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Old 25th August 2012, 06:23 PM   #261
bcarso is offline bcarso  United States
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Quote:
Originally Posted by john curl View Post
Let me clarify things for you all: Over the decades, I have had a series of 'requirements' for making an audio gain stage, developed after years of making prototypes. This designs meets MY criteria, but then I designed the whole thing and put it into a power amplfier, 30 years ago. I guess I should mention that, but that is NOT why I said that it was 'rational'. I meant that this design example meets MY criteria, and does not fall down in some way. Now, if I am allowed, I will try to show where many other designs fall away from meeting the criteria that I find useful and important. Not ALL designs are failures in some way, but they have yet not been presented here to any degree, recently.
Thanks! Now I know to what you were referring.
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Old 25th August 2012, 08:10 PM   #262
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Originally Posted by bcarso View Post
I wish you'd stop calling me Surely.

No, I was speaking with poetic license (although until Scott posted I had no idea what they really did, but knew that positional adjacency information was either lost or not taken into account).
What's funny is that back in 1974 they had a slice of a huge steel rod about 4" in diameter that they rolled over the wafer to break all the scribes.
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Old 25th August 2012, 09:01 PM   #263
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Originally Posted by ChocoHolic View Post

I bet this was the answer everybody was seeking for...

In addition to Ken's question, I am wondering whether the natural output impedance (without feedback) is low enough to be called a OP amp output stage. To me it appears more like both output MosFets would act like inverse controlled currents sources and consequently would deliver a high output impedance.
...on the other hand, there is no proof that only circuits with a natural low output impedance sound good.
Yes, that's how the output stage appears to operate to me as well. It seems the totem pole output is designed as a high impedance node, not the current-source loaded follower it might appear to be on first glance. This was the design aspect I was most hoping John might speak to. Lowering of the output impedance would (I presume) come via negative-feedback. If the output does in fact behave like a high impedance transconductance node, I wonder how it might perform using a simple I/V gain setting resistor connected to ground, without a negative feedback loop.
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Old 25th August 2012, 09:13 PM   #264
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Please tell Wavebourn what you found, Ken! '-)
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Old 25th August 2012, 10:48 PM   #265
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Originally Posted by john curl View Post
Please tell Wavebourn what you found, Ken! '-)
I am eager to know! Did he found that differential pair of MOSFETs loaded on many times different resistances does not exhibit 2'nd order distortions? Or did he found that as drawn that amp will be quite thermally stable?

By the way, I was thinking about thermal stability issue yesterday, and found some ideas.
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Old 25th August 2012, 10:54 PM   #266
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Originally Posted by scott wurcer View Post
What's funny is that back in 1974 they had a slice of a huge steel rod about 4" in diameter that they rolled over the wafer to break all the scribes.
I heard an anecdote in 1970'th that some Japanese company was buying lots of Soviet transistors that were not used in their production. It turned to be, they cleaned dies to make many transistors from one, and melted cases on scrap metal.
One more Soviet anecdote. When Shokin become Minister of Electronic Industry, visiting one plant he spotted a huge red banner above the entrance, "Praise Soviet Transistors, the Biggest Transistors in the World!" He whispered, "Remove that shame immediately!"
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Old 25th August 2012, 11:46 PM   #267
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Originally Posted by john curl View Post
Please tell Wavebourn what you found, Ken! '-)
Huh?
What did I find?
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Old 25th August 2012, 11:59 PM   #268
bcarso is offline bcarso  United States
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Originally Posted by Ken Newton View Post
I wonder how it might perform using a simple I/V gain setting resistor connected to ground, without a negative feedback loop.
Speaking now of the Constellation-like simplified schematic from JC's post #168:

Too much gain for open loop, without a lot of local feedback/degeneration.

As an amp around which feedback will be applied:

The concerns about output impedance and stability are well-taken, but there are ~fixes.

For an example I threw down of BF862 input pair, ZVP3306 second stage pair, and ZVN3306 ouputs, 10mA in input tail, 10mA each in P channel, 220 ohm bias R across each output G-S. I see an open loop output impedance resistive to about 92kHz of 2.1k ohms. But one of the problems is that the lower output device must supply all of the input bias current of the upper device, so this intrinsically unbalances the P pair. Throw down a current sink from the output (or just a resistor to the negative rail), then the second stage can be balanced. It's also good to put some voltage drop in the one drain to equalize the drain voltages of the P devices.

Compensation can be done, as some (e.g. Borbely) like, across the input pair drains, a series R-C tailored to the desired closed-loop gain; slew rate doesn't suffer much, since there is a large voltage gain following, for moderate output loads. A disadvantage is that at high frequencies the input pair can't do much to reduce the contribution to noise of the second stage (see discussions about the AD797). But DMOS isn't bad at high frequencies, only at low (and less so as processes continue to improve, but be wary).

Now some can argue that the bias resistors could be made larger, but then we run the second stage at lower current, and the input capacitance of the lower N output will slow that section down.

At least, if the two output devices are reasonably well-matched, and perhaps with some ballasting R in the sources, the quiescent bias can be not too unstable.

EDIT: schematic attached
Attached Images
File Type: jpg Constellation-like opamp 08-25-12 001.jpg (222.9 KB, 479 views)

Last edited by bcarso; 26th August 2012 at 12:20 AM. Reason: attachment
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Old 26th August 2012, 03:11 AM   #269
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Originally Posted by bcarso View Post
At least, if the two output devices are reasonably well-matched, and perhaps with some ballasting R in the sources, the quiescent bias can be not too unstable.
Plus couple of diodes in thermal contact with output devices controlling current source in the tail of the 1'st stage.
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Old 26th August 2012, 05:11 AM   #270
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Default ideas? did someone say ideas?

Quote:
Originally Posted by Wavebourn View Post
I am eager to know! Did he found that differential pair of MOSFETs loaded on many times different resistances does not exhibit 2'nd order distortions? Or did he found that as drawn that amp will be quite thermally stable?

By the way, I was thinking about thermal stability issue yesterday, and found some ideas.
Great.. what are your ideas?!
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