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I have two feedback signals. I want to create a circuit that outputs the larger of the two. I know I could do this with diodes, but I'm unwilling to accept the diode drops. I suspect some op amp circuit would do the job, but I'm not sure how. What should this circuit look like?

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    \$\begingroup\$ Are you comparing ac signals? What (rough) frequency? \$\endgroup\$ – Peter Smith Nov 9 '15 at 13:15
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    \$\begingroup\$ @PeterSmith DC signals, 0-5V, feedbacks to a control circuit for a switching power supply running at 200 kHz. \$\endgroup\$ – Stephen Collings Nov 9 '15 at 13:21
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    \$\begingroup\$ A comparator controlling an analog switch? \$\endgroup\$ – Brian Drummond Nov 9 '15 at 13:22
  • \$\begingroup\$ @BrianDrummond that's neat - just like a synchronous rectifier. I'd make that an answer (before I do muhuhahaha) \$\endgroup\$ – Andy aka Nov 9 '15 at 13:29
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Use an op-amp precision rectifier, well, actually use two of these: -

enter image description here

Shown above is the circuit for one feedback signal - use another identical circuit for the other feedback signal. Then connect the outputs together - highest input dominates the output.

Because the basic circuit inverts you might probably want to invert the inputs to each precision rectifier first.

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Sketch of a solution : for it to work well, the devil's in the details...

A comparator decides which signal is larger. Its output controls an analog switch, aka 2:1 multiplexer, connecting the selected signal (the larger one) to the output.

schematic

simulate this circuit – Schematic created using CircuitLab

You probably want a "make before break" switch (read the datasheets) so that at the moment of switching, the output is the average of the two input values. As they are by definition the same voltage at the moment of switching, this prevents glitches or spikes.

Note : depending on the actual devigce you use as a comparator, you may need a pullup resistor on its output (the MUX "select" line). Details of comparator, MUX control voltages etc aren't shown, they will depend on the actual devices selected.

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