I have a project requiring a switch (I supply 4.8V, the switch stays off. I supply 5V, it turns on.) After some testing and asking several questions, I realized that the MOSFET I was using would not cut it. In EE.SE chat, I was informed that a comparator could solve my problems. Doing some research, I found a schematic for an OP Amp Comparator. Changing it a bit, I got this: OP Amp Comparator

Assuming this works, how would I choose an OP amp to work here? Looking on Digikey and several other online electronics stores showed me that I know next to nothing about OP amp specifications.

Generally, I want it to be fast (Using the FET for high-speed switching), and can handle 5V plus a margin (call it 6-7V?) What should I look for on a spec sheet that represents those things? Also, since there were about 10 different option tables, what would be a good, nothing fancy, set of specs to work from?

  • \$\begingroup\$ What do you mean by fast (1 ms or 1 ns)? Why do you want to use an op-amp when a comparator would work much better? Why did you add the diode between the op-amp/comparator and the FET? \$\endgroup\$ – The Photon Oct 10 '14 at 4:17
  • \$\begingroup\$ @ThePhoton 1ms should be fine. Where would I look to find? I looked on Digikey, but perhaps I overlooked? Oh, yeah. FETs require negative voltage to turn off, right? Forgot that. **1ns better than 1ms but I think both should work. \$\endgroup\$ – CoilKid Oct 10 '14 at 4:22

First, you should use a comparator if you want to do comparisons.

Choose one with a switching time substantially less than what you want the overall switching time of the circuit to be (with 1 ms spec, you should be able to choose almost any of them).

Second, choose one that can drive enough output current to pull the FET gate full swing quickly enough. On chat, you mentioned the DMG9M65CT, whose datasheet gives a gate capacitance of 2.3 nF. To switch this by 5 V in 1 ms requires about 10 uA of current. Again, you should have no problems finding a comparator that can do this. (Your problem would be harder if you wanted to switch in less than a microsecond, for example).

Third, choose whichever comparator has the lowest price, is available for immediate delivery, has the package you want, comes from a vendor you trust, etc.

Final note, remember that many comparators have open drain outputs. That means they can only pull their output low. You'll need to have a pull-up resistor to pull the output high when it should be high. Choose the highest-value pull-up resistor that gives an adequately fast switching time for the low-high transitions. With a 15 V positive supply available, this could be as high as 50 kohms, but if your available supply is not very much above 5 V, you might need a much lower value.

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  • \$\begingroup\$ 10uA current. What type? I see... Current - Input Bias, Current - Output, and Current - Quiescent. I assume that it would be Current -Output with a voltage of 5V? \$\endgroup\$ – CoilKid Oct 10 '14 at 4:48
  • \$\begingroup\$ It's the current the comparator (or pull-up resistor) will be driving in or out of the gate to change it's voltage during a switching event. \$\endgroup\$ – The Photon Oct 10 '14 at 4:49
  • \$\begingroup\$ So, output current? \$\endgroup\$ – CoilKid Oct 10 '14 at 4:57
  • \$\begingroup\$ Yes, output current of the comparator. \$\endgroup\$ – The Photon Oct 10 '14 at 4:58
  • \$\begingroup\$ Would this work, with the listed FET? digikey.com/product-detail/en/LMV331IDBVR/296-6633-2-ND/372773 \$\endgroup\$ – CoilKid Oct 10 '14 at 15:22

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