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Old 25th August 2012, 01:41 PM   #241
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Quote:
Originally Posted by ChocoHolic View Post
What would you expect from a configuration like this?

The input stage does not need additional components for biasing.
It will offer high input impedance and low noise.
R1 and R2 allow adjustment of bias independently from adjustment
of gain and degeneration which is solved by R3 and R4....
Last but not least you could use such an OP amp for voltage feedback configurations and current feedback applications as well. In current feedback applications one could use the nodes of R1 as current feedback inputs, or center tap R1...
All the rest of the circuit is mostly standard.
That looks really interesting. I almost understand it.
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Old 25th August 2012, 01:49 PM   #242
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Originally Posted by ChocoHolic View Post
What would you expect from a configuration like this? ...... All the rest of the circuit is mostly standard.
I'm not sure what you are asking from us. As you observe yourself, the circuit is mostly standard (essentially being a complementary folded-cascode + source follower).
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Last edited by Ken Newton; 25th August 2012 at 01:52 PM.
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Old 25th August 2012, 02:02 PM   #243
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Two more candidates for the MosFet search.
N Chanel and P Chanel as well.
...for the circuitry as proposed by John in posting #169...
Attached Files
File Type: pdf NDC7002N.pdf (84.2 KB, 64 views)
File Type: pdf NDC7003P.pdf (114.7 KB, 36 views)
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Old 25th August 2012, 02:09 PM   #244
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Originally Posted by Ken Newton View Post
I'm not sure what you are asking from us. As you observe yourself, the circuit is mostly standard (essentially being a complementary folded-cascode + source follower).
The input stage is not that much standard.
...hm, and when we combine this input stage with the second and third stage of John's proposal in posting #179 then it starts to look really nice.

Edit:
Nevertheless the basic circuitry of John's posting #169 (I guess source resistors at the output MosFets would be necessary for a real life set up to stabilize and linearize the output stage) is still my favorite, because it has essentially just 6 transistors and you only need matched, what you can get matched. Means it can be optimized without having exactly complementary types. Definitely worth to spend some time with !

Last edited by ChocoHolic; 25th August 2012 at 02:32 PM. Reason: Additional comment
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Old 25th August 2012, 02:47 PM   #245
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Quote:
Originally Posted by john curl View Post
Let me offer an example of a simple, elegant, balanced circuit that can be made into the highest quality amplifier, preamplifier, or whatever, with a minimum number of parts.
This is NOT my circuit, and it is at least 40 years old, but Constellation makes world class amps with it, today. I give the credit to Bascom King, but it might go back earlier, I don't know.
John,

For me, the most interesting aspect of this topology is the totem pole output stage design and it's interface to the diff. VAS. Could you make any comments specifically about the output stage to VAS interface and asymmetrical operation?
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Old 25th August 2012, 02:52 PM   #246
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This is a VERY RATIONAL design.
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Old 25th August 2012, 02:57 PM   #247
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Quote:
Originally Posted by EUVL View Post
Scott,

A SMD dual JFET in SOT23 is too good for me to give up, so I went back to the Toshiba website.
It says (see attached) internal connection common source.

So it can at least be used for input LTP.

Toshiba never says whether their FETs can be used with D/S swap.
All the ones I tried can, even though some has a slightly different Idss when reversed.
But this is solvable even if the lower FET has lower Idss when reversed.
Just tie a resistor or a CRD from Vout to the negative rail.

So I am still on for this one.


Patrick
Someone should test a few of the duals. If they isolate the halves (before hardwiring the channels together) the same way they do in the obsoleted parts like the 389, there are no parasitic diodes to conduct other than via reverse leakage (I tested this with the 389, facilitated by their bringing out a lead for the substrate --- which they then tell you to float ). I think they just have blinkers on as far as the possible different configurations. But maybe not ---perhaps the hard wired "sources" are part of a structural change to simplify isolation. Testing will tell.

It's interesting that the model published for the BF862 shows markedly different parasitic (i.e., "contact") resistances in series with the designated "drain" and "source". However, when I tested a few there was virtually no difference in on resistance for normal or reversed connections. BTW those parasitic resistances start to become important at high drain currents and limit the already-slow rate of improvement of transconductance and noise reduction.
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Old 25th August 2012, 03:14 PM   #248
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Originally Posted by john curl View Post
This is a VERY RATIONAL design.

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.
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Old 25th August 2012, 03:17 PM   #249
bcarso is offline bcarso  United States
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Originally Posted by jneutron View Post
Dumped into a big hopper??? Shirley you jest..

I'da thunk either an expanded wafer, or waffle pack..

jn
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).
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Old 25th August 2012, 03:17 PM   #250
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Quote:
Originally Posted by john curl View Post
This is a VERY RATIONAL design.
Well, that was succinct.
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