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Chip Amps Amplifiers based on integrated circuits

Composite amplifier: LM3886 + LME49710
Composite amplifier: LM3886 + LME49710
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Old 22nd February 2019, 06:41 AM   #11
simonra is offline simonra  United Kingdom
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Quote:
Originally Posted by jeanlemotan View Post
So it's not clear for me at all: do I have to do the PM > 60 deg (or at least 0 deg) and GM < -10dB (or at least 0 dB) with both the loop closed and open?
That seems impossible
My thinking is that the closed loop is what the op amp will see in reality. So if your negative feedback turns positive at a frequency that has >0 gain then it will cause instability.

Quote:
Originally Posted by jeanlemotan View Post
So you're saying that I should choose the inner/outer gain so that the 49710 will clip first?
It's probably easier for you to simulate than for me to try to explain... send a sine wave at a voltage where the output just starts to clip and monitor the output of both the 3886 & 49710 by running a transient analysis.
When the 3886 hits the rail the 49710 will follow suit because of the imbalance between the input and feedback signals. Then the 49710 recovers quicker than the 3886 so it hits the other rail coming out of clipping.
Now adjust your gain so that the 3886 never hits the rail, this leaves the 49710 in complete control so recovery is smooth.

As I said before I'm new to this too so others may wish to correct me in my thinking or explain it better.

Good luck.
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Old 22nd February 2019, 07:35 AM   #12
jeanlemotan is offline jeanlemotan
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Quote:
Originally Posted by simonra View Post
My thinking is that the closed loop is what the op amp will see in reality. So if your negative feedback turns positive at a frequency that has >0 gain then it will cause instability.
This was my reasoning when I started the simulations - I went for calculating the PM and GM with the loop closed and I was unable to get it stable for a week.
Then I realized that maybe I'm doing it wrong so I re-read tomchr's page (LM3886 Chip Amp Stability Analysis) and this (Negative Feedback, Part 9: Breaking the Loop) and they both mention the PM/GM of the open loop.

Quote:
Originally Posted by simonra View Post
It's probably easier for you to simulate than for me to try to explain... send a sine wave at a voltage where the output just starts to clip and monitor the output of both the 3886 & 49710 by running a transient analysis.
When the 3886 hits the rail the 49710 will follow suit because of the imbalance between the input and feedback signals. Then the 49710 recovers quicker than the 3886 so it hits the other rail coming out of clipping.
Now adjust your gain so that the 3886 never hits the rail, this leaves the 49710 in complete control so recovery is smooth.
For 49710 to clip first I see 2 options:
1. Reduce the gain of the 3886 so that the 49710 has to swing more. This is not a good idea as the 3886 is not stable below 10x gain according to the datasheet
2. Add a voltage divider between the 2 opamps so that the 3886 sees only a fraction of the 49710 swing.

I'll try #2 and see where that takes me.

I'll post back the results.
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Old 22nd February 2019, 07:47 AM   #13
jeanlemotan is offline jeanlemotan
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Originally Posted by tomchr View Post
If you don't need it, why include it?
It was the way I found to stabilize the amp. By adding that divider I was able to add the C17 capacitor and the PM got way better.


Quote:
Originally Posted by tomchr View Post
That's a good place to start. You have to open the loop in the right place for the simulation, though. If opening the loop affects the load impedances in the circuit, you won't get valid results.
I opened the loop exactly as you recommend on your website.
But as simonra was saying - is it enough to simulate PM/GM of the open loop? Don't I have to do the same with the closed loop to get a stable amp?

Screenshot from 2019-02-22 09-44-41.png

For example with the loop closed I get a PM of 131 and GM of 13 which sounds really bad, but the same amp with the loop open was getting way better PM/GM.
What's the correct way to characterize the amp stability?

Quote:
Originally Posted by tomchr View Post
You can often see THD in the simulation due to the simulator's choice of time step. You need to make sure you sample exactly one period of the input sine wave with the exact number of samples as you have points in your FFT. Otherwise, you'll get artifacts from discontinuities. I can find a few articles on that on TI's E2E forum.
Yes, simulator aliasing seems like a good way to get THD.
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Old 22nd February 2019, 08:47 AM   #14
simonra is offline simonra  United Kingdom
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Quote:
Originally Posted by jeanlemotan View Post
2. Add a voltage divider between the 2 opamps so that the 3886 sees only a fraction of the 49710 swing.
That's what I'd try... you have a divider already (R12 & R5)
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Old 22nd February 2019, 02:21 PM   #15
wallyboy is offline wallyboy  Canada
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have you read the article from the November 1992 issue of Electronics Magazine on composite amps? It has some decent info on composites. Here is the link:
Internet Archive: Page Not Found
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Old 22nd February 2019, 03:57 PM   #16
mlloyd1 is offline mlloyd1  United States
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Composite amplifier: LM3886 + LME49710
thanks for the pointer wallyboy!

this link worked a little better for me: https://ia801701.us.archive.org/15/i...ember_1992.pdf

mlloyd1
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Old 22nd February 2019, 04:05 PM   #17
wallyboy is offline wallyboy  Canada
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Sorry: I should have checked the link before I posted it
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Old 22nd February 2019, 05:48 PM   #18
jeanlemotan is offline jeanlemotan
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I applied your idea simonra and did the simulations again:
Here is the step response:
Screenshot from 2019-02-22 19-31-46.png
Here is the gain peaking:
Screenshot from 2019-02-22 19-31-49.png
And here is the phase margin calculated according to this (https://training.ti.com/ti-precision-labs-op-amps-noise-3)
Screenshot from 2019-02-22 19-44-11.png
This is the schematic used to calculate the phase margin:
Screenshot from 2019-02-22 19-46-12.png

I get a margin of 65 deg, confirmed by the step response which shows no overshoot at all and the gain peaking - which is zero.

Next I will test the clipping characteristics of both configs: clipping handled by the 47910 or by the 3886.

I did the math like this:
1. The 3886 is configured with a gain of 12 and can reach (Vsupply - 3V) on each rail before clipping. For +/- 28v supply, this means I want to drive it to max +/- 25V. Say 24V to make it round. So a gain of +/- 2V and a gain of 12x will result in +/- 24V swing.
2. The 49710 can swing +/-14V before clipping so to get the +/-2V for the 3886, I have to divide it by 7.

Does this make any sense? Did I miss smth?

Last edited by jeanlemotan; 22nd February 2019 at 05:55 PM.
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Old 22nd February 2019, 05:52 PM   #19
jeanlemotan is offline jeanlemotan
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Originally Posted by wallyboy View Post
have you read the article from the November 1992 issue of Electronics Magazine on composite amps? It has some decent info on composites. Here is the link:
Internet Archive: Page Not Found
Nope, didn't read it. Very interesting article although I don't really understand the topology in figure 3: where does the gain come from? Does this mean the LM1875 is a current amplifier only and the overall amp has no voltage gain?
Very good read, thanks for the article.
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Old 22nd February 2019, 05:59 PM   #20
tomchr is offline tomchr  Canada
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Quote:
Originally Posted by jeanlemotan View Post
Then I realized that maybe I'm doing it wrong so I re-read tomchr's page (LM3886 Chip Amp Stability Analysis) and this (Negative Feedback, Part 9: Breaking the Loop) and they both mention the PM/GM of the open loop.
PM, GM are simulated open loop.

Quote:
Originally Posted by jeanlemotan View Post
I opened the loop exactly as you recommend on your website.
I don't show any simulations of a composite amp on my website. I show loop simulations of the LM3886.

You always need to make sure that you break the loop in a way that has as little (preferably no) impact on the loop as possible. This generally means that you'll be breaking the loop at a point where the drive impedance is low and the load impedance is high. The "LC way" of breaking the loop that I like to use generally works well enough. There are fancier ways, however, which yield more accurate results. You can see Middlebrook's method for an example. There's a newer analysis by Tian (et al.) that's supposedly better or simpler to implement in the simulator. You can find some good links and references here: Education AC and Stability Analysis in NGSPICE. - Education

If you have access to IEEE (or are willing to pay for the paper), this looks worth checking out: Striving for small-signal stability - IEEE Journals & Magazine

Quote:
Originally Posted by jeanlemotan View Post
But as simonra was saying - is it enough to simulate PM/GM of the open loop? Don't I have to do the same with the closed loop to get a stable amp?
No. The PM/GM (which are loop parameters, hence simulated open loop) are just one piece of the puzzle.

I start with the loop analysis, because if the loop isn't stable there's no point in simulating anything else. I then simulate the closed loop response to make sure I don't have excessive peaking and to pick out potential issues that I may have missed. I simulate the transient response to see how the amp behaves when slewing, when it reaches clipping, and on hard clipping. I look at the DC operating point to see the DC offset. I look at the noise performance. And take a quick glance at the THD to see if anything is fundamentally broken.

In a production environment I sweep these parameters across supply voltage and temperature as many of these characteristics vary across temperature. I don't recall if the LM3886 macro model takes temperature into account. It might. I know it laughs at the power supply voltage. Nothing power supply related is simulated with the LM3886 macro model.

Once I have a circuit that I feel reasonably confident in, I'll build it. I then correlate reality with simulation. Over time, I've developed a sense of the strengths and weaknesses of the LM3886 model, which allows me to shorten the development time. Basically, I know how far off target to aim in order to hit the target in reality.

Based on your questions so far, I recommend taking half a step back and brushing up on some stability theory. Chapter 8 in Franco is a good place to start. I'm sure there's a corresponding chapter in Yung. You can find the full references here: References.
In general, I highly recommend Franco for anyone interested in working with opamps and amplifiers in general. The latest edition (3rd) is absurdly expensive, in particular since it hasn't changed in 15+ years, but the 2nd edition is every bit as good (there were very few changes between editions) and can be found on the used market for less.

Tom
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