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I have an idea that requires switching based on one of three input voltages. The basic idea is to have three different circuit behaviours based on three different voltage inputs. I've been doing this successfully with comparators, but that takes a lot of space. I'd like to do the same thing with discrete transistors or transistor arrays. Does this sound reasonable, or are the comparators actually the right solution?

It seems to me that if I had N and P transistors with one rather low voltage threshold and also had N and P transistors with a higher voltage threshold I could arrange them in various configurations to switch all sorts of ways as long as the voltage thresholds didn't overlap. However, in searching digikey, I couldn't find transistors with differing voltage thresholds clearly delineated. Are they all pretty much the same based on some physical fact of their manufacture or am I just looking for something really unusual?

Thanks much!

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  • \$\begingroup\$ If comparators are taking too much space, maybe find one in a smaller package. The first page of the product comparison table has at least one part in a 1x1 mm QFN. Or use a quad part (4 comparators per package). Either way you're likely to end up with a smaller solution than using transistors with all the required biasing components. \$\endgroup\$ – The Photon Jul 30 '15 at 2:53
  • \$\begingroup\$ can you tell us the 3 possible input voltages? Depending on how apart they are, using other components (like zeners) might be the way to go. \$\endgroup\$ – FrancoVS Jul 30 '15 at 5:24
  • \$\begingroup\$ BTW, instead of asking how you can make an hypothetical solution work, tell us the exact problem you are trying to solve. We might be able to give more useful advice. In other words, if you post a schematic or a block diagram with some specs of what you are trying to do, that would help a lot. \$\endgroup\$ – Lorenzo Donati Jul 30 '15 at 12:25
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It's not impossible (depending on what voltages you had in mind) - it's possible to get precision-threshold MOSFETs +/-20mV 0, 0.2, 0.4, 0.8, 1.4V but it seems like really silly thing to do when comparators and resistors are cheap, tiny (in the right packages) and very accurate.

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    \$\begingroup\$ Agreed. If he doesn't want to use comparators for some odd reason, there is always a tiny MCU with an ADC, but this seems like huge overkill. \$\endgroup\$ – Kurt E. Clothier Jul 30 '15 at 2:48
  • \$\begingroup\$ @KurtE.Clothier We don't know voltage, speed etc, but for slower speed applications that's an excellent suggestion. One part and he can set thresholds, hysteresis etc. with a few lines of code and no resistors. \$\endgroup\$ – Spehro Pefhany Jul 30 '15 at 2:54
  • \$\begingroup\$ You pretty much confirmed my suspicion. I've been using comparators (LM319) successfully. My problem is that I need lots and lots and lots of them and they just take so much board space! Each one has two comparators and each comparator is pretty much just standing in for a single transistor. If I could find a reliable way to use discrete transistors or arrays of them, I could save a lot of board space. Two transistors is only 6 pins. Two comparators (1 lm319) is 16 pins. \$\endgroup\$ – Rory O'Hare Jul 30 '15 at 7:41
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    \$\begingroup\$ The LM339 in SO14 has 4 comparators - if that would work for you. Quad resistor arrays can come in handy too if you can use several resistors of the same value. \$\endgroup\$ – Spehro Pefhany Jul 30 '15 at 12:16
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When you say "transistor" and "threshold voltage" I assume you mean MOSFETs. Anyway, basing any project on the precision of the threshold voltage of a FET is foolish, because the manufacturing spread is huge. You either need to measure accurately the threshold voltage (or make the manufacturer do it for you, but that's expensive). Moreover threshold voltages can change with temperature and that makes a design critical, unless you can guarantee that the temperature of the circuit won't change much.

EDIT (prompted by a comment on the answer of @SpehroPefhany)

You said:

If I could find a reliable way to use discrete transistors or arrays of them, I could save a lot of board space. Two transistors is only 6 pins. Two comparators (1 lm319) is 16 pins.

If you really need you can use transistors as comparators, but not relying on their characteristics, but on a particular circuit topology. You can build a decent comparator (not extremely precise, though) using a so-called long-tailed pair, i.e. a transistor differential amplifier, which in its cheapest, less precise incarnation, can be built with just two transistors. See this answer (and the related question) on EE.SE, which refers to BJTs (but something similar could be made with FETs, too).

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