What Is a MOSFET Amplifier?

MOSFET

• A metal oxide field-effect transistor, or MOSFET, is an electronic component that amplifies and switches electrical currents. In the 1970s, electronics manufacturers made them with metal oxides, but more recently, silicon has been the material of choice. The device has three terminals, called the gate, source and drain. A voltage at the gate terminal controls the current flow between the source and drain. A MOSFET can detect and boost very weak signals, making it a useful amplifier.

Amplifiers

• Audio amplifiers boost the strength of weak sound signals. For example, the signal in a radio starts at an antenna, which produces a signal too weak to drive a speaker directly. An amplifier increases the strength of the signal so you can hear it clearly. In electronic amplifier design, a trade-off exists between efficiency and fidelity. Amplifiers called Class A reproduce a signal faithfully but consume a great deal of power. Class B and C amplifiers have better efficiency but introduce distortion into the sound. Many MOSFET amplifiers are Class D, but with changes that improve sound.

Class D

• A Class D amplifier simply switches on and off rapidly in response to an input signal. Normally, this would produce an intolerably distorted sound, but in modern MOSFET amplifiers, this is done at frequencies of about 500,000 hertz. The switching frequency is far beyond the range of human hearing, which normally does not go beyond 20,000 Hz. The MOSFET amplifier removes the high-frequency signal from the amplified sound and the result has very good fidelity.

Uses

• In sound reproduction, the lowest frequencies, such as those for thunder and explosions, take the most power from an amplifier. Woofers and subwoofers are speakers specially designed for these low frequencies, and they take more power than other kinds of speakers. A MOSFET amplifier can drive speakers to kilowatt power levels with good clarity and fidelity. A class A or B amplifier would be impractical at these levels because it would consume many more kilowatts than a MOSFET design, and produce enough heat to roast the amplifier. MOSFET amplifiers take reasonable amounts of power and run cool enough to be practical.