I discovered I could easily make an H-bridge (or something that effectively behaved as one) by cross coupling the inputs of two op amps. So for op amps 1 and 2, 1+ is connected to 2- and 1- is connected to 2+. This worked great, but since I am not really amplifying a signal so much as comparing them, this seemed like a better job for comparators. However, when I tried the same thing with comparators, it did not work. After studying comparators a bit, I realized the problem had to do with the fact that a comparator's output cannot not reverse polarity like an op amp, but can only act as a switch. I get the feeling I would need four comparators for this to work, but I could use some help. Since I am using them in an odd way, the recourses online did hot help much.

My comparator is LM2903P. My op amp is TL082.

Here is the circuit I am using:

enter image description here

It is being used as a multivibrator. I would like to be able to get higher frequencies, so being able to use comparators would be useful.

  • \$\begingroup\$ If your circuit worked great using opamps, why do you want to use a comparator? Many opamps work just fine at low speeds when run open loop. Most IC comparators have an open-collector output, which will make using them in such an application difficult. \$\endgroup\$ – Bitrex Apr 18 '12 at 2:15
  • \$\begingroup\$ I would like to say, "because this is a job for comparators" or something, but to be honest, I ordered a bunch of comparators before I realized there was a difference and I am trying to see if there is a way to use them while I wait for my order of op amps to come. \$\endgroup\$ – Alex Eftimiades Apr 18 '12 at 2:22
  • \$\begingroup\$ Actually you said they work fine at low speeds. Well, I was using this as part of a multivibrator and would like to be able to get to higher frequencies if possible. So I do have an excuse to use comparators other than making a dumb mistake in ordering the wrong product. \$\endgroup\$ – Alex Eftimiades Apr 18 '12 at 2:24
  • \$\begingroup\$ interesting circuit. It looks sort of like the classical multivibrator en.wikipedia.org/wiki/Multivibrator upload.wikimedia.org/wikipedia/commons/thumb/6/6a/… \$\endgroup\$ – Jason S Apr 18 '12 at 3:05
  • \$\begingroup\$ Also keep in mind that in general, a TL082 will only operate properly from a split supply. \$\endgroup\$ – Bitrex Apr 18 '12 at 4:13

Ideally comparators CAN switch output polarity.

Most real world ones can too.

If yours can't you may be using open collector / open drain ones like an LM393 or its cousin.

As usual, you need to supply a circuit diagram and par number to have best chance of people understanding what you are really doing.

  • \$\begingroup\$ I included a circuit diagram and part numbers. \$\endgroup\$ – Alex Eftimiades Apr 18 '12 at 2:53
  • \$\begingroup\$ Could you recommend a particular part number based on the circuit--anything that is cheap and NOT LM393 or its cousins? \$\endgroup\$ – Alex Eftimiades Apr 18 '12 at 2:55
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    \$\begingroup\$ look up "push pull comparators" google.com/search?q=push+pull+comparators LMV762 comes up near top \$\endgroup\$ – Jason S Apr 18 '12 at 3:02

comparators are poor choices for totem pole cascode H mode drivers. You have to manage the transition times so that you dont end up shorting out the V+ to ground or create a deadtime or break before make. Unfortunately open collector drivers such as the IC you chose have fast ON times but relative slower OFF times controlled by pullup current and junction capacitance.

The best H bridge drivers use MOSFETs and even then it only looks trivial, but you could make something work with 4 NAND gates if you wanted and make sure you use the 2nd gate input to control the dead time of the H bridge drivers.

Remember to decide what you want to switch, over what range of power and voltage and temperature and ensure it works for worst case conditions. ;)When in doubt put in PTC polyfuses.;) My son in-law, EE Prof @ U of T says his students blow bridge drivers all the time... !!! ha ( not cheap when they are 10KW drivers)


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