An amplifier is an electronic device used to increase signal power. The amount of amplification in an amplifier is measured by its gain, that is, the ratio of output voltage, current, or power to input voltage, current, or power is greater than one. RF amplifiers are similar to audio amplifiers but are equipped with components suitable for RF frequencies. Because radio frequency amplification involves higher frequencies, the impedance level of radio frequency amplifiers is usually much lower than that of audio amplifiers. The frequency range of a radio frequency amplifier is 20kHz to 300GHz, and is usually coupled with an input or output impedance that matches the transmission line impedance and the ratio of voltage to current. Usually 50 ohms and 75 ohms are common impedance values of radio frequency ports, so radio frequency amplifiers are usually specified to provide a given output power level under these impedances. Although the radio frequency amplifier is characterized by amplifying voltage or current, it is essentially amplifying power.
The ideal amplifier is a linear device, although the amplifier is actually only linear within a limited range. When a sufficiently strong input signal is given, the linear system will reach a critical point that deviates from the linear relationship between input and output. At this point, the system can be considered to be in compression or beginning to saturate. After the critical point is exceeded, there is no longer an effective linear relationship between input and output, and the amplifier is no longer considered linear, or in other words, deviates from linearity by -1dB at this point. The output power of the amplifier cannot increase indefinitely, and when the increase in input power no longer leads to a significant increase in output power, the amplifier reaches saturation, and the output signal is no longer proportional to the input signal. This value is usually called Psat in the data sheet.
The amplifier also increases the power of the signal and the noise at the input. Large-scale amplification requirements make the circuit susceptible to interference from noise, distortion, and other non-linear effects. The spurious signal product has nothing to do with the signal itself, and may be caused by low-level instability in the amplifier, or introduced into the amplifier from outside through power, radiation interference, or other intermodulation sources. When the signal driving the amplifier increases, the output will also increase, until a certain point is reached, a certain part of the amplifier is saturated and the output cannot be increased, resulting in distortion.
The low noise amplifier (LNA) can amplify very low power signals without significantly reducing its signal-to-noise ratio, with the aim of minimizing additional noise. Low noise amplifiers are mostly used in communication systems, medical instruments and electronic equipment. The low noise amplifier is designed for low noise figure, and its component size is very small, and it is not suitable for handling high input power. Although low-noise amplifiers are suitable for processing low-frequency signals slightly above the noise floor, they must also reduce the presence of larger signals, otherwise they will cause intermodulation distortion. The RF limiter is used to protect the low noise amplifier in the receiver chain. For example, if the input power is within the limit range of the low-noise amplifier, the previous limiter is in a low-loss state, so it transmits the radio frequency signal to the input of the low-noise amplifier, and its insertion loss is also negligible. When the input of the low noise amplifier encounters a stronger signal, which may cause damage, the limiter attenuates the signal level to ensure that the low noise amplifier or other components in the chain are not damaged.
In radio frequency or microwave circuits, especially in receiver circuits, the amount of power that circuit elements can handle is limited. Generally, electronic warfare (EW), signal intelligence (SIGINT), radar, and interference-limited wireless communication systems are equipped with highly sensitive receivers that are vulnerable to desensitization and excessive signal power. In the environment where these receivers are deployed, the receivers are also susceptible to interference, congestion, or other environmental factors that may threaten their operation. Because of this, the function of the limiting amplifier is that, regardless of the input signal power, the limiting amplifier can ensure that the total signal power in the specified frequency range is always below the desired threshold.
There are many different types of limiting amplifiers, mainly including two: one is to shunt excessive signal energy outside the sensitive circuit, and the other is to rely on continuous gain compression stages to provide constant output signal energy. Like introducing any additional components in the signal chain, the key to choosing a limiting amplifier is linearity and frequency performance.