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RF power amplifier

An RF power amplifier
Class C VHF power amplifier based on the transistor MRF317.

A radio frequency power amplifier (RF power amplifier) is a type of electronic amplifier that converts a low-power radio-frequency signal into a higher power signal. Typically, RF power amplifiers drive the antenna of a transmitter. Design goals often include gain, power output, bandwidth, power efficiency, linearity (low signal compression at rated output), input and output impedance matching, and heat dissipation.

Amplifier classes

Many modern RF amplifiers operate in different modes, called “classes”, to help achieve different design goals. Some classes are class A, class AB, class B, class C, which are considered the linear amplifier classes. In these classes the active device is used as a controlled current source. The bias at the input determines the class of the amplifier. A common trade-off in power amplifier design is the trade-off between efficiency and linearity. The previously named classes become more efficient, but less linear in the order they are listed. Operating the active device as a switch results in higher efficiency, theoretically up to 100%, but lower linearity.[1] Among the switch-mode classes are Class D, Class F and class E.[2] The Class D amplifier is not often used in RF applications, because the finite switching speed of the active devices and possible charge storage in saturation could lead to a large I-V product[1], which deteriorates efficiency.

Solid state vs. vacuum tube amplifiers

Modern RF power amplifiers use solid-state devices such as bipolar junction transistors and MOSFETs.[3] Transistors and other modern solid-state devices have replaced vacuum tubes in most electronic devices, but tubes are still used in some high-power transmitters (see Valve RF amplifier). Although mechanically robust, transistors are electrically fragile – they are easilly damaged by excess voltage or current. Tubes are mechanically fragile, but electrically robust – they can handle remarkably high electrical overloads without appreciable damage.

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).[citation needed]

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

is defined as the breakdown voltage

is defined as the knee voltage

and typically

The loadline method is often used in RF power amplifier design.[4]

References

  1. ^ a b Lee, Thomas (2003). The Design of CMOS Radio-Frequency Integrated Circuits. Cambridge: Cambridge University Press. pp. 494–503.
  2. ^ Cloutier, Stephen R. "Class E AM Transmitter Descriptions, Circuits, Etc". www.classeradio.com. WA1QIX. Retrieved 6 June 2015.
  3. ^ MFJ Enterprises. "Ameritron ALS-1300 1200-watt NO TUNE TMOS-FET AMPLIFIER". MFJ Enterprises. Retrieved 6 June 2015.
  4. ^ Matthew Ozalas (January 14, 2015). "How to Design an RF Power Amplifier: The Basics". youtube.com. Retrieved 2015-02-10.

External links