An RF (radio frequency) power amplifier is a type of amplifier that boosts the input low-power RF signal into a higher power RF output signal. RF power amplifiers are used with transmitters to amplify the power of RF signal for transmitting through the antenna.
To learn more about RF communication systems, we recommend you to check the following article.
To understand the RF power amplifiers in a better way, we recommend you check the details of the following components.
In this RF and Microwave amplifier article, we will cover,
Different operation modes of an RF amplifier is called " amplifier classes". These modes will help the designers to achieve design goals.
Some commonly used linear amplifier classes are class A, class AB, class B, class C, which uses an active device as a controlled current source. The bias at the input of the device determines the amplifier class.
Other modes of amplifier classes which have higher efficiency, theoretically up to 100% by using the active device as a switch are class D, class F and class E, but will have lower linearity.
In a class-A amplifier, all the time the active element remains conducting mode, hence 100% of the input signal is used and amplified. I.e, class-A amplifier conduct over the full range of the input frequency cycle.
The active device used in the design of the class-B amplifier conducts for 180 degrees of the input cycle. Two devices are used in the design to avoid intolerable distortion, especially amplifiers at audio frequencies. Each device conducts for one half (180° cycle) of the signal cycle, so the full signal will be amplified.
In a class-AB amplifier, the conduction period is between class A and B design; I.e. each active device conducts for more than half of the time. Class AB design is considered as a good class of design for amplifiers, as it has the advantage of class A and Class B design. In class AB design, by using negative feedback, the crossover distortion can be reduced.
In Class-C amplifier design, will have only less than 50% of the input signal conduction angle. The efficiency of the class C design can reach up to 80% for RF applications. The class-C amplifiers are used generally with RF transmitters that operate at a single fixed carrier frequency.
Class-D amplifiers use pulse-width modulation to control the output devices for power amplification. The conduction angle varies in pulse width and is not directly related to the input signal. Because of the finite switching speed of the active devices, the class D amplifier is not suitable for RF applications.
The class-E amplifier is considered as the highly efficient tuned switching power amplifier for RF applications. The class-E design uses a tuned reactive network between the single-pole switching element and the load. The design circuit obtains high efficiency by operating the internal switching element at zero current points(on to off switching).
Class F amplifier can be driven by using a sine or square wave. To increase gain, the input sine wave can be tuned by an inductor. If class F design is implemented with a single transistor, the filter becomes complicated to short the harmonics.
The class-AB output stages can be enhanced by a variety of amplifier designs to achieve greater efficiency with low distortion. The terms "class G" and "class H" design are used interchangeably to different designs reference. Class-G amplifiers are more efficient than class-AB amplifiers. Class-H amplifiers usually generate an infinitely variable supply rail (analogue signal).
RF Amplifiers are classified on the basis of the purpose and applications of RF Amplifiers in the industry.
Broadband RF amplifiers provide a moderate gain by keeping the noise figure low over a wider bandwidth. These RF amplifiers are used within the antenna front-end of the receiver circuity, where low noise amplifiers are not available.
Coaxial RF Power Amplifiers are the basic amplifier that will have a very high power output level with corresponding gain. RF power amplifiers are generally used with the RF front-end of the transmitters. Power amplifiers convert smaller power level signals from communications transmitters to high power need to transmit through an antenna.
Low noise amplifiers have very high gain, with lower power output levels in comparison with normal RF power amplifiers. These amplifiers are used at the receiver input port after the antenna to amplify a very low signal without introducing noise to the original signal.
Hi-Rel amplifiers are very reliable amplifiers designed to meet the required reliability standards in automotive, aerospace, and military communication applications. These amplifiers will have a higher lifespan even under various operational and environmental conditions. Due to their reliability factor, these amplifiers are costlier than most industrial amplifiers.
Gain block amplifiers are available with narrow or wide bandwidths and offer higher gain than broadband Amplifiers. Gain block amplifiers are used with RF and microwave transmitters even though noise figure is verser than broadband amplifiers.
The output gain of a log amplifier is the natural log of the input voltage. This RF amplifier was designed for applications that required very high gain.
Variable gain amplifiers offer a programmable gain in the output. They will have a built-in programmable attenuator or variable gain circuits depending on the application.
Waveguide power amplifiers will boost the smaller power level signals to the very high power need for transmission of the signal through the waveguide. These amplifiers will have direct waveguide output ports. These RF power amplifiers are optimized for pulsed / CW radar or for digital communication systems and are capable of handling different loads. These amplifiers will have reflection matching and protective circuits with proper cooling arrangements for heat dissipation.
A linear amplifier maintains the proportional linearity in the output power level with respect to the input power and provides high-quality linear performance over the operating range. These amplifiers will maintain linearity over different load conditions and are primarily used for the test and measurement application where higher linear power is required.
A Bi-directional amplifier will amplify the signal in both directions and provide low-noise performance in a particular direction. Bi-directional amplifiers can be directly connected to a transceiver and it will act as an intermediate node for receiving very weak signals and amplifying the signal for long-distance transmission. Bi-directional amplifier designed to use in the digital communication system to ensure the reception of the highest signal power at the destination.
RF power amplifiers can be manufactured using a solid-state device like LDMOS, GaN etc or using vacuum tubes.
Currently, Solid State Power Amplifiers use GaN devices for the amplifier design. GaN device provides higher GaN in smaller packages. In earlier times MOSFET (metal-oxide-semiconductor field-effect transistors) or Bipolar junction transistors like LDMOS were used for the amplifier design. Even though LDMOS transistors offer superior RF performance and are easy to design, GaN devices are used for the design of high power RF amplifiers due to gain and compact size. The drawback of the SSPA is the chance of failure is more due to reflection from the loads.
For very high power amplifier requirements like 10KW, 20KW power output, vacuum tubes are still in use for the design due to their mechanically robust nature. Amplifiers made of vacuum tubes are electrically robust and can withstand high electrical overloads. For high power applications, vacuum tubes are a more cost-effective solution.
RF amplifiers are used in all RF circuits and systems where the power level of the signal needs to increase. Below are a few of the common applications of the amplifiers.
When selecting RF Amplifiers for a system design, the designer needs to consider the following features for the perfect overall communication system design.
The frequency range of the power amplifier is the range of frequency signal it can amplify by meeting all other electrical specifications. In general, wideband amplifiers will have a higher frequency range and high power amplifiers will have a narrow band of frequency of operation due to the requirement of input and output matching.
The gain of the RF power amplifier is the ratio of the output to the input power level. The gain of RF amplifiers is expressed in decibels (dB).
Gain flatness shows the level of the gain variation of the amplified signal over the entire range of operation of the RF amplifier. Lower variation of the gain is good for communication applications.
Output power is one of the prime selection specifications of the RF amplifiers. It indicates the signal power level of the output from the amplifier under specified conditions. Now, most of the high power amplifiers are available with the options to vary and control the output power level as per the requirements.
The noise figure is the measure of the amount of noise added to the signal during the amplification of the signal in the RF amplifier. A lower noise figure leads to a quality output signal from the power amplifier.
As the amplifier may need to connect to various loads, it is better to have good Input and output VSWR. Better the VSWR, the life of the amplifiers will be more. Most of the high power amplifiers will have internal reflection protection circuits to save from the reflections from the loads.
As RF amplifiers consist of many active devices like GaN, LDMOS etc, the power consumption needs to be considered during the selection for optimising the power consumption of overall systems.
RF amplifiers are available with different packages and dimensions with various RF connector options for the RF output and input signal.
As the RF and microwave high power amplifiers generate a large amount of heat internally during the operation, it is necessary to have a proper cooling arrangement to control the temperature. Internal or external higher temperatures may lead to the breakdown of the entire amplifier circuit.
RF amplifiers are active devices that will amplify the input signal in the specified frequency range to a definite gain level as per the design. RF amplifiers are available from many manufacturers in a wide range of operations, below are the listed few.
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