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[SOLVED] Using a single FET as analog switch?

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tmcone

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Hi,

I want to control the gain of an inverting amplifier(just two gain configurations).Since the board size in my design is very restricted, I can't use a 2 or more channel analog switch. What I need is only a FET as a SPST analog switch.Is it possible to use a FET as an analog switch?
 

It is, but it will have common-mode voltage induced variation
in on-resistance that may bother the linearity. Smart choice
of how you use it can help that. Like attaching it at a virtual
ground point where signal is always roughly zero. Shunt vs
series, etc.

There are also packaging options out there that are smaller
than a TO-92 package, surface mount. Probably single
channel analog switches too. Presuming you like the voltage
range of the itty bitty ones which may be likelier to come in
low voltage.
 
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    tmcone

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A tiny SPDT in SOT23-6 may be an alternative to a discrete transistor. It opens circuit options for gain switching not affected by switch Rdson. You'll find a large repository e.g. at Analog Devices. You should also consider standard single gate logic devices like 74LVC1G3157. Smallest package is 0.9 x 1.4 mm chip scale.
 
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    tmcone

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Thanks for replies. I will look for single channel SPST or SPDT switch.
 

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