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Considerations In Choosing An RF Amplifier



April 11, 2016

What Is An RF Amplifier?

An RF amplifier is similar to an audio amplifier, but applied at much higher frequencies. The frequency boundary where an amplifier is considered to be an RF type is typically vague. Applications requiring a frequency over 100 kHz often defines this boundary. We will address this in the amplifier types and frequency response section. Being an amplifier we will be concerned with power output, or wattage, frequency response, noise figure, efficiency, gain, and distortion / linearity.[1]

Amplifier Types

Amplifier technologies come in several types. Each type will have a different frequency response characteristic. The major technologies of RF power amps include: vacuum tube, TWT, and solid state. The solid-state types can be further classified as bipolar junction or BJT, metal oxide silicon field effect transistor or MOSFET. There are several other technologies used for solid-state semiconductor RF amplifiers. Please see the footnote for more information on the other types.[2] A TWT amplifier is also known as a traveling wave tube amplifier. These have very wide bandwidth and can be relatively high power, up to 2.5kW. Another feature of TWT amplifiers is that they are very low noise so they are very useful in EMI testing as well as clean amplification. The main issue with TWT amplifiers is that they must be properly implemented as they can be damaged if not. Vacuum tube amplifiers have been phased out except for very high power applications such as broadcast transmitters. Solid state and TWT amplifiers are preferred in all but the highest power uses.

Power

The output power can range from as small as 0.1 watt up to megawatts. Regardless of the level of output power both the input and output impedances will be 50 ohms. The lower power amplifiers are considered to be pre-amps or driver amplifiers. From approximately 10 watts and up can be considered power amplifiers, which are meant to drive a large amount of signal into various kinds of loads.

Efficiency

This can also be called amplifier class as it referrers to the output duty cycle. Class A means that the amplifier will be able run with 100% duty cycle. Other classes offer progressively less duty cycle in exchange for increased output power and distortion. These other classes are also discussed in footnote 2. Please refer to it for more information on classes of amplifiers.

Frequency response

The frequency response will be well defined depending on the type of amplifier being used. TWT amplifiers can typically be on the order of two octaves. Solid state can even be a wider bandwidth. Regardless of how wide of a bandwidth an amplifier has, they can also be tuned to a narrow bandwidth for specific purposes. This is usually done to reduce the harmonic content of the output. Pay attention to the dBc specification of the amplifier because in many cases harmonic content may be larger than you expect. It is important to understand your bandwidth requirements as you are choosing an amplifier for your application.

RF Amplifier Applications

There are many applications for RF amplifiers; some are beyond the typical expectation of simple signal amplification. Cellular networks are a major user of RF amplifiers and they need to be linear, i.e. non-distorting, because the signal contains modulations such as QPSK, QAM, OFDM. Constant envelope signals such as CW, FM, or PM do not require linear amplification and so these RF amplifiers will typically be higher power. One of the many applications for RF amplifiers is for inductive heating, typically in an industrial setting. One of the main applications of RF amplifiers is EMI, or electro/magnetic interference, testing. In this application the amplifier is typically connected to an antenna that is co-located inside an anechoic chamber together with the article under test. Certainly video is another area where RF amplification is predominant. They are used in video distribution systems and broadcast. Speaking of broadcast, RF amps are used to drive the antennas of all over the air broadcast channels, for both radio and television. Wifi is another use for the smaller RF amps.

RF Amplifier Cautions

You should be aware of several issues in using RF amplifiers, especially higher power ones over 10 watts. RF radiation produced by RF amplifiers can be ionizing. That is it can literally cook you. Think of a microwave oven. It operates at approximately 2.4 GHz from 400 to 1000 watts. There is a reason it is shielded!

 

Never operate a high powered, or any RF amplifier for that matter, with the output un-terminated!

 

Not only will you prevent a hazardous situation, you will prevent possible damage to the amplifier. All of the energy generated will be reflected back into the amplifier if it is un-terminated, possibly destroying the output devices. Also make sure you do not overdrive the input as this can also damage the output stages. Make sure your output load is capable of handling the amount of power that the amplifier is able to produce, even if you do not intend to use its full output. Momentary full output excursions are possible and that can destroy the circuit you might be driving. Make sure that the coaxial connection from the amplifier to the load is capable of carrying the power level you intend to deliver. Speaking of power, you need to provide adequate cooling for the amplifier. Some types can generate significant amounts of heat that will need to be dissipated to avoid damaging the amp.

RF Amplifier Suppliers/Manufacturers

Since 2005, Axiom Test Equipment has been providing a variety of RF amplifiers for rental or sale. Additionally, we offer repair on most brands and types of RF amplifiers. Major manufacturers we have in stock include Keysight (formerly Agilent), Amplifier Research, Ophir, IFI, and Milmega. Axiom Test Equipment offers a complete range of RF amplifiers for your needs, with power capability ranging from 30 W up to more than 1 kW as well as frequency response up to 18 GHz. View these online at, http://www.axiomtest.com/Amplifiers-_-RF-and-Microwave/.

 

[1] Rosu, Iulian. RF Power Amplifiers. N.p.: QSL.net, n.d. PDF.

 

[2] Raab, Frederick H., Peter Asbeck, and Steve Cripps. "RF and Microwave Power Amplifier and Transmitter Technologies — Part 1." High Frequency Electronics (2003): 22-36. Ecee.colorado.edu. Summit Technical Media, May 2003. Web. 2 Feb. 2016. <http://ecee.colorado.edu/microwave/docs/publications/2003/HFE0503_RaabPart1.pdf>.



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