Overview
Many readers are interested in amplifier topics. RF amplifiers are complex devices and can be difficult to explain clearly. This article summarizes basic concepts related to RF power amplifiers.
Function and role
An RF amplifier is fundamentally a forward amplifier in an RF system, typically located in the transmit chain. Due to path loss in wireless links, the transmitter must radiate sufficient power to achieve the required range. The RF amplifier increases the power level before feeding the antenna, making it a core component of a communication system. What are the main types of RF amplifiers?
Main types of RF power amplifiers
1) By operating frequency band
Classified by operating band, amplifiers are divided into narrowband power amplifiers and broadband power amplifiers. Narrowband power amplifiers typically use frequency-selective networks as load circuits, such as LC resonant networks. Broadband power amplifiers use wideband transmission lines as the load rather than frequency-selective networks.
2) By matching network characteristic
Based on the matching network, power amplifiers can be non-resonant or resonant. Non-resonant power amplifiers use non-resonant matching networks, for example RF transformers or transmission-line transformers, and present an effectively resistive load. Resonant power amplifiers use resonant matching networks and present reactive load characteristics.
3) By current conduction angle
By current conduction angle, RF power amplifiers are classified into A, AB, B, C, D, E, and so on. Among these classes, Class C typically achieves the highest output power and efficiency in RF applications, and many RF amplifiers operate in Class C.
Key performance metrics
1. Operating frequency range
This generally refers to the linear operating frequency range of the amplifier. If the frequency response starts from DC, the amplifier is considered a DC amplifier.
2. Gain
Gain is the primary metric for amplifier amplification capability. Gain is defined as the ratio of the power delivered by the amplifier output port to the load to the power actually delivered by the signal source to the amplifier input port. Gain flatness, the variation of gain across the operating band at a given temperature, is also a key specification.
3. Output power and 1 dB compression point (P1dB)
When input power exceeds a certain level, a transistor's small-signal gain begins to fall and the output power approaches saturation. The point where the amplifier gain drops by 1 dB relative to its small-signal gain is the 1 dB compression point (P1dB). P1dB is commonly used to indicate the amplifier's power capability.
4. Efficiency
Power amplifiers consume supply current, so efficiency is important for overall system power. Power efficiency is the ratio of RF output power to the DC power supplied to the transistor: ηp = RF output power / DC input power.
5. Intermodulation distortion (IMD)
Intermodulation distortion refers to mixing products produced when two or more input tones pass through the nonlinear amplifier. Third-order intermodulation products are particularly important because they lie close to the fundamental signals and have the largest impact. Lower third-order IMD is preferable.
6. Third-order intercept point (IP3)
The third-order intercept point (IP3) is the intersection of the extrapolated fundamental output power line and the extrapolated third-order intermodulation line. IP3 is an important measure of amplifier linearity: for a given output power, a higher IP3 indicates better linearity.
7. Dynamic range
Dynamic range is the difference between the minimum detectable signal and the maximum input power within the linear operating region. Larger dynamic range is generally preferable.
8. Harmonic distortion
At high input levels the amplifier operates in the nonlinear region and generates harmonic components. In high-power systems, filters are typically required to reduce harmonics to below 60 dBc.
9. Input/output VSWR
Input and output voltage standing wave ratio (VSWR) indicate the matching between the amplifier and the system. Poor VSWR degrades gain flatness and group delay. Amplifiers with high VSWR are more difficult to design; typical system requirements often specify input VSWR below 2:1.
