|27th March 2013, 12:37 PM||#12|
Join Date: Mar 2009
Location: Buenos Aires - Argentina
I *have* read the article, but I have a few objections, pity it's a graphic image and can't cut an paste.
Objections which by the way have also been raised by other posters, both here and in Simple hybrid amplifier (35W/8Ohm)
1) the LM3886 is not unity gain stable, that's set internally, there's nothing you can do "outside" to change that.
Or, to be more precise, that can be done in Op Amps where you either use an external compensation cap, (in which case you would use the one needed for "real" unity gain stability) *or* by some design quirk you have access to some internal part of the circuit where you can add it, even if that's not the "official" way to do it.
None of these cases is possible with LM3886.
2) I've read the "trick" described, what he does is exactly what we all suggest: attenuate signal (5X in the example), then provide same 5X gain (what you are trying to avoid) to compensate.
All he does is, instead of having one resistive voltage divider (a.k.a. "attenuator") at the input and another, same ratio one as "gain setting" connected ad NFB, he does both functions with the same resistor set (Rf/Rc).
It does not "look" like conventional attenuating 5X, then having 5X gain, but that's exactly what he does.
He even does all the calculation to prove he has 5X gain , to meet stability criteria.
And he also takes great care to show that you do *not* have the bandwith which would be obtained with "real" unity gain (80 MHz in his example) but the 5X gain one (16 MHz).
Clear as water.
That "hidden in plain sight" attenuator/gain network carries other problems, that's why he writes, say, 80 lines explaining the "trick" (which ends up not being such a big deal after all) and then writes, say, 200 lines explaining "what's the catch?", showing all problems wit this configuration.
Which in your particular case *is* troublesome.
a) the main one: that resistor from your (+) input (high impedance) to the (-) input, which by itself is high impedance too, but is connected to a low impedance NFB network, sure wreaks havoc with your input impedance.
The third line , first column in page 13 says exactly that: "the input Resistor Ri (which would set input impedance) is in parallel with Rc (around 100 times smaller)" WTF?
Which was noticed by jcx in post #3 .
You simply can't connect this "compensated" LM3886 to a tube plate or to a sensible value volume pot.
By the way, if it happened only at "high frequencies", or it made to appear , say, 100pF across the input, it would be quite bearable, but since it really needs 1000X the capacitor value needed for compensation, you end having it "at ALL frequencies"; more precisely in your case (didn't check your Math but that's not the point anyway, I trust you) you are effectively placing 1K in series with 2.2uF across your input
b) since you are *really* having the "stable" gain anyway (11X minimum) you are still having all noise/distortion/whatever implicit .
In fact you are still having dreaded "SS gain", what you are desperately trying to avoid.
Am I dissing your design or goals?
Not at all, I find it a very interesting idea, only I see you won't achieve it with, of all chips in tne World, an LM3886
By the way, what's wrong with the unity gain stable Power Op Amps mentioned in the original design?
|27th March 2013, 12:48 PM||#13|
Join Date: Jan 2008
Thank you very much for your explanation. I do not have enough knowledge to fully understand the article. Thatīs pity I canīt use LM3886.
There is nothing wrong with OPA548/549, except that they have max 35W/8 Ohm power, worse distortion than LM3886 and they are quite expensive. OPA549 sounds fine, no doubt, but it could have been better.
I am just looking for unity-gain stable power opamp for two reasons:
1) simplicity of build
2) the buffer has NFB around itself, I donīt want class AB open loop buffer
I know there was or maybe still is unity-gain stable power opamp - LM12, but I canīt find them in store anywhere.
Thanks for your help.
|27th March 2013, 02:49 PM||#15|
Join Date: Feb 2006
Location: On a hill, in a wooden shack, next to the woods, in Somerset.
There may be a way you could do this. Mauro gets to use a 3886 in unity gain in his My_Ampli project which I think is also called Ref Freeman of similar and he did it by using an op amp (a humble 318 IIRC). I imagine it worked because it could use the Miller pole in the op amp and just had the 3886 as a follower, though I never bothered to really examine it, it seems to work well.
Do you have somewhere to pin the feedback in your pre-amp? Ie. so you could simply put the 3886 between the preamp output and the feedback loop. It may blow up both units for all I know but someone familiar with your preamp might know.
Or there is a more complicated solution in which you do the same as Mauro but feed the whole signal to the 3886, have the op amp correct it because it has been given a sample of the output and gets a sample of the input too.
Or, another or, you could bridge two of your OPA549s and get the voltage swing you want that way.
|30th May 2013, 08:03 PM||#19|
Join Date: Oct 2006
|31st May 2013, 02:15 AM||#20|
Join Date: Jun 2009
Yes, open loop measurement shows the 3886 model has 81 degrees phase margin and 32dB gain margin (details attached, though it's easy enough to do one's own). This is in obvious disagreement with the open loop figure on page 16 in the datasheet so I don't think the model is going to be very useful for this discussion.
One can, in principle, stabilize a 3886 at unity voltage gain via noise gain compensation. Refer, for example, to part 8 of Tim Green's series on op amp stability. The noise can be lower than the more popular approach of using a voltage gain of 10-20 but the required feedback capacitance ends up on the order of 10pF. This is quite difficult for the typical DIYer to hit, much less with the accuracy needed to maintain a 20dB/decade rate of closure over the part's GBP spread. Even with a good PCB fab and surface mount parts it's not something I'd try for; too easy to end up off by several pF due to stray capacitance on traces or the 3886 package's pins.
An easier solution is to operate the 3886 at 26dB local gain (or whatever) for stability within a composite amplifier with a global gain of unity. This is the same basic idea as the attenuation approach proposed earlier in this thread but with better error correction due to the global feedback---this fixes the 3886's poor small signal THD, delivers a noise gain of 2 instead of 21, and limits the output's DC offset to the controlling op amp's input offset.
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