RF power amplifier

Not to be confused with Audio power amplifier.

An RF power amplifier

Class C VHF power amplifier based on the transistor MRF317.

A radio frequency power amplifier is a type of electronic amplifier used to convert a low-power radio-frequency signal into a larger signal of significant power, typically for driving the antenna of a transmitter. It is usually optimized to have high efficiency, high output Power(P1dB) compression, good return loss on the input and output, good gain, and optimum heat dissipation.

RF amplifiers formerly were all made used vacuum tubes (called valves in some English-speaking countries). Tubes are still used for exceptionally high-power transmissions (see Valve RF amplifier). Many modern RF amplifiers of over 1000 Watts are solid state using various classes, class A (in various forms, Class AB, AB1, etc.), class B, class C and class E. Class D amplifiers are rarely used for RF purposes, as it would require an even higher frequency device to be used. Solid state devices used in these solid state RF amplifiers are MOSFET, TMOS-FET, Bipolar junction transistors, and IGBT.

Applications

The basic applications of the RF power amplifier include driving to another high power source, driving a transmitting antenna and exciting microwave cavity resonators. Among these applications, driving transmitter antennas is most well known. The transmitter–receivers are used not only for voice and data communication but also for weather sensing (in the form of a radar).

Wideband amplifier design

Impedance transformations over large bandwidth are difficult to realize, thus most wideband amplifiers use 50 Ω output loading. Transistor output power is then limited to

$P_{out} \le \frac{(V_{br} - V_k)^2}{8Z_o}$

$V_{br}$ is defined as the breakdown voltage

$V_k$ is defined as the knee voltage

and typically $Z_o = 50\Omega\,$

The loadline method is often used in RF power amplifier design.