Discrete Opamp Open Design

[dirkwright]

Originally Posted by Hentai

The biggest advantage that i see in discrete opamps would be higher power supply voltage capability and thus higher output voltage swing.

Right. There's no need for a folded cascode, for example, since that particular circuit was invented to solve a problem that we don't have.

Don't forget current drive. While there are plenty chip buffers that will drive 30 ohms, I'm not aware of very many opamps that can do that (my awareness is very limited however).

[scott wurcer]

Originally Posted by RNMarsh

Please explain in more detail the rising thd with frequency cause. Thx - RNM

The crossover is an "S" shaped voltage discontinuity across the output stage. Inside the loop the current needed in the compensation capacitance is the derivative of this voltage or a "hump" at zero crossing which gets larger as the dV/dT increases (higher input frequency). The more bias (also degeneration tricks) will reduce the size of the discontinuity in the first place.

[PMA]

One more.

[scott wurcer]

Originally Posted by hitsware

The H in LH was for hybrid (IIRC)

That is correct.

[jneutron]

Originally Posted by scott wurcer

That is correct.

IIRC, it was an active trimmed 4 chip puppy. Back in the 80's, my boss was trying to duplicate it for use in some mil test equipment, programmable ATE.

j

[scott wurcer]

Originally Posted by jneutron

IIRC, it was an active trimmed 4 chip puppy. Back in the 80's, my boss was trying to duplicate it for use in some mil test equipment, programmable ATE.

j

Remember the LH0028 "Design a better inamp and we'll give you a job" campaign?

[jneutron]

Originally Posted by scott wurcer

Remember the LH0028 "Design a better inamp and we'll give you a job" campaign?

No, don't remember that. At the time, I was a lowly captive hybrid design engineer, they never let me out...

j

[zinsula]

Originally Posted by PMA

One more.

Like it. Kinda mix between +input stage of AD844 and diff amp of LT1364
No Wilson mirror though.

[bcarso]

Originally Posted by Hentai

The biggest advantage that i see in discrete opamps would be higher power supply voltage capability and thus higher output voltage swing.

Also remember the constraints of chip area, number of process steps, and yield from a wafer when making ICs. Scott remarked that the 2SK170 is larger than some of ADI's recent video amps!

And then there are a host of components that discrete or hybrid designers are free to use that are not available for integration: big caps, semiconductor devices with very different composition (e.g. GaAsP, SiC, GaN), batteries, transformers. It's not a hard and fast dichotomy as clever IC folks are coming up with ever-more diverse means for mixing things. And discrete constructions have benefits: at one time it was junction isolation or nothing, which led to all sorts of subsidiary issues; dielectric isolation (see the OPA627 in question for example) was a great advance. Another tough one in early days was getting good stable resistors.

Thermal issues cut both ways, as inherent matching of monolithic is a plus for ICs, but thermal feedback must be controlled through careful layout and design.


Brad

[scott wurcer]

Originally Posted by zinsula

Like it. Kinda mix between +input stage of AD844 and diff amp of LT1364
No Wilson mirror though.

PMA's circuit looks like two CFA inputs looking at eachother, it lends itself very well to full differential connection.

For that matter that simplified 844 made with discrete duals could be tuned up nicely for several applications. I think there are cheap SMT dual's in the BXXXX series. That's 8 SOT6's and an output pair and you have access to all nodes so you play with Charles Hansen's ideas.