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I was wondering why would anyone use a MOSFET amplifier?

Since a MOSFET amplifier is nonlinear, it produces significant amount of distortion.

To minimize distortion, textbooks suggest using small voltage variation around the Q-point, which makes amplifier very inefficient.

Why not then use a standard BJT amplifier instead?

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    \$\begingroup\$ FETs are usually used in closed-loop designs where the nonlinearity can be made almost irrelevant. \$\endgroup\$
    – pjc50
    Feb 12 '21 at 9:19
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    \$\begingroup\$ All transistors are non-linear. We work around this by using a high gain configuration but with negative feedback to correct the non-linearities. You are describing a particular situation where the device is being used "open-loop". Does that help? \$\endgroup\$
    – Transistor
    Feb 12 '21 at 9:23
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    \$\begingroup\$ What you write is true and that means MOSFETs and BJT based amplifiers without feedback distort the signal and are only usable for small signals. However, using feedback can fix these issues resulting in efficient amplifiers with little distortion. \$\endgroup\$ Feb 12 '21 at 10:58
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    \$\begingroup\$ It's not just MOSFETs and transistors. Valves are non-linear too. Feedback is part of the answer, but choosing the right circuit topology is important too. \$\endgroup\$ Feb 12 '21 at 14:25
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    \$\begingroup\$ @RodionDegtyar In that case the person who taught you were careless. \$\endgroup\$
    – pipe
    Feb 12 '21 at 18:25
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Well, BJT are not linear too, that's why we usually add a negative feedback path.

MOSFETs have a huge gate impedance which is useful in many occasions. Also their distortion behavious is… well different. MOSFET also need a lot less bias current so they are useful in battery applications.

As discrete component you almost never use a mosfet amplifier at low power (except in special cases, like some RF applications). However in power circuits it's a different story, efficiency it's usually better.

The real importance of mosfet in linear mode is in IC design where the process simplifies mixed signal design (logic and analog on the same chip). When you use opamps these days you are either using a mosfet amplifier or some hybrid (like BiCMOS processes).

Both MOSFETs and BJTs (JFETs, too!) have their use in both low power and high power applications. It's just that discrete designs are not prevalent these days.

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The main benefit of FET amplifiers is the high input impedance (at least for low frequenies).

Because BJT amplifiers always need some input current, they are not very well suited for amplifying signal sources with a high source impedance. Take for example biopotential measurements, with electrode to skin impedance as high as many megaohms.
FET amplifiers on the other hand can be designed to have extremely high input impedances of teraohms and even more. This makes it possible to measure the signal of very weak sources.

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  • \$\begingroup\$ Beware that FET are generally have higher rms input-referred voltage noise values, hence for 'weak sources' they may not be the best choice and bipolar transistors might be. \$\endgroup\$
    – edmz
    Feb 12 '21 at 21:21
  • \$\begingroup\$ @edmz Depends on the source. For low impedance voltage sources you are right, but for charge sources FETs are good. Tiny FETs can have noise levels in single digits, measured in electron charges. \$\endgroup\$
    – John Doty
    Feb 13 '21 at 0:41

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