A plethora of puzzling op amp puzzles

RE: A plethora of puzzling op amp puzzles

Tim Green1 — Mon, 17 Feb 2014 21:45:41 GMT

Answers are now posted so your theories and observations can now easily be either confirmed or denied.....Thanks for taking the time to puzzle.

RE: A plethora of puzzling op amp puzzles

Bruce Ploetz — Mon, 17 Feb 2014 13:44:52 GMT

Ken, if you need network theorems and SPICE to deal with these you must be a bit wet behind the ears. They are all standard common circuits well known since the 60's. The kicker on these is the data sheet, and your network theorems won't help you there! It's non-linear! Hint: most of them are jammed against the rails.

RE: A plethora of puzzling op amp puzzles

Tim Green1 — Mon, 03 Feb 2014 17:04:55 GMT

Right on positve feedback, Ken. Justed wanted to see if you were aware of it...

RE: A plethora of puzzling op amp puzzles

Tim Green1 — Mon, 03 Feb 2014 17:03:48 GMT

Richard Price Feb 03 2014 02:14 AM In the table of specifications, "Voltage Output Swing from Rail" makes no sense with those numbers - they must be output swing from 0V. The actual OPA735 datasheet does specify output swing from rails, but that is 20mV typ, 50mV max at 10kOhm load.

Response: Table of specifications given in the blog are the output voltage of the op amp when powered form +/-5V and delivering the noted current. The more output current delivered, the farther away from +/-5V the output will be or larger drop from the rail. The specifications I give in the blog are located in the data hseet curves, specifically "Voltage Output Swing from Rail". Hope this helps.

RE: A plethora of puzzling op amp puzzles

Ken Dillinger — Mon, 03 Feb 2014 14:53:58 GMT

I know there are a few other places where positive feedback may be used with op-amps. If you want to use an op-amp as a comparator you can certainly use positive feedback for hysteresis. The Sallen-Key filter also uses positive feedback too.

As long as it is the negative feedback path that dominates the loop gain then the amplifier should be stable.

-Ken

RE: A plethora of puzzling op amp puzzles

Richard Price — Mon, 03 Feb 2014 08:14:47 GMT

In the table of specifications, "Voltage Output Swing from Rail" makes no sense with those numbers - they must be output swing from 0V. The actual OPA735 datasheet does specify output swing from rails, but that is 20mV typ, 50mV max at 10kOhm load.

RE: A plethora of puzzling op amp puzzles

Salil Tembe — Sun, 02 Feb 2014 15:55:27 GMT

I think I have to dig out my textbook to refresh my knowledge :D Nice puzzles!

RE: A plethora of puzzling op amp puzzles

Tim Green1 — Sat, 01 Feb 2014 01:15:16 GMT

Glad to see someone out there is puzzling, Ken. But hey doesn't Puzzle 1 have "positive feedback"? But i remember from college that positive feedback usually casues instability in op amps. What a puzzle?

RE: A plethora of puzzling op amp puzzles

Ken Dillinger — Fri, 31 Jan 2014 03:27:42 GMT

Figure 1 is the longest to derive Vout. I think it is supposed to be a constant current source.

The remaining figures are rather simple, save for some of the real world gotchas. ;-)

-Ken

RE: A plethora of puzzling op amp puzzles

Tim Green1 — Thu, 30 Jan 2014 18:43:58 GMT

I would hope you would not use the SPICE space but rather the space between your ears and the back of a napkin and a pen like us "old school" engineers! Good luck on the puzzling!

RE: A plethora of puzzling op amp puzzles

Ken Dillinger — Thu, 30 Jan 2014 17:59:07 GMT

Do we get bonus points if we do this using network theorems rather than plopping these in SPICE?

Any self respecting analog engineer would use KCL, KVL, Thevenin or Superposition!

-Ken