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Old 17th December 2012, 02:31 PM   #2561
jcx is online now jcx  United States
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TPC already really needs an enhanced VAS/TIS to show any big difference

even higher order compensation schemes really only benefit amps with even more gain stages - NDFL, "error correction" local loops - the PGP amp

and higher order schemes have added nonlinear clipping, slewrate limit triggered oscillation modes that require attention
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Old 5th February 2013, 11:35 AM   #2562
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Default Differential current mirror?

The differential current mirror shown on p. 146 of the book seems a clever idea.
A quick Internet search did not turn up much, does any one have an analysis or references, or was it invented by Bob himself?

David
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Old 5th February 2013, 12:31 PM   #2563
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The topology is surely well known in IC design, although I can't say which opamp used this first. The name is somewhat misleading, as this is not actually a current mirror. A current mirror has a current input (low Z) and a current output (high Z), and the currents are modulated by the signal. Here there are just two outputs--with common-mode feedback, but no differential signal modulation. Perhaps "active load with common-mode feedback" is more accurate.

Samuel
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Old 5th February 2013, 02:23 PM   #2564
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Quote:
Originally Posted by Dave Zan View Post
The differential current mirror shown on p. 146 of the book seems a clever idea.
A quick Internet search did not turn up much, does any one have an analysis or references, or was it invented by Bob himself?

David
Hi Dave,

I first used it in my MOSFET Power Amplifier with Error Correction in 1983. I designed it myself, but that doesn't mean that someone else did not do the same thing or something similar before. If they did, I just was not aware of it.

The semantics of the term differential current mirror may be questoned, but I came up with the term as a derivative of what I consider to be the ordinary (single-ended) current mirror load that we are all familiar with in the input stages of power amplifiers, where it acts as a load for one side of the LTP and its output is connected to feed the net current to the input base of the VAS. This circuit does essentially the same thing, but differentially from both sides.

If it was used in op amps in the 70's or early 80's, you might be able to find it in a patent search, since such stuff was regularly patented back then. Even the Widlar current mirror was patented way back when, even though once one sees it, it looks pretty obvious. However, searching for it might require the use of numerous different semantics to find it.

Calling it something like a differential current source load with common mode feedback is certainly accurate as well.

Best,
Bob
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Old 5th February 2013, 02:55 PM   #2565
rsavas is offline rsavas  Canada
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Hi Bob,

Reading your book, Santa gave it to me for xmas, this year, what a nice guy. Excellent book, wish I had a book as well written as this one, when I attended College, back in the late 70's.
P.S. Thx for the LTSPICE additions as well!!

Cheers
Rick
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Old 5th February 2013, 03:42 PM   #2566
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US patents 4,271,394 and 4,272,728 came to my mind, but there's probably prior art.

Samuel
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Old 5th February 2013, 04:35 PM   #2567
Mooly is online now Mooly  United Kingdom
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The trouble with a book is that you don't know what's in it 'till it's too late

(Quote- Jeanette Winterson)
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Old 5th February 2013, 06:26 PM   #2568
wahab is offline wahab  Algeria
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One thing i do not agree with in Bob s book is that it is said that
"parralelling mosfets is not as straightforward as with BJTs".

Might be true for verticals but laterals can be parralelled even easier
than BJTs.

Indeed , the part devoted to laterals is somewhat symbolic ,
it would have been great to give thoses devices a little
more attention.

Perhaps in the next edition?.

Last edited by wahab; 5th February 2013 at 06:29 PM.
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Old 6th February 2013, 03:11 AM   #2569
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Quote:
Originally Posted by Samuel Groner View Post
... Perhaps "active load with common-mode feedback" is more accurate.
Quote:
Originally Posted by Bob Cordell View Post

Even the Widlar current mirror was patented way back...once one sees it, it looks pretty obvious.
That's what makes a circuit brilliantly simple rather than just simple! I would be more proud of a circuit that looks obvious after you have seen it, but that nobody did see, than any other. I hope my current idea turns out like that.

Quote:
However, searching for it might require the use of numerous different semantics to find it.

Calling it something like a differential current source load with common mode feedback is certainly accurate as well.
Both your and Samuel's phrases are helpful descriptions of different aspects so it is a pity that multiple different terms makes searches tedious. That was part of the reason I enquired here - in case there was another term I missed.

Thank you both for your replies. I will study the patents that Samuel referenced.
I have some questions on LTP to VAS connections that may also be in your respective areas, but I started a new thread for simplicity.

Best wishes
David

Last edited by Dave Zan; 6th February 2013 at 03:15 AM.
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Old 7th February 2013, 10:40 AM   #2570
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Quote:
Originally Posted by rsavas View Post
Hi Bob,

Reading your book, Santa gave it to me for xmas, this year, what a nice guy. Excellent book, wish I had a book as well written as this one, when I attended College, back in the late 70's.
P.S. Thx for the LTSPICE additions as well!!

Cheers
Rick
Hi Rick,

Thanks for those kind words.

Cheers,
Bob
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