Amplifiers Gain Power And Minimize Noise

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Amplifiers come in many shapes and sizes, from audio through optical wavelengths. For RF/microwave communications amplifiers, the two main duties have to do with strengthening low-level signals for receivers and boosting high-level output signals for transmitters. Although they differ in function, as well as in size and power requirements, both types of amplifiers benefit from continuing improvements in transistor technologies.

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