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An n-channel MOSFET is an inexpensive way to connect and disconnect circuits to 0V. But if I have a fixed voltage (in this case, 2.5V) and want to connect fixed resistances of higher or lower voltage, the diode in the MOSFET will interfere?

There are analog switches, but they are expensive by comparison. I am looking for the equivalent of an inexpensive MOSFET that is 1Mohm when the gate is 0V, 0.5ohm or less when the gate is 5V (or 10V) above the reference voltage, but I want to handle voltages both higher and lower than the reference voltage.

The power in the circuit is tiny, in the milliwatt range, so dissipation is not an issue.

Is there any such device?

Here is an op amp circuit. It will allow variable gain depending on the whether the gates of M1 and M2 are high or low. This works when the resistors are connected to ground, and it should work when the resistors are connected to 2.5V as shown, as long as Vin is higher than 2.5V. I would like to know what to do if Vin is less than 2.5V.

enter image description here

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    \$\begingroup\$ A common solution to this dilemma is to use 2 MOSFETS back-to-back in series, so that their diodes point in opposite directions. \$\endgroup\$
    – brhans
    Oct 13, 2016 at 13:36
  • \$\begingroup\$ Have you considered JFETs? \$\endgroup\$
    – Ignacio Vazquez-Abrams
    Oct 13, 2016 at 13:41
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    \$\begingroup\$ Why don't you describe your application more. There is probably a better solution than the one you have constrained yourself to. \$\endgroup\$
    – Daniel
    Oct 13, 2016 at 14:45
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    \$\begingroup\$ It looks like OP is trying to built a variable-gain amplifier with binary-code control, and looking for analog switches. Next questions will be about parasitic capacitance... I just don't get the cost considerations... \$\endgroup\$
    – Ale..chenski
    Oct 13, 2016 at 15:51
  • \$\begingroup\$ Hmm. A very 'beefy' 4066 would be in order. Too bad 4-term discrete mosfets are unobtainium. \$\endgroup\$
    – jonk
    Oct 13, 2016 at 18:32

3 Answers 3

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HEF4066B <200 Ω @12V only cost $0.15usd/1k

schematic

simulate this circuit – Schematic created using CircuitLab

enter image description here

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You can use a solid state relay.

TLP222AF (2 ohms, $1.08)
http://www.digikey.com/product-search/en?keywords=TLP222AF

VO14642AT (1/4 ohm, $2.66)
http://www.digikey.com/product-detail/en/vishay-semiconductor-opto-division/VO14642AT/VO14642AT-ND/4074374

Or use a regular mechanical relay.
Coto 9007-05-00 (0.2 ohms, $1.27)
http://www.digikey.com/product-detail/en/coto-technology/9007-05-00/306-1062-ND/301696

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  • \$\begingroup\$ \$2\:\Omega\$ from datasheet. OP says \$\le \tfrac{1}{2}\:\Omega\$, as I read it. \$\endgroup\$
    – jonk
    Oct 13, 2016 at 19:24
  • \$\begingroup\$ @jonk you are correct. I added some other parts that meet the 1/2 ohm spec. \$\endgroup\$
    – user4574
    Oct 13, 2016 at 20:53
  • \$\begingroup\$ Sometimes what the OP wants is not what they need \$\endgroup\$
    – Tony Stewart EE75
    Oct 13, 2016 at 21:53
  • \$\begingroup\$ A relay? I want to change amplification more than 10 times/second. I liked the 2n7000 @ .05, so I'm not in love with these solutions, but thanks. \$\endgroup\$
    – Dov
    Oct 14, 2016 at 1:33
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If you need a really good DC isolation from any offsets, and speed of switching is not important, you can use "reed relays", like this one, and this one. These devices will give you parasitic capacitances under 1-2pF and will provide good bandwidth for variable-gain designs.

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