Depending on output requirements, amplifiers range from tiny chips to complete subsystems with digital interfaces. In general, the trend toward higher levels of integration, such as embedding an amplifier on a chip with other transceiver components, still favors small-signal designs. Large-signal or power amplifiers are still mostly designed around discrete transistors and with discrete matching components. Power transistors are simply larger than low-noise or small-signal transistors. They dissipate more heat than low-noise transistors and require larger supporting (impedance-matching, power-supply) passive circuit elements, making a power amplifier larger than a low-noise amplifier (LNA). Power amplifiers operate on current in the ampere (A) range, compared to LNAs that only require milliamperes (mA).
At one time, both microwave low-noise and power amplifier designs were dominated by their use of GaAs field-effect transistors (FETs). But the development of other transistor architectures, such as GaAs heterojunction bipolar transistors (HBTs) and high-electron-mobility transistors (HEMTs), has given amplifier designers alternatives to traditional high-frequency silicon bipolar transistors and GaAs FETs for high-frequency LNAs. For power amplifiers at higher frequencies, GaAs FETs are still the device of choice. At lower frequencies, however, silicon laterally diffused, metal-oxide-semiconductor (LDMOS), silicon-carbide (SiC), and gallium-nitride (GaN) transistors offer impressive power densities.
Specifiers of high-frequency amplifiers have probably never had as wide a selection. Many suppliers offer both LNAs and power amplifiers, while others may specialize in one type or the other. Most companies feature selections
