Characterizing RF Power Amplifiers: Introduction
Introduction
A radio-frequency (RF) power amplifier (PA) is essential to wireless communication systems. It can “amplify” (or convert) a low-power signal within a particular RF “band” (or range of frequencies) to a higher power level. Radio transmitters typically use power amplifiers to drive an antenna before transmitting a signal out of the medium (usually free space). Receivers can also contain power amplifiers. However, they are designed with characteristics important for a receiver rather than the transmitter, such as better “noise immunity” (since most signals that arrive at a receiver are noisy, have low power, and are distorted). The following image shows the location of a power amplifier in a typical RF circuit at both the transmitter and receiver.
Typical RF system block diagram
We can see that the power amplifier is the component closest to the antenna. Amplification is the last operation performed before transmission and the first during reception.
Most power amplifiers use transistor circuits such as Bipolar Junction Transistor (BJT) and Metal-Oxide Semiconductor Field-Effector Transistor (MOSFET). A supply voltage excites charge carriers in these circuits and induces the required conductivity to amplify a signal.
In this blog post, we will discuss key characteristics that must be considered when designing or choosing a power amplifier for our product and application.
Design
The following outlines the steps involved in designing or selecting a power amplifier for an RF circuit:
Selecting the appropriate class: Power amplifier classes use letters to denote various types. Each class offers a broad indication of an amplifier’s characteristics and performance. Class A, B, and AB power amplifiers are usually used for “linear designs.” Class D and E power amplifiers are used for “switching designs.” Additional amplifier classes, such as G and H, represent variations of the classes above. Choosing the appropriate class depends heavily on the final application.
Input and output impedance matching: Efficient impedance matching maximizes power transfer from the amplifier stages to the load, reducing distortions and noise. In usual RF circuits, 50 ohms is the typical impedance. “Matching networks,” usually consisting of discrete components such as capacitors, inductors, and transmission lines, are chosen based on the operating frequency.
Output power and gain: The selection of output power levels depends on the specific application. The gain of an RF power amplifier, expressed in decibels (dB), is the logarithm of the ratio of the output power to the input power. This parameter is usually defined for the “linear region” of the power amplifier. The linear region of the power amplifier is where the relationship between the output power and the input power is linear. As we increase the power level of an incoming signal, the power amplifier may shift and operate in the “non-linear region,” where the linear relationship no longer holds. Additionally, in this region, undesired effects may be generated by the power amplifier, such as “harmonics” and “intermodulation products.”
Bandwidth: RF power amplifiers are usually selected based on their “bandwidth,” defined by the optimal gain for a particular set of continuous frequencies. The bandwidth of an RF power amplifier dictates the range of optimal frequencies that the final product or application can operate. Power amplifiers can be designed for narrow or wide frequency bands.
Efficiency: Efficiency is the ratio of output power to the input power. A high-efficiency power amplifier consumes less energy and produces less heat, making it ideal for battery-powered and compact devices. However, achieving high efficiency usually entails operating the PA in the non-linear region, potentially introducing undesired artifacts. Since 100% efficiency cannot be achieved (due to the Laws of Thermodynamics), achieving high efficiency of a PA depends on numerous factors, such as architecture, operating frequency, biasing schemes, and the nature of the signal itself.
Linearity: Linearity refers to the power amplifier’s capability to amplify a signal without distorting its shape or changing its operating frequency. A linear PA maintains the information content and integrity of the signal while minimizing interference with other signals in the same frequency band. However, achieving linearity often involves sacrificing efficiency. Various metrics are used to assess the linearity of a PA, depending on the application and signal characteristics. Common metrics include:
P1dB: The “1 dB compression point” defines the output power level at which the gain of the PA drops by 1 dB from its linear value.
IP3: The “third order intercept point” represents the theoretical output power level where “third-order intermodulation products” (or IM3) match the power level of the output signal.
EVM: The “error vector magnitude” measures the “root mean square” (RMS) error between the ideal and actual output signal vectors and the RMS magnitude of the ideal signal vector.
The following image shows how the above metrics are defined in relation to the power levels of the input and output signal:
Linearity metrics of an RF PA
Various techniques are employed to enhance the linearity of a PA, such as biasing, feedback, feedforward, pre-distortion, and envelope tracking.
In the next series of blog posts, we will discuss how to appropriately design or choose an RF PA for our application in further detail. We will learn how to read a PA’s datasheet to understand whether its suitable for our application. We will learn about experiments that can be conducted to validate these characteristics.
For any RF design needs, please reach out to mab@mab-labs.com.
