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Amplifier with nested Miller compensation
Amplifier with nested Miller compensation
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Old 30th March 2013, 07:22 PM   #1
matze is offline matze  Europe
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Default Amplifier with nested Miller compensation

After some years of experimentation with different amplifier topologies I would like to share with you the following schematic. I post this result here, because I have much benefited from the discussions here as a silent listener.

Discussion of current design variant starts at post #100 >here<

Edit 22nd April 2013:
Maybe, I first have to clarify the main idea proposed in this thread.
With the advent of low-voltage operational amplifiers, especially CMOS, the designers had the problem that the active devices did not provide enough transconductance in order to get good DC gain figures. This was due to the standard OPA architecture with only two effective gain stages: differential transconductance plus transimpedance stage (VAS). Thus, they were forced to throw in more gain stages that had to be compensated somehow. Nested Miller compensation is in my eyes just the simplest scheme, more are discussed e.g. in Johan Huijsing's book. (BTW, also thrilling there: biasing circuits for rail-to-rail A/B output stages [without cut-off], linearisation of rail-to-rail input stages, multipath frequency compensation and discussion of many design examples, including a few important standard OPA)
Lacking DC gain is of course not our problem. But each nested Miller compensation loop does provide, as a side-effect, extra feedback around the output stage. This is what I'm proposing to exploit.
End Edit 22nd April 2013


In the current version, the amplifier is a breadboard design delivering only a few watts for room listeneing levels. The supply voltages are 20 volt, the mains transformer only delivers 30 W. The amplifier drives, however, a really difficult and highly resolving load (Dynaudio Crafft) with excellent bass quality (comparable to Bryston) and incredible overall naturality and smoothness.

I cannot measure its objective performance. Simulation indicates, however, good results (very low THD20 at low to medium levels, very clean response to rectangular signals). The rectangular response looks as nice on the oscillograph as in the simulation.

I have written a small text matzes-amp.pdf that shall describe the ideas behind the design. Although the overall schematic looks quite complex, it is just a repeated application of the same simple principle. The schematics referred to in the text are fig1.asc to fig5.asc.

matzes-amp.asc is the overall LTSpice schematic. It uses trans.txt (models for D44/D45 from Harry Dymon, adapted BD139/40 models) and bc-co.txt (models for bc-npn und bc-pnp which are the BC550/60 models from Bob Cordell). All resistors in series with blocking capacities are just ESR placeholders.


Best regards,
Matze

Note: Schematics fig1.asc to fig5.asc contain mistakes that have been corrected in post #3. In post #7, the figures are given as screenshots.

matzes-amp.asc

matzes-amp.pdf

bc-co.txt

fig1.asc

fig2.asc

fig3.asc

fig4.asc

fig5.asc

trans.txt

Last edited by matze; 11th January 2018 at 01:37 PM.
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Old 31st March 2013, 05:14 AM   #2
Dave Zan is offline Dave Zan  Australia
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Nice to see some ideas about fundamentals. Pretty rare
But your ASCs don't actually show any feedback connection?
And the polarity of the "VAS" and the "OPS" look inconsistent.
I remember that some of the Spice sources have polarity conventions that seemed counter-intuitive but that would not explain inconsistency.

Best wishes
David
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Old 31st March 2013, 08:04 AM   #3
matze is offline matze  Europe
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Default Correction of principle schematics

Dear Dave,

thank you for the comments. Of course, you are right. It is funny that the simulation of bode plots even may bring the right results if one does not get the basics right

The "VAS" has to be inverting, and even the input stage G1 had the wrong sign of amplification. Se below the corrected schematics fig1 to fig4. For instance, in fig1.asc, if we apply a positive voltage to '-' of G1, a (positive) current is injected into the left side of C2, forcing E2 to output a positive voltage so that its '+' input remains near ground as C2 charges.

Concerning your question with the feedback path, I have tried to clarify the schematics. The "real" input of the whole amplifier is '+' of G1, and R2/3 form the feedback network. As the amplifier output is not really loaded by the feddback path, I have used this simple scheme. In order to see the closed-loop response, connect V1 to R1 and R3 to OUT.
In fig4.asc, we can see the bode plot of the inner loop by disconnecting R5 from OUT and connecting it to V1, leaving R3 "in the air". (Compared to fig3, the gm of G2 has increased ten times, because the connection to OUT now again goes via a divider R4/5)

Matze

fig1.asc

fig2.asc

fig3.asc

fig4.asc
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Old 31st March 2013, 08:55 AM   #4
matze is offline matze  Europe
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Sorry, correction(2) to the previous post. In the discussion of fig1.asc, E2 is forced to output a negative voltage. The left side of C2 is charged positive.

Matze
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Old 31st March 2013, 05:58 PM   #5
matze is offline matze  Europe
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Default Performance examples

Here are two examples that may demonstrate the good performance despite the *very* simply "VAS" and output stages.
The first is an FFT of a 20 kHz sine with 1V rms into 4 Ohms. The second shows the response to rectangular input (rise/fall time 1ns, of course somewhat flattened by the input low-pass). The additionally plotted behaviour of one LTP current demonstrates the good internal stability: virtually no overshoot on any node in the amplifier internals.

[[ To my ears, the amplifier just sounds marvelous: authorative, transparent, grain-free, colorful depending on the material. ]]

Matze


fft20-1vrms-4ohms.gif

10k-rect-4ohms.gif
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Old 31st March 2013, 06:20 PM   #6
lineup is offline lineup  Sweden
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I can not see .asc files.
Why not make an ordinary image for everybody to see. = screencapture
No, I wont install LTspice only for to see your .asc schematic.

Regards
__________________
lineup
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Old 31st March 2013, 06:36 PM   #7
matze is offline matze  Europe
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Default Screenshots for schematics

Dear lineup,

sorry, this is my first substantial input to the forum. I just thought it to be practical to have the sim input files. But I agree, just for reasoning it may be more practical to have a simple image.

Here are the schematics as screenshots. Fig1 to Fig5 relate to the small "paper" matzes-amp.pdf

Greetings,
Matze


matzes-amp.gif

fig1.gif

fig2.gif

fig3.gif

fig4.gif

fig5.gif
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Old 31st March 2013, 07:26 PM   #8
dchisholm is offline dchisholm  United States
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Quote:
Originally Posted by lineup View Post
I can not see .asc files.
Why not make an ordinary image for everybody to see. = screencapture
No, I wont install LTspice only for to see your .asc schematic.
It is difficult to decide what kind of files should be attached to a post. I would wager that most of us use circuit simulators to investigate ideas and to design projects, and LTSpice is probably the most popular simulator . . . but, like "lineup" said, that isn't true for everybody. And, the " *.asc " file extension may cause problems because it is not unique to LTSPice. (The " *.sch " extension used to denote schematic drawings in some programs is even worse!)

Using something like the no-charge "CutePDF Writer" to "print" the LTSpice schematic is about the same amount of work, and probably more versatile, than using something like the Windows "Snipping Tool" to get a screen shot - especially on large or very crowded schematics.

On the other hand . . . when a post such as this thread says, "Hey, I have been playing with this circuit in simulation and I'd like to have your comments . . . ", many of us would like to at least verify your simulation conclusions, check your options and settings to be sure the simulation is giving accurate results, see how our favorite devices behave in the proposed circuit, etc. For these members it is VERY helpful (and you are more likely to get useful comments) if you attach the simulator file (and don't forget to list the device models you used!).

It just proves that you can't please everybody!

Dale
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Old 6th April 2013, 09:40 AM   #9
matze is offline matze  Europe
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Default Example from Bob Cordell's Book

Probably I have a bit overdone with my initial example, that nevertheless fills the living room with highly enjoyable sounds.

Here is an application to the example amplifier from Bob's book, page 63.

basic-schem-bode.png is the schamtic used to get the bode plot for the main feedback loop (basic-bode.png). One sees a unity gain crossover of around 400 kHz, the gain margin is more than 20 dB, phase margin is around 85 degrees.
basic-schem-fft.png shows the schematic for the FFT, 10kHz, 1 V rms into 4 ohm. My way to enforce a low simulation step is the additional dummy voltage source V4, spinning at 1 MHz.
basic-fft.png shows the FFT, the reading from the Spice .four command is 0.037%.

Example with one nested Miller compensation loop comes in the next post.

Matze


basic-schem-bode.png

basic-bode.png

basic-schem-fft.png

basic-fft.png
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Old 6th April 2013, 10:08 AM   #10
matze is offline matze  Europe
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Default ... now with nested Miller compensation

The schamtic is nested-schem-fft.png. As discussed in the text matzed-amp.pdf in the first post, the simple transconductance stage with Q16, Q17 works quite well, here with the current source I1 as load. In order to ensure balanced voltages at the LTP output and sufficient Vce over Q16, I have reduced the negative supply voltage of the input stage. The additional voltage source V6 further reduces Vce of Q5.

nested-schem-bode-inner.png schows the schematic used to get the Bode plot of the inner feedback loop. In nested-bode-inner.png, we see a unitiy gain crossover of about 400 kHz, phase margin a bit more than 85 degrees. Gain margin is more than 30 dB, since the zero-degree frequency is quite high (very very probably, this does not translate to reality ...).
nested-schem-bode-outer.png is the schematic for the Bode plot nested-bode-outer.png of the outer loop. It very closely resembles the Bode plot of the basic amplifier from the last post.

nested-schem-fft.png and nested-fft.png are for the FFT, 10kHz, 1V rms into 4 ohm. The reading from the .four Spice command is 0.0038% .

I did not yet investigate clipping behaviour and behaviour on turn-on and turn-off.

Matze

nested-schem-bode-inner.png

nested-bode-inner.png

nested-schem-bode-outer.png

nested-bode-outer.png

nested-schem-fft.png

nested-fft.png
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