[falkon72]

How does a MOSFET amp work? What is it used for? and How is it different from opamp and valve?

 

[morsel]

MOSFETs are voltage gain devices with characteristics closer to vacuum tubes than BJTs. MOSFETs have a metallized gate separated from a semiconductor channel by a thin insulating layer. Charge applied to the gate controls the flow of electrons through the channel.

MOSFET .vs. BJT .vs. tube amplifiers is a subject of holy wars. I am firmly in the MOSFET camp. Typically, when MOSFET or tube amps distort, they enter a gradually increasing nonlinear mode, resulting in mostly sweet-sounding even order harmonics, whereas when BJT amps distort it is usually hard clipping, which results in harsh odd order harmonics. MOSFETS, unlike tubes, don't need periodic replacement, or ring when jarred.

Having said that, you are probably more interested in headphone amps. MOSFET and tube headphone amps have a fair amount in common, although many tube amps intentionally create even order harmonic distortion to sweeten the sound. Opamp based amps are all BJT output based, but really, opamps are in a class by themselves due to the peculiar nature of how they work.

An ideal opamp has infinite input impedance and gain, zero output impedance, zero DC offset, and probably some other ridiculous specs I don't recall, go read a book on it for more details. Anyway, opamps have a plus and minus input, and amplify the difference between the inputs. In order to get a "real world" gain, a feedback loop is incorporated using two resistors. Opamps are relatively easy to use and versatile, so they are common in electronics. We use them in headphone amps because of this convenience factor.

I hope this at least partially answers your questions.

 

[rohitbd]

A MOSFET is a device (other than a BJT) that is used to create op-amps and other circuits either in discrete or integrated form. Even tubes can be used to create op-amps, in discrete form. For op-amps that come in IC form only BJTs or FETs can be used. MOSFETS are not that common for low power op-amps, and are mostly used for power op-amps in their o/p stages. An op-amp basically is any ckt (amplifier) that can do "operations" like electrical addition, subtraction, integration, differentiation, multiplication of signals. Hence the name operational amplifier.

 

[tangent]

Quote:

other ridiculous specs I don't recall

Also infinite bandwidth, infinite slew rate, a linear gain-bandwidth curve, infinite PSRR and CMRR, and 90 degrees of phase margin. Ah, what a wonderful dream....

Quote:

For op-amps that come in IC form only BJTs or FETs can be used.

There are CMOS-based IC op-amps, which is a MOSFET type process, no? The problem is, no one uses them for audio because the main point of CMOS is low-power operation, which entails sonic compromises. I'm sure there's no electrical reason you couldn't make a good-sounding MOSFET-based op-amp. I'm just saying that I've never heard of such a thing.

 

[rohitbd]

Tangent,
What you say is right...it should have been phrased as "For op-amps that come in IC form, more commonly BJTs or FETs are used"...