# electronic switch (like MOSFET, but without diode?)

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.

• A common solution to this dilemma is to use 2 MOSFETS back-to-back in series, so that their diodes point in opposite directions. – brhans Oct 13 '16 at 13:36
• Have you considered JFETs? – Ignacio Vazquez-Abrams Oct 13 '16 at 13:41
• Why don't you describe your application more. There is probably a better solution than the one you have constrained yourself to. – Daniel Oct 13 '16 at 14:45
• 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... – Ale..chenski Oct 13 '16 at 15:51
• Hmm. A very 'beefy' 4066 would be in order. Too bad 4-term discrete mosfets are unobtainium. – jonk Oct 13 '16 at 18:32

HEF4066B <200 Ω @12V only cost $0.15usd/1k simulate this circuit – Schematic created using CircuitLab You can use a solid state relay. TLP222AF (2 ohms,$1.08)
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)
• $2\:\Omega$ from datasheet. OP says $\le \tfrac{1}{2}\:\Omega$, as I read it. – jonk Oct 13 '16 at 19:24