Discrete Opamp Open Design

[Wavebourn]

Is the I2 connection to the output follower correct? Don't think I've ever seen that before.

That 20 mA source supplies current to asymmetric class A output stage.

Bootstrapped current mirror supplies voltage between gate and source of the output device. It would be interesting to compare results with a single pnp transistor loaded on resistor between source and gate.

 

[PaulBysouth]

Chris,
Yes it's correct - see the AD797 opamp (designed by Scott Wursor). In the schematic shown, if 5mA runs through each of the resistors R5, R6 then, about 5.5mA will flow out of the Q3,Q4 emitters, leaving the remainder of the I2's 20mA to come from the output Mosfet M3. The entire circuits self biases up automatically.
Paul Bysouth

 

[Chris Hornbeck]

Oh, Ok, thanks! That's cool. So the output stage isn't a follower, but rather a "transconductance output" (high Z).

Much thanks to you both,
Chris

 

[31697B]

Looks pretty good to me -

If for the moment we allow complimentary JFET’s there is complimentary symmetry on the circuit I posted. Here for those that are interested. Load the output with 90k and the open-loop BW is 20k while the OPEN-loop distortion is -70dB. I thought these things were good folk medicine for the sound. The triple-darlington is virtually no load to this and can be biased to drive a heavy load too. At 20k the base current re-capture lowers the open-loop distortion by 30dB.

This is a nice design. can you give the transistor types you recommend using here, pls. Thx - RNM

 

[31697B]

I was speaking from the perspective of my career as a patent examiner for the U.S. Patent and Trademark Office. In the patent world, they are considered not analogous. I would never look in steam engines for a control scheme of an opamp and it would never hold up upon appeal.

So, what you consider analogous is not what I consider analogous.

Which is exactly how i consider it as well or anyone working in corporate America. If it hasnt been done in THIS field, it is patentable.

 

[hitsware]

How much emitter resistance is needed
to degenerate the gain of a 2n4401 down to
an (enhancement-version) 2sk170 ?
(since the enlightened know that bi-polars are voltage controlled)

 

[Wavebourn]

How much emitter resistance is needed
to degenerate the gain of a 2n4401 down to
an (enhancement-version) 2sk170 ?
(since the enlightened know that bi-polars are voltage controlled)

Try 47 Ohm: 2sk170 = 22 mA/V

 

[hitsware]

Try 47 Ohm: 2sk170 = 22 mA/V

how did you derive that ?

 

[EUVL]

1/22mS = 46 ohm.

And 22mS comes from first page of data sheet :
2SK170?Junction FETs (Single/Dual)?TOSHIBA Semiconductor & Storage Products Company

Mr. Wavebourn is assuming the NPN has zero emitter resistance, which is of course not 100% true.
It depends on the bias.


Patrick

 

[hitsware]

1/22mS = 46 ohm.

And 22mS comes from first page of data sheet :
2SK170?Junction FETs (Single/Dual)?TOSHIBA Semiconductor & Storage Products Company

Mr. Wavebourn is assuming the NPN has zero emitter resistance, which is of course not 100% true.
It depends on the bias.


Patrick

Is there a formula for Hfe to Gm ?

 

[abraxalito]

No, Hfe isn't one of the fundamentals of transistor operation, there's significant process variation.

 

[Wavebourn]

1/22mS = 46 ohm.

And 22mS comes from first page of data sheet :
2SK170?Junction FETs (Single/Dual)?TOSHIBA Semiconductor & Storage Products Company

Mr. Wavebourn is assuming the NPN has zero emitter resistance, which is of course not 100% true.
It depends on the bias.

No, Mr Wavebourn does not assume that emitter resistance is zero, as well as that beta is infinite. But I assume that 47K would be close enough.

 

[EUVL]

47 ohm I guess, not quite so close to 47k.

 

[Wavebourn]

47 ohm I guess, not quite so close to 47k.

Yes, sorry for the typo.

 

[hitsware]

No, Hfe isn't one of the fundamentals of transistor operation, there's significant process variation.

Pardon my (evedently) **** poor education on the subject.

 

[31697B]

Spec goal in design -

Here is were some spec numbers came from as goal - see line #279 of Marsh headphone amp forum.

 

[hitsware]

No, Mr Wavebourn does not assume that emitter resistance is zero, as well as that beta is infinite. But I assume that 47K would be close enough.

47 Ohm or 47 kOhm ?

 

[abraxalito]

Pardon my (evedently) **** poor education on the subject.

But you have been educated on this subject - you yourself said that 'the enlightened know transistors are voltage controlled' That Hfe isn't well defined is a corollary of this I believe.

 

[jan.didden]

At 20k the base current re-capture lowers the open-loop distortion by 30dB.

Cute. Brings memories of Malcolm Hawksford.
So far the only real novelty in the barrage of 'me-too' circuitry

jan

 

[bcarso]

Cute. Brings memories of Malcolm Hawksford.
So far the only real novelty in the barrage of 'me-too' circuitry

jan

This is a critical tool, and as I and Jung have shown elsewhere, dates back well before Hawksford to Boxall and Aldridge. Walt is working on the all-JFET history so I won't go into that here.