitance presenting a nonlinear load to the diff amp modulating the gain of the diff amp. Driving the VAS with more of a voltage source helps, this is why an emitter follower is sometimes used between the diff amp and VAS. A cascode configuation at the VAS also nearly eliminates this problem because it holds the VAS collector voltage constant, and the cascode load is a common base configuration with its base at ground potential from an AC perspective.
There are output stage HF nonlinearity considerations but for amplifiers with high enough current gain in the output stage the VAS junction Miller cap is the main contributor. This assumes high gain in the VAS, really any stage with high voltage gain will contribute. Running stages with high bias currents tends to improve the situation since there's more current available to charge the capacitances.
Thinking back to the Tiger amplifiers we see that with the output devices configured in common emitter mode that the Miller capacitance will present a nonlinear load to the drivers, however the VAS does not have as large output voltage swings since there's gain in the ouput stage. It's a tradeoff. |
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| thanh |
| quote: | Originally posted by jhead
speaking of Miller cap I have a BK ST-140 amp that use' s a 320pf silver mica cap the reddish epoxy covered type I believe to be rather generic has anyone tryed useing polystrene or what might be considered higher quality silver mica's I.E. Soshin silver mica's as miller compensation caps ? |
yes ,i'm using ceramic capacitor |
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| thanh |
| quote: | Originally posted by janneman
Hi,
Would that not be because as frequency rises, the cap impedance goes down. That has two effects I think: first, it shunts the output Z of the a gain stage therefore giving less gain thus less for the feedback (assuming global fb) to work its magic. Second, the stage itself could have more distortion working in an increasingly non-linear load (the cap is also level dependent).
Dimitri?
Jan Didden |
oh good |
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| thanh |
mikeb! i used 470p for cap from vas to ground and my amp work but my amp has much noise when it has no music signal
when the volume is set at low level (no signal) , my speaker is very noisy ,there are wind which running through my speaker but at high level ,the sound decrease |
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| thanh |
hi mikeb! i have just modified my amp with 1k5+330ohms resistorload at input stage and a 2n capacitor from vas to ground .In orcad ,my open loop bw is only 3khz
you don't use miler capa and how is your amp's o/l bw? |
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