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What is an RF power amplifier? Power Amplifier Classification Reasons for Using Power Amplifiers How to Select a Power Amplifier Power Amplifier Process Key Specifications of Power Amplifiers |
A power amplifier is a specialized electronic device used to convert a low-power electrical signal from a source into a more powerful signal. In wireless devices, we often hear the term "power in watts," and we often assume that the higher the power, the better. Is this true? Today, we'll learn about power amplifiers.
In RF circuits, a power amplifier (PA), also known as a power amplifier, amplifies weak signals by converting DC power into AC RF signal power.
A power amplifier amplifies the input signal and delivers sufficient power to the load. The importance of the RF power amplifier (RF PA) is a key component of the transmitter system. In the transmitter's pre-amplifier circuit, the RF signal generated by the modulated oscillator circuit is very low in power and requires a series of amplification stages (buffer stage, intermediate amplification stage, and final power amplifier stage) to achieve sufficient RF power before it can be fed to the antenna for radiation. To achieve sufficient RF output power, an RF power amplifier is essential. After the modulator generates the RF signal, the RF PA amplifies it to sufficient power, passes through a matching network, and is then transmitted by the antenna.
The function of an amplifier is to amplify its input and output it. These input and output signals are called "signals" and are often expressed as voltage or power. The key technical specifications of RF power amplifiers are output power and efficiency. Improving these two factors is a core design goal. In RF power amplifiers, an LC resonant circuit is typically used to select the fundamental frequency or a specific harmonic to achieve distortion-free amplification. Furthermore, the harmonic components in the output should be minimized to avoid interference with other channels.
Power Amplifier Classification
① Based on the power generated by the load, they can be classified as: low power (up to 0.3 W); medium power (0.3 to 3 W); high power (above 3 W).
② Based on the type of amplifier device: bipolar transistors; field-effect transistors; vacuum tubes, etc.
③ Based on the type of matching element: transformer input, transformer output; transformer input, non-transformer output; non-transformer input, transformer output; and no transformer.
④ Based on the type of amplified signal: single-phase excitation and dual-phase excitation.
⑤ Based on the operating mode of the boost device: single-stroke and two-stroke.
Based on their operating principle, power amplifiers can be divided into three categories: linear power amplifiers, switching power amplifiers, and digital power amplifiers.
Linear Amplifiers:
A linear amplifier is a type of power amplifier whose operation depends on the conduction angle of the transistor or the input signal ratio. Class A, Class B, Class AB, and Class C amplifiers are all examples of linear power amplifiers. These amplifiers can be inefficient or produce output distortion because high current causes a voltage drop across the resistor. Linear power amplifiers operate at high frequencies but have a relatively narrow bandwidth. RF power amplifiers generally use a frequency-selective network as a load circuit.
Switching-Mode Power Amplifiers:
Switching-Mode Power Amplifiers (SMPAs) operate electronic devices in a switching state. Common examples include Class D (D) and Class E (E) amplifiers. Class D amplifiers have higher efficiency than Class C amplifiers. SMPAs drive active transistors in switching mode. The transistors operate either in an on or off state, with no overlap in the time domain waveforms of their voltage and current. This results in zero DC power consumption and ideally 100% efficiency. Switching amplifiers can rapidly turn power transistors on and off to achieve high-frequency operation. Switching amplifiers do not rely on on-time to amplify the input signal. Therefore, compared to linear amplifiers, switching power amplifiers offer high efficiency, low distortion, low power consumption, and smaller size.
Digital Amplifiers:
The aforementioned linear and switching power amplifiers use analog technology for amplification. Digital amplifiers employ DSP (digital signal processing) to amplify signals and generate digital outputs. However, the digital output can be converted to an analog signal using a DAC in the output stage. Class D and Class E amplifiers are partially digital but still analog amplifiers. Class T amplifiers are "digital switching" amplifiers.
Based on their application, power amplifiers can be further categorized as audio amplifiers and RF amplifiers.
Audio Power Amplifiers:
Audio power amplifiers are specifically designed to amplify low-frequency signals within the audible frequency range (20 Hz - 20 kHz) for driving loudspeakers (speakers). The world's first audio power amplifier was invented by American inventor Lee de Forest in 1912. It was capable of converting low-power audio signals from radios and electric guitars into sound that we hear from speakers! For example, National Semiconductor's LM380 is a popular power audio amplifier.
RF Power Amplifiers:
RF power amplifiers amplify high-frequency radio signals for transmission via antennas. RF power amplifiers (RF) operate in the frequency range of 30 kHz to 300 GHz. Typically, power amplifiers with tuned LC circuits are used in RF applications. Class C and Class E amplifiers are examples of RF amplifiers. However, "true" RF amplifiers are not typical power amplifiers, as their manufacturing and design differ significantly for high-frequency functions.
Reasons for Using Power Amplifiers
Power amplifiers are a crucial component of power electronics systems, converting low-power input signals into high-power output signals. In high-power applications, the load requires high power to drive the output devices. High power means high current. Low-power amplifiers cannot perform reliably in such applications. Therefore, various low-power amplifiers are connected to the input side of power electronics systems.
All low-power amplifiers are voltage amplifiers. All voltage amplifiers are small-signal amplifiers. Simply put, voltage amplifiers can only output high-voltage, low-current signals. Due to their limited output current capability, voltage amplifiers are not applicable or "inadequate" in power electronics. General-purpose operational amplifiers also fall into the category of voltage amplifiers.
Power amplifiers are located on the output side of electronic systems, connected directly to the load. This is because power amplifiers can deliver high power to the load. Power amplifiers are current amplifiers because they convert low-power input signals into high-power output signals. Sometimes, power amplifiers are also called large-signal amplifiers.
How to Select a Power Amplifier
Power amplifier selection is primarily based on key parameters of the power amplifier.
① Operating Frequency Band
Select the power amplifier's operating frequency band based on the frequency band your product needs to support, and select a power amplifier for the corresponding frequency band.
② Power
Evaluate the power amplifier's maximum output power. This is based on the product's maximum output power requirements to ensure it meets the project's maximum output power requirements. Evaluate the power amplifier's maximum input power. This is based on the power at the RF transceiver chip's output port to ensure the input power is not saturated.
③ Control Interface
The control interface is the interface used by the RF transceiver chip to communicate or control the power amplifier. When evaluating the interface type, pay attention to the power amplifier's logic control table. Also, pay attention to the interface level. Try to select a power amplifier with the same level as the RF transceiver chip's interface to avoid level shifting.
The logic control table, also known as the truth table, is the logic table used by the RF transceiver chip to control the various states of the power amplifier. Ensure it complies with the RF transceiver chip's requirements.
④ Gain
Evaluate the power amplifier's gain based on the output power of the RF transceiver chip's output port and the product's maximum output power to ensure it meets the product's output power requirements.
⑤ Power Supply
Evaluate the operating voltage of the power amplifier and select the appropriate power amplifier voltage based on the overall power supply planning, conversion efficiency, and cost. Evaluate the power amplifier's power consumption and, for the same output power, choose a low-power amplifier.
⑥ Non linearity
The smaller the nonlinear product of the power amplifier, the better.
⑦ Impedance
Select the required impedance of the power amplifier based on the port impedance of the RF transceiver chip.
Power Amplifier Process
Currently, the mainstream process for power amplifiers remains GaAs. In addition, GaAs HBTs (GaAs heterojunction bipolar transistors) are bipolar transistors constructed of gallium arsenide (GaAs) and aluminum gallium arsenide (AlGaAs) layers. Although CMOS technology is relatively mature, Si CMOS power amplifiers are not widely used. Regarding cost, while CMOS silicon wafers are relatively inexpensive, the layout area of CMOS PAs is relatively large. Coupled with the high R&D costs associated with the complex design of CMOS PAs, the overall cost advantage of CMOS PAs is less pronounced. In terms of performance, CMOS power amplifiers exhibit poor linearity, output power, and efficiency, compounded by inherent drawbacks of the CMOS process: high knee voltage, low breakdown voltage, and low resistivity of the CMOS substrate.
| Project | Power amplifier type | |||
| GaAs | SiGe | GaN | CMOS | |
| Advantages | High speed, high frequency, high power, and low noise figure | High frequency, safe materials, good thermal conductivity, high integration, and cost advantages | Higher efficiency, wider bandwidth, higher breakdown voltage, high power density, and high thermal conductivity. | Easy to integrate with other circuits, low cost |
| Disadvantages | Slightly higher cost, unsafe materials, and high process requirements | Low breakdown voltage, low cutoff frequency, and high power consumption | Higher cost, strict process requirements, and low yield. | High noise, high power consumption, low breakdown voltage, poor heat dissipation, poor linearity, and low efficiency |
| Application | Widely used in mobile communications | Mainly used in Wi-Fi, radar, and other applications | Mainly used in high-frequency, high-power applications such as military, base stations, microwave, and millimeter-wave applications. | Mainly used in low-power applications such as Bluetooth and ZigBee |
Key Specifications of Power Amplifiers
Operating Frequency Range: Generally speaking, this refers to the linear operating frequency range of the amplifier. If the frequency starts at DC, the amplifier is considered a DC amplifier.
Gain: Operating gain is the primary metric for measuring an amplifier's amplification capability. Gain is defined as the ratio of the power delivered to the load from the amplifier's output port to the power actually delivered to the amplifier's input port from the signal source. Gain flatness, which refers to the range of amplifier gain variation across the entire operating frequency band at a given temperature, is also a key amplifier specification.
Output Power and 1dB Compression Point (P1dB): When the input power exceeds a certain value, the transistor's gain begins to decrease, ultimately resulting in output power saturation. When the amplifier's gain deviates from a constant or is 1dB lower than the gain of other small signals, this point is known as the 1dB compression point (P1dB).
Efficiency: Because the power amplifier is a power component and consumes supply current, its efficiency is crucial to the overall system efficiency. Power efficiency is the ratio of the amplifier's RF output power to the DC power supplied to the transistor.
Intermodulation distortion: Intermodulation distortion is the mixing of two or more input signals of different frequencies through the power amplifier. This is caused by the nonlinear nature of the power amplifier.
Third-order intermodulation intercept (IP3): IP3 is also a key indicator of power amplifier nonlinearity. For a given output power, the higher the third-order intermodulation intercept, the better the amplifier's linearity.
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 its linear operating region. Naturally, the higher this value, the better.
Harmonic distortion: When the input signal increases to a certain level, the power amplifier will generate a series of harmonics due to its nonlinear operation. In high-power amplifier systems, filters are generally required to reduce harmonics to below 60dBc.
Input/Output Standing Wave Ratio (VSWR): This indicates the degree of match between the amplifier and the overall system. A poor input/output ratio can lead to system gain fluctuations and poor group delay. However, amplifiers with high VSWRs are difficult to design. In general, systems require an input VSWR below 2:1.
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