Newsroom
Power Amplifier: Guide to Choose and More
In the front-end circuitry of a transmitter, the RF signal power generated by the modulation oscillator circuit is very small. Considering the link attenuation in wireless transmission, the transmitter needs to radiate sufficiently high power to achieve a relatively long communication distance. After a series of amplification stages (buffer stage, intermediate amplification stage, and final power amplification stage) to obtain sufficient RF power, the signal is fed to the antenna via a matching network for radiation.
Therefore, the RF amplifier is primarily responsible for amplifying power and is a core component of the communication system, as well as the most power-consuming component. In this article, we will mainly discuss the relevant knowledge of power amplifiers in detail.
What is the Power Amplifier?
The power amplifier is a crucial component in the RF front-end, its performance directly determining signal strength, stability, power consumption, and other important factors, thus influencing user experience. RF power amplifiers are primarily used in mobile terminals, communication base stations, IoT devices, and military weather radar.
In the transmitter’s pre-amplifier circuitry, the RF signal generated by the modulation oscillator circuit has very low power. It needs to undergo a series of amplification stages (buffer stage, intermediate amplification stage, and final power amplification stage) to obtain sufficient RF power before being fed to the antenna for radiation.
To obtain sufficiently high RF output power, an RF power amplifier is essential. After the modulator generates the RF signal, the modulated RF signal is amplified to sufficient power by the RF PA, passed through a matching network, and then transmitted by the antenna.
The main technical specifications of an RF power amplifier are output power and efficiency. Improving output power and efficiency is the core design goal of RF power amplifiers. Typically, in RF power amplifiers, an LC resonant circuit can be used to select the fundamental frequency or a specific harmonic to achieve distortion-free amplification. In addition, the harmonic components in the output should be as small as possible to avoid interference with other channels.
What are the Different Types of Power Amplifier?
Different RF power amplifiers exhibit differences in semiconductor materials and transistor manufacturing processes. RF semiconductor materials have evolved from the first generation to the third generation, while transistor manufacturing processes have progressed from basic BJTs and FEFs to more complex HBTs, LDMOS, and HEMTs.
First-generation semiconductor materials: including Si and Ge, employing the BJT transistor manufacturing process. Global reserves of Si are abundant, and it offers advantages such as high temperature resistance, high stability, and low cost. However, Si’s low electron mobility limits its operation to low-frequency environments, effective only within a frequency range not exceeding 3.5 GHz.
Second-generation semiconductor materials: including compounds such as GaAs and InP, which possess high saturation electron velocity and high electron mobility. Therefore, RF power amplifiers based on these materials can operate in high-frequency bands and exhibit radiation resistance, low noise, and high linearity. Transistor manufacturing processes that followed the development of second-generation semiconductor materials include MESFET, HEMT, PHEMT, and HBT. Power amplifiers manufactured using second-generation semiconductor materials could not meet the power requirements of macro base stations.
Third-generation semiconductor materials: including compounds such as SiC and GaN, possess higher electron mobility. GaN-based RF power amplifiers offer advantages such as high power, high gain, high efficiency, and high operating frequency, while also exhibiting good heat dissipation, high temperature resistance, and radiation resistance. The primary manufacturing process for third-generation semiconductor transistors is HEMT.
Common Technical Specifications of RF Power Amplifiers
1. Operating Frequency Range
Generally, this refers to the linear operating frequency range of the amplifier. If the frequency starts from DC, the amplifier is considered a DC amplifier.
2. Gain
Operating gain is the main indicator of an amplifier’s amplification capability. Gain is defined as the ratio of the power delivered from the amplifier’s output port to the load to the power actually delivered from the signal source to the amplifier’s input port. Gain flatness refers to the range of amplifier gain variation across the entire operating frequency band at a given temperature, and is also a key indicator of an amplifier.
3. Output Power and 1dB Compression Point (P1dB)
When the input power exceeds a certain value, the transistor’s gain begins to decrease, eventually resulting in output power saturation. When the amplifier’s gain deviates from a constant or is 1dB lower than other small-signal gains, this point is the well-known 1dB compression point (P1dB). Generally, the power capacity of an amplifier is expressed using the 1dB compression point.
4. Efficiency
Since a power amplifier is a power component, it consumes supply current. Therefore, the efficiency of the power amplifier is extremely important to the overall system efficiency. Power efficiency is the ratio of the power amplifier’s RF output power to the DC power supplied to the transistors. ηp = RF Output Power / DC Input Power
5. Intermodulation Distortion (IMD)
Intermodulation distortion refers to the mixed components generated when two or more input signals with different frequencies pass through a power amplifier. This is due to the nonlinear characteristics of the power amplifier. Third-order intermodulation products have the greatest impact because they are very close to the fundamental signal; therefore, third-order intermodulation is the most important factor to consider. Lower third-order intermodulation products are better.
6. Third-Order Intermodulation Cutoff Point (IP3)
The intersection of the extended line of the fundamental signal output power and the extended line of the third-order intermodulation is called the third-order intermodulation cutoff point, denoted by IP3. IP3 is also an important indicator of power amplifier nonlinearity. When the output power is constant, a higher third-order intermodulation cutoff point output power indicates better linearity of the power amplifier.
7. Dynamic Range
The dynamic range of a power amplifier generally refers to the difference between the minimum detectable signal and the maximum input power in the linear operating region. Naturally, a higher value is better.
8. Harmonic Distortion
When the input signal increases to a certain level, the power amplifier will generate a series of harmonics due to operating in the nonlinear region. In high-power amplifier systems, filters are typically needed to reduce harmonics to below 60dBc.
9. Input/Output VSWR (Standing Wave Ratio)
This is also a very important indicator, reflecting the matching degree between the power amplifier and the overall system. A deteriorating input/output VSWR will lead to gain fluctuations and worsened group delay. However, power amplifiers with high VSWRs are relatively difficult to design; generally, systems require the power amplifier’s input VSWR to be below 2:1.
What are the Different Design Types of Power Amplifiers?
Classification by Operating Frequency Band
Based on operating frequency band, power amplifiers can be divided into narrowband and wideband RF power amplifiers. Narrowband RF power amplifiers generally use a frequency-selective network as the load circuit, such as an LC resonant circuit. Wideband RF power amplifiers do not use a frequency-selective network as the load circuit; instead, they use a transmission line with a wide frequency response as the load.
Classification by Matching Network Properties
Based on the properties of the matching network, power amplifiers can be divided into non-resonant power amplifiers and resonant power amplifiers. The matching network of a non-resonant power amplifier is a non-resonant system, such as a high-frequency transformer or a transmission line transformer. Its load characteristics are purely resistive, while the matching network of a resonant power amplifier is a resonant system, and its load characteristics are reactive.
Classification by Current Activation Angle
According to the current activation angle, RF power amplifiers can be divided into Class A, Class AB, Class B, Class C, Class D, and Class E. Class C has the highest output power and efficiency among these classes, and most RF amplifiers operate in Class C (or Class C-type). Power amplifiers are further divided into traditional power amplifiers (such as Class A, B, AB, C, etc.) and switching power amplifiers (such as Class D, E, F, etc.).
- Class A: Conduction angle = 360°. The amplifier maintains linear operation (linear region) across the entire input/output range, and all transistors remain on. Theoretically, the highest efficiency is 50%, but in practice, it’s around 30%. Commonly used for small-signal, low-power amplification.
- Class B: Conduction angle = 180°. The bias is set so that the output device is only on for half a cycle. Efficiency is improved by having two transistors work alternately, each transistor on for half a cycle. In practice, efficiency is generally no more than 60%. It is a linear amplifier, but its linearity is lower than Class A. Commonly used in high-power operation.
- Class AB: 180° < Conduction angle < 360°. It combines some linearity (better than Class B) with relatively high efficiency (generally between 30% and 60%). Small-signal operation is in Class A, and large-signal operation is in Class B.
- Class C: Conduction angle less than 180°, high efficiency (actually around 60%), but poor linearity; it is a nonlinear amplifier and commonly used in high-power applications.
- Class D: The transistor operates only in two states: cutoff and full conduction (linear region), i.e., in switching mode. Current and voltage waveforms do not overlap, and theoretically, the transistor consumes no power (ideal efficiency 80%-90%, actual around 80%). However, switching losses exist in practical applications, and its linearity is suitable for lower frequencies.
- Class E: Compared to Class D, it has some special design features, such as very small drain current when the drain-source voltage is high; very small drain-source voltage when current flows through the transistor; and minimized switching time between conduction and cutoff states. Ideal efficiency 100%, actual 90%; it is a fully nonlinear amplifier.
- Class F: Improves the voltage waveform across the switch by controlling higher harmonic components in the circuit, making it steeper and reducing power loss.
Working Principle of RF Power Amplifier
Unlike other Class A, Class B, and Class C tube RF power amplifiers, RF power amplifiers do not require high voltage; nor do they have much bandwidth, unlike other low-frequency power amplifiers. In Class D amplifiers, the field-effect transistors (FETs) operate in a switching state, resulting in very low drain power dissipation. Although the power is high during the on/off transition in the linear region, the high operating frequency and very short transition period significantly improve efficiency compared to previous power amplifiers, practically achieving over 90%.
Class D FET amplifiers consist of two or more pairs of transistors that alternately conduct in two groups to cooperate in power amplification. The excitation voltage controlling the switching state of the FETs can be a sine wave or a square wave. There are two practical circuit types: current-switching and voltage-switching.
Because the output current in a current-switching circuit is a square wave, the switching time of the FETs cannot be ignored at high operating frequencies. Therefore, medium-wave broadcast transmitters use voltage-switching circuits, which are available in full-bridge and half-bridge operating modes.
A bridge amplifier is the RF power amplifier used in modern medium-wave broadcast transmitters. It consists of four field-effect transistors connected in a bridge configuration, operating in a Class D switching amplification mode. This full-bridge connection is an H-type, hence also called an H-type Class D amplifier.
A full-bridge circuit is composed of two half-bridges. The outputs of the left and right halves are connected end-to-end to the primary coils of opposite synthesizer transformers, similar to a traditional push-pull circuit. The two RF power amplifiers are designed to be powered by independent power supplies, and the drive signals are input independently to each part of the bridge. This half-bridge operation of the RF power amplifiers is utilized in the pre-drive stage.
What is a Power Amplifier Used for? The Applications
RF power amplifiers have a wide range of applications, including radar, communications, navigation, satellite ground stations, and electronic warfare equipment.
For example, RF power amplifiers play a crucial role in active phased array radars. As a vital component of the side-mounted radar system, the RF power amplifier directly determines the aforementioned technical parameters. RF power amplifiers can also be used to create solid-state transmitters. In electronic warfare, RF power amplifiers can be used as active decoys to protect aircraft from missile attacks. In communications, RF power amplifiers are widely used in low-power or low-data-rate terminals; for instance, the efficiency of the RF power amplifier largely determines the talk time and standby time of personal mobile phones.
In short, RF power amplifiers are indispensable in any device that requires amplification of RF signals. Compared to low noise amplifiers, in addition to meeting certain gain, VSWR, and bandwidth requirements, power amplifiers are characterized by high output power, high conversion efficiency, and reduced nonlinear distortion.
How to Ensure the Stability of an RF Power Amplifier?
Every transistor is potentially unstable. Good stabilization circuitry integrates with the transistor to create a “sustainable operating” mode. Stabilization circuits can be implemented in two ways: narrowband and wideband.
Narrowband stabilization circuits involve some gain reduction. This is achieved by adding loss-reduction and selectivity circuitry. This circuitry restricts the transistor’s contribution to a very small frequency range. Wideband stabilization, on the other hand, introduces negative feedback. This circuitry can operate over a very wide frequency range.
The root of instability is positive feedback. Narrowband stabilization aims to suppress some positive feedback, which, of course, also inhibits its contribution. Well-implemented negative feedback can generate many additional desirable advantages. For example, negative feedback may exempt the transistor from matching, allowing it to interact well with the external environment without needing to be matched. Furthermore, the introduction of negative feedback can improve the transistor’s linearity.
How to Choose an RF Power Amplifier?
When selecting an RF power amplifier chip, multiple factors need to be considered to ensure that the chosen chip meets the requirements of the specific application scenario. Here are some key selection principles:
1. Frequency Range
The operating frequency range of an RF power amplifier chip is one of its most fundamental technical parameters. When selecting, it’s necessary to ensure that the chosen chip’s frequency range covers the frequency bands required by the target application. For example, for mobile communication systems, an RF power amplifier chip that covers the frequency bands required by the corresponding communication standards (such as 2G, 3G, 4G, 5G, etc.) needs to be selected.
2. Output Power
Output power is one of the important indicators of an RF power amplifier chip, determining the signal transmission strength and coverage range. When selecting, the required output power needs to be determined based on the communication distance and coverage range of the target application. Generally, the higher the output power, the longer the communication distance, but the corresponding power consumption and heat dissipation requirements will also be higher. Therefore, while meeting communication requirements, it’s necessary to select an RF power amplifier chip with good power consumption and heat dissipation performance.
3. Efficiency
Efficiency is one of the important indicators for evaluating the performance of an RF power amplifier chip. It represents the chip’s ability to convert input power into output power. High-efficiency RF power amplifier chips can reduce power consumption, heat generation, and extend device battery life. When selecting an RF power amplifier chip, priority should be given to those with high efficiency characteristics.
4. Linearity
Linearity is another important indicator of RF power amplifier chips, determining the degree of signal distortion during amplification. Good linearity ensures minimal signal distortion during amplification, thereby improving communication quality. When selecting an RF power amplifier chip, choose one with excellent linearity characteristics to ensure signal accuracy and reliability.
5. Noise Performance
Noise performance is an important aspect of RF power amplifier chip performance. It represents the noise level generated by the chip when amplifying signals. Low-noise RF power amplifier chips reduce interference and distortion during signal transmission, thereby improving communication quality. When selecting an RF power amplifier chip, priority should be given to those with low noise characteristics.
6. Package Form
The package form of an RF power amplifier chip has a significant impact on its performance and reliability. When selecting, the appropriate package form should be determined based on factors such as the installation space required for the target application, heat dissipation requirements, and cost. Common packaging forms include surface mount technology (SMD), ceramic packages, and metal packages. Each packaging form has its unique advantages and applications, and the choice should be made based on specific requirements.
7. Reliability
Reliability is a crucial indicator of RF power amplifier chip quality. It represents the chip’s stability and reliability during long-term operation. When selecting a chip, it’s essential to choose one that has undergone rigorous testing and certification and possesses high reliability to ensure the stability and reliability of the communication system.
8. Cost
Cost is another important factor to consider when selecting RF power amplifier chips. Different chips can vary significantly in price, depending on their performance, brand, manufacturing process, and other factors. When choosing a chip, it’s necessary to select one with the lowest possible cost and highest performance-to-price ratio while still meeting performance requirements.
9. Technical Support and Service
When selecting RF power amplifier chips, the manufacturer‘s technical support and service capabilities should also be considered. A reputable power amplifier manufacturer provides comprehensive technical support and after-sales service, including support for product design, testing, production, and subsequent maintenance. This helps reduce product development cycles and costs, and improves product quality and reliability.
About ZR Hi-Tech: Leading Power Amplifier Manufacturer
ZR Hi-Tech is a company specializing in integrated circuit manufacturing, committed to providing customers with high-performance, high-reliability integrated circuit solutions. Our team consists of experienced professionals.
If you are looking for a reliable power amplifier manufacturer solution for your project, please consult with ZR Hi-Tech experts!
ZR Hi-Tech’s Product Advantages and Core Capabilities
With deep technological expertise, we have built a comprehensive product portfolio of power amplifiers, dedicated to providing professional solutions for our customers.
High Reliability Assurance: Our power amplifier product line adheres to stringent quality standards, undergoes comprehensive environmental stress screening and reliability testing, and complies with CE, RoHS, and REACH standards.
Deep Customization Services: We possess strong custom development capabilities and can provide flexible customization services based on specific customer needs. For customized specifications or special requirements, please contact us directly.
