What is class a fet? And what is low-noise fet? Which is better?

Answer 1

Class A refers to the class of amplifier. "Class A FET" refers to a Class A amplifier that uses a FET in the circuit.

A preamplifier should use low-noise components, like a low noise FET. FETs generally provide higher input impedances than bipolar transistors, which is good for non-loading of microphones and pickups.

Class A amplifiers have the lowest distortion of all linear amplifier classes (A, AB, B, or C).

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Answer 2

Class of amplifier operation refers to the period of conduction of the transistor. In class A, the transistor is biased so that it stays in conduction for 360 degrees of signal phase. For Class AB, conduction is less than 360, but greater than 180 degrees. Class B is 180 degrees, and Class C is less than 180 degrees.

Since a Class A biased transistor is constantly in conduction, you tend to have better linearity and no crossover distortion. However, it is less efficient than the other bias configurations, which is why they are also useful.

A low noise FET is usually used for things like receiver front-ends, where you want to amplify a very small signal without adding noise that could bury the signal. A commonly used figure of merit for this is called Noise Figure, and simplistically speaking, it is SNR in/SNR out, where SNR is the signal to noise ratio. The lower the Noise Figure, the better.