# A plethora of puzzling op amp puzzles

In honor of National Puzzle day here in the United States of America, I humbly present six op amp puzzles for you to puzzle on until your puzzler is sore – or at least until next week when I will publish the right answers!

I’ve even provided you with some key op amp specifications to help in Figure 1.

 PARAMETER CONDITION UNIT MIN TYP MAX Vs=±5V unless otherwise noted Operating Voltage Range VS ±1.35 ±6 V Common-Mode Voltage Range VCM (V-) -0.1 (V+) - 1.5 V Short Circuit Current ISC ±23.5 mA Voltage Output Swing from Rail VO IOUT=+5ma (source) 4.70 V VO IOUT= +10ma (source) 4.44 V VO IOUT= +20ma (source) 3.85 V VO IOUT= -5ma (sink) -4.81 V VO IOUT= -10ma (sink) -4.62 V VO IOUT= -20ma (sink) -4.00 V Input Offset Voltage VOS 1 5 uV Input Bias Current IB VCM=VS/2 ±100 ±200 pA Input Offset Current IB VCM=VS/2 ±200 ±300 pA

Figure 1: Key Op Amp Specification for OPA735

Without any further ado, let’s get puzzling!

Puzzle #1: What is Vout?

Puzzle #2: What is Vout?

Puzzle #3: What is Vout?

Puzzle #4: What is Vout?

Puzzle #5: What is Vout?

Puzzle #6: What is Vout?

Please do not post your answers so that those who are still puzzling can keep on puzzling, until next week when I publish the answers in my next blog post. I’ll be monitoring the comments and will jump in to answer questions and hints, if you get stuck.

A special thanks to my boss, Art Kay, for the starter seed of the idea for these op amp puzzles.

Happy puzzling!

PS: If you thought this was fun, you might want to check out these posts from long-time TIer and retiree Bruce Trump.

• 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

• 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!

• 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

• 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?

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

• 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.

• 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

• 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.

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

• 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.

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