RF Power Amplifiers: Efficiency vs. Linearity

If your signal is fading over a distance, then an RF power amplifier can solve this problem.

RF and microwave power amplifiers are the types of electronic amplifiers commonly found in today’s electronics. Its main work is to amplify a weak input signal to a greater power level over a long-distance transmission. 

In order to handle high power efficiently, these types of RF and microwave amplifiers are good especially due to various factors like linearity, good signal quality and reduced distortions. 

The aim of RF amplifiers is to increase radio frequency signals’ power levels while maintaining their signal waveform intact. They amplify weak signals to enable them to travel through long distances or for application in other electronic circuits.

You can find RF and microwave amplifiers of the following types below:

  • Broadband Amplifiers: They are used to amplify signals over a wide frequency range. They are often used in applications where the frequency of the input signal varies significantly.
  • NarrowBand Amplifiers: It amplifies the signals within a restricted frequency band. They have a high gain and low noise within their operating frequency range. This makes them ideal for applications such as radio receivers.
  • Power Amplifiers (PAs): They amplify the signals to high power levels for transmission. They are critical components in transmitters. They are basically designed to deliver maximum output power while maintaining signal quality.

Components Used in RF and Microwave Amplifier 

  • Transistors: Commonly used active components in RF amplifiers, such as bipolar junction transistors (BJTs), field-effect transistors (FETs), and laterally diffused metal-oxide-semiconductor (LDMOS) transistors.
  • Passive Components: Including resistors, capacitors, and inductors, used for biasing, impedance matching, and filtering
  • Feedback Networks: Used to control gain, stability, and linearity.


Linearity in RF and Microwave Power Amplifiers

Linearity implies that the equipment possesses the capability of producing an accurate replica of the input signal. It is vital for systems where exact duplication of a signal is needed – e.g., communication, radar systems, and measuring devices. 

In the context of amplifiers, linearity usually involves sustaining the quality of a transmission signal, e.g., minimizing distortions due to noise or other signals within the same frequency band. But we need to remember that such efficiency stands on the platform of linearity. For example, amplifiers with this property waste less power on heat, thus yielding more output power, an advantage to portable devices like cell phones. 

Efficiency generally involves running the amplifier in a nonlinear region in which the output does not scale with the input signal, leading to distortion and harmonics. 

Linearization techniques are used to enhance the linearity of the Power Amplifier (PA), preventing inband distortion and adjacent band interference. These methods often will involve one of the amplitude modulation schemes such as QAM (Quadrature Amplitude Modulation).

Efficiency in RF Power Amplifiers

Efficiency in RF and microwave power amplifiers is the ratio of RF output power to DC input power, typically expressed as a percentage.


It quantifies how much of the input power is converted into useful RF output power while minimizing power losses. Inefficient RF power amplifiers waste a significant amount of input power as heat, reducing overall system efficiency and potentially causing thermal issues. 

The operating class of an RF power amplifier has significant effects on its efficiency. Different operating classes, such as Class A, Class AB, Class B, Class C, and Class D, provide various options between efficiency and linearity.

  • Class A: The Class A amplifier provides high linearity and low distortion. Here, the active elements (transistors) are biased, so their quiescent operating point is in the linear part of the conducting region. So, they have low efficiency (25-50%) but offer excellent linearity.
  • Class B: The Class B amplifier can have efficiency in the 50-70% range, far better than Class A although it is still unacceptably high for some applications.
  • Class AB: The Class AB amplifier combines Class A and Class B to provide a mix of efficiency and performance.
  • Class C: Class C amplifiers are highly efficient (70–90%). They have a conduction angle of less than 180° and are biased to turn on only upon large signal fluctuations. So, they are suitable only for applications where high linearity is not critical, such as FM transmission.
  • Class D: Class D amplifiers can achieve very high efficiency (90-99%) by rapidly switching between on and off states. Instead of a linear or quasi-linear approach, as with the other classes, it functions as a nonlinear switching amplifier. The active element is either fully on or fully off and is pulse-width modulated by the input signal.

How Can You Improve Efficiency and Linearity in an RF Power Amplifier?

Some of the most common techniques to improve efficiency and linearity in RF power amplifiers are discussed below:

1. Operating Class selection: The choice of operating class has a considerable impact on the linearity-efficiency trade-off in RF power amplifiers. Different operating classes have various trade-offs. Thus, the selection of an operating class involves careful consideration of the desired balance between efficiency and linearity. 

One way to create an RF amplifier to improve both factors is by proper selection of operating class and optimizing biasing techniques. This way, your RF amplifier can fulfil the unique performance needs of a wide range of applications. This also results in maximum efficiency while retaining appropriate levels of linearity.

2. Advanced Transistor Techniques: Advanced transistor techniques involve the use of modern semiconductor technologies such as Gallium Nitride (GaN) Transistors, Gallium Arsenide (GaAs) Transistors, Heterojunction Bipolar Transistors (HBTs), & Silicon Germanium (SiGe) Bipolar transistors, as well as new design procedures to improve the performance of transistors used in RF power amplifiers and other electronic circuits. The aim of all these strategies is to enhance the efficiency and linearity power-handling capability of these devices while making them more reliable.

3. Feedforward and Feedback: In RF power amplifiers, feedforward and feedback are two important strategies in RF power amplifiers. This is a way of using extra paths for signals to detect and correct any errors produced by the primary amplifier.

In feedforward, some amount of RF power amplifier output signal is taken and compared to what goes in the amplifier (input signal). In the feedback technique, a portion of the output signal from the RF or microwave amplifier comes back into it again at its start.

4. Envelope Tracking: Envelope Tracking basically detects the amplitude envelope of the RF signal and adjusts the bias voltage correspondingly in order to ensure that the amplifier works with the highest efficiency at every point of its instantaneous power level. In fact, this method functions by dynamically adjusting the supply power or bias voltage of RF and microwave power amplifiers with regard to the envelope of the input signal.


As you design a power amplifier, you need a trade-off between linearity and efficiency. Among various power amplifiers, there exists a choice between linearity and efficiency. For instance, in the previous discussion, Class A provides high linearity with low efficiency, yet Class D has a low linear nature with high efficiency. Consequently, several sophisticated methods such as GaN, HBTs, GaAs, and SiGe have been proposed. This method improves linearity, efficiency, and power handling capability performance. This is dependent on the application’s specific requirements. 

The choice of appropriate methods relies on what a certain application needs. As a result, the RF power amplifier may achieve the balance that is desired between linearity and efficiency by looking at the trade-offs and applying the right methods thoughtfully.