Discrete Opamp Open Design

[magnoman]

Seems like this can be a real fun and entertaining excersize if just not taken too seriously.

Is there any consideration to try and exploit any potential advantages of "newer" available discrete components, which I can only think of the small signal mosfets (Siliconix-Vishay..). Mixing the best characteristics of FETS/BJT/MOSFETs also has some appeal for discrete.

Thanks
-Antonio

[bcarso]

Originally Posted by magnoman

Is there any consideration to try and exploit any potential advantages of "newer" available discrete components, which I can only think of the small signal mosfets (Siliconix-Vishay..).
-Antonio

For example?

[RNMarsh]

Scott - you have the floor/ball..... take it and run. Are you going to do a discrete version of the 627 with us? Or? -RNM

[elektroj]

Originally Posted by Joachim Gerhard

Samuel Groner has shown the most popular topologies for Opamps.

....but did not include in his analysis important one that's been around for ages as discrete design.

[RNMarsh]

Originally Posted by magnoman

Seems like this can be a real fun and entertaining excersize if just not taken too seriously.

Is there any consideration to try and exploit any potential advantages of "newer" available discrete components, which I can only think of the small signal mosfets (Siliconix-Vishay..). Mixing the best characteristics of FETS/BJT/MOSFETs also has some appeal for discrete.

Thanks
-Antonio

That would be good to know and learn thier potential.

[RNMarsh]

Originally Posted by elektroj

....but did not include in his analysis important one that's been around for ages as discrete design.

I used that topology for a passively equalized phono stage (with fet input) - TAA 3/80. Good basic circuit.

[bcarso]

Originally Posted by elektroj

....but did not include in his analysis important one that's been around for ages as discrete design.

That's (post 94 schematic) also close to what Rich May did as power amplifiers in the Harman/Kardon TC400, although he inserted a common-base stage in the one second stage collector to equalize the dissipations of the second diff pair. He also compensated with a series RC network across the input collectors, and had some emitter degen in both diff pairs iirc. The resulting design had o.k. static distortion performance, and very good and symmetrical slew rate. The output buffer was I think cascaded common-collectors with the fast Toshiba output devices (2SA1302? etc).

I think I inherited it and screwed it up a bit This was in the early days. The line of automotive amps didn't sell very well; they were too expensive, and because of the "transverse tunnel cooling" didn't have the large expanse of heatsink beloved of the kids.

[magnoman]

Originally Posted by bcarso

For example?

Along the lines of SI1029X and or subsequentially larger devices (problem is even when I was using these a few years ago the parts wer becoming obsolete so quick, but at least they were being replaced with newer part numbers).
So I dont know what is readily available today but there seems to be a number of mfgr's and series to speculate from.

Hope this helps
-Tony

[bcarso]

Originally Posted by magnoman

Along the lines of SI1029X and or subsequentially larger devices (problem is even when I was using these a few years ago the parts wer becoming obsolete so quick, but at least they were being replaced with newer part numbers).
So I dont know what is readily available today but there seems to be a number of mfgr's and series to speculate from.

Hope this helps
-Tony

Thanks, didn't know of those. Most of the work in such parts is for rather lower voltage ones.

[bcarso]

Since virtually all of the likely designs to pour forth are going to want an output buffer, I dredged up and dusted off the attached.

It can be scaled back to lower rails without too much trouble. The bandwidth per se from a 1k source R is about 35MHz. There is the advantage of reduced thermal distortion when used with about a 515 ohm load, since the output devices are equidissipational*, that is, their dissipations roughly track. But when, as we anticipate, this is inside of an overall global loop with plenty of loop gain, the rest of the works will easily correct for dissipation-induced shifts, as the shifts should be fairly slow, and thus the thermal concerns should be minor.

The bootstrapping of the input devices raises the input Z, but also requires some lumped C to reduce peaking. Since such C is constant, the distortion at high frequencies at the input is reduced.

Short-circuit protection is left as the proverbial exercise to the reader.

Brad

*the approximate condition for this is that the peak output current into the load at just before onset of clipping is twice the quiescent current. Things are slightly more complicated by the emitter ballasting and the fact that the input devices draw their operating current from the output devices due to the bootstrap arrangement.