How to choose the right RF amplifier?
When selecting the appropriate RF amplifier for a specific application, it is essential to consider characteristics such as gain, noise, bandwidth, and efficiency.
This article will review the most commonly used RF amplifiers and explain how gain, noise, bandwidth, efficiency, and various functional features influence the choice of amplifiers for different applications.
RF amplifiers come in many types and forms, designed to meet various application scenarios. However, choosing the right RF amplifier for a specific application is not easy due to the wide variety of RF amplifiers available. Although gain is a key characteristic of almost all RF amplifiers, it is not the only parameter to consider when selecting the right device, and it is often not the most important one.
Gain indicates the amount of amplification an amplifier can provide to a signal, expressed as the ratio of output power to input power (in dB). It is usually specified for the amplifier’s linear mode (where changes in output power correspond linearly to changes in input power) (see Figure 1). If the input signal power level to the RF amplifier continues to increase, the device will begin to enter a nonlinear mode, generating spurious frequency components. These interference components include harmonics and intermodulation products (see HD2, HD3, IMD2, and IMD3 in Figure 2), representing intermodulation distortion (IMD) appearing at the output of the RF amplifier. The ability of an RF amplifier to handle different input power levels without introducing significant distortion reflects its linearity performance, which can be expressed by various parameters (see Figure 1), including:
1. Output 1 dB Compression Point (OP1dB), which defines the output power when the system gain is reduced by 1 dB.
2. Saturated Output Power (PSAT), which is the output power when changes in input power no longer alter the output power.
- Second-Order Intercept Point (IP2) and Third-Order Intercept Point (IP3), which are hypothetical points of input (IIP2, IIP3) and output (OIP2, OIP3) signal power levels where the corresponding spurious components would reach the same level as the fundamental component.
Although gain describes the key function of an RF amplifier, linearity and other characteristics play an important role in determining the choice of an RF amplifier. In fact, selecting the type of RF amplifier always involves trade-offs between different design parameters. Below is a brief guide for choosing the right type of RF amplifier for your target use case.
Low Noise Amplifier (LNA)
Low Noise Amplifiers (LNAs) are often used in receiver applications to amplify weak signals at the front end of the signal chain interfacing with the antenna. This type of RF amplifier is optimized to introduce minimal noise while performing this function. Minimizing noise is particularly important in the early stages of the signal chain, as these stages have the greatest impact on the overall system noise figure.
Low Phase Noise Amplifier
Low Phase Noise Amplifiers introduce minimal additional phase noise, making them ideal for RF signal chains that require high signal integrity. Phase noise is near-carrier noise that manifests as jitter, characterized by small phase fluctuations of the signal in the time domain. Therefore, Low Phase Noise Amplifiers are well-suited for use with high-performance PLL frequency synthesizers in high-speed clock and LO networks.
Power Amplifier (PA)
Power Amplifiers (PAs) are optimized for power handling performance and are suitable for applications that require high power output, such as transmitter systems. These amplifiers typically have high OP1dB or PSAT characteristics and offer high efficiency, enabling them to maintain low heat dissipation.
High Linearity Amplifier
High Linearity Amplifiers are used to provide a high third-order intercept point over a wide range of input power levels with very low spurious levels. This type of device is commonly chosen for communication applications using complex modulation signals, where the RF amplifier must handle high peak-to-average ratios with minimal signal distortion to maintain a low bit error rate.
Variable Gain Amplifier (VGA)
Variable Gain Amplifiers (VGAs) are used in applications requiring flexible gain adjustment to accommodate varying signal levels. VGAs achieve this function by offering adjustable gain, which can be varied digitally in steps using a digitally controlled VGA or continuously using an analog-controlled VGA. These amplifiers are often used in Automatic Gain Control (AGC) and to compensate for gain drift caused by temperature or characteristic changes in other components.
Broadband Amplifier
Broadband Amplifiers can provide moderate gain over a wide frequency range (often spanning several octaves), benefiting multiple broadband applications. These amplifiers offer high gain-bandwidth products, often at the expense of efficiency and noise performance.
Gain Blocks
Other general-purpose RF applications may also rely on Gain Blocks, which represent a broad category of RF amplifiers that can cover various frequencies, bandwidths, gain, and output power levels. These amplifiers typically provide flat gain response and good return loss. Their design often includes matching and bias circuits, requiring minimal external components for integration into the signal chain, simplifying operation.
Conclusion
This article provides several examples of RF amplifiers and their applications. However, given the numerous types of these devices and the countless applications they target, some information is inevitably omitted. RF amplifiers can be designed using different assembly and process technologies to provide various integration features, support specific operating modes, and achieve optimized performance to meet the needs of diverse applications – from communication and industrial systems to test and measurement equipment and aerospace systems.