Cool Pease circuit - floating simulated inductor

[c4757p]

Just stumbled on this and thought I would share it, in case it's useful to anyone. It's certainly clever. We've all seen the op-amp simulated inductor, but one end of that inductor is grounded. Bob Pease gives a floating version in this video



I had to tweak the series resistance to get it to follow a real inductor. Offsets need to be closely matched (microvolts).

[mtdoc]

Great video. Some of it over my beginner head but ya gotta love the "Bogosity Factor" !

Pease is the best!

[codeboy2k]

"If analog people can do stupid things, can stupid people do analog things?"   
-- best line ever!

[GreyWoolfe]

My thought is that stupid people live in a digital world.  Yes and no=yes, I am going to do that anyway and no, I don't care what you say, I won't do that.  Yes being +5 VDC and no being 0 VDC.  There is no DC offset needed, it is hard enough to get them to comprehend yes and no and we don't want to get into shades of grey if we include an offset voltage. 

[skipjackrc4]

Yeah, a few years ago I found a similar circuit in the IEEE archives from the 60s.  Definitely a cool idea, though I've never actually used it. 

[IntegratedValve]

I think this is an application to....Gyrator?

http://en.wikipedia.org/wiki/Gyrator

[codeboy2k]

I think this is an application to....Gyrator?

http://en.wikipedia.org/wiki/Gyrator

Yep, as the wiki says they are popular in phone circuits, called DAA (modems, fax machines) and FXO (Foreign eXchange Office) which is the phone end of the circuit, which may in actuality be a physical phone connected to a central office (CO), or the line cards in the racks at the customer premises of a private business exchange(PBX)  that go to a CO... basically the FXO is your end of the line to the CO... the CO end is called an FXS (although a PBX served by a digital line these days may have local FXS line cards that go to local POTS phones that are then the FXO end.

The gyrator used in the DAA/FXO holds the DC line current , so the DC current doesn't have to flow through the transformer that is actually on the line. This allows the transformer to be made smaller and cheaper, and thus more line cards can fit in a rack. By the way.. more old terminology.. this is called "wet" and "dry" ... "wet" means the transformer is designed to carry the DC current without saturating , "dry" means it does not.  All modern telephone circuits use dry transformers and a gyrator circuit for the DC current loop.