A low noise amplifier (LNA) is an essential active RF device in virtually all receiver (RX) front-ends (FE) and throughout the signal chain in many sensitive designs. LNAs are designed to amplify weak RF signals while introducing minimal noise into the signal chain. For these reasons, LNAs can be found at the front-end for essentially all test and measurement receivers, such as vector network analyzers, spectrum/signal analyzers, and digitizers. Moreover, LNAs are ubiquitous in the front-ends for RF sensing applications, for instance, radar and millimeter-wave imaging.
When should you use a low noise amplifier?
LNAs are most commonly used to amplify the very weak signals captured by an antenna. To avoid noise and other undesirable interference also being amplified, a LNA should generally be placed as close to the receiving antenna or the receive side switch/circulator/filter as possible. It may be necessary to place the LNA after a limiter, balun, or other impedance matching circuit, given other specific requirements of the given receiver or signal chain.
In other cases, especially after highly selective filtering or attenuation for impedance matching purposes, an LNA may be useful in adding gain to the signal chain. This may include adding gain with minimal added noise before a mixer or other active device that requires a given minimum power level that may not be readily achieved without amplification. This could also be at the input of an analog-to-digital converter (ADC) for direct digital conversion (DDC) or direct RF conversion (DRFC).
How do I choose a low noise amplifier?
LNAs are not all created equal. Like with other active electronic devices, there are a myriad of performance features and criteria for a given application. The most important features of an LNA are the frequency range (bandwidth), small signal gain, and added noise figure (NF). Other often important figures-of-merit (FOM) for LNAs include size, packaging, phase noise, linearity parameters, input power, gain flatness, VSWR, temperature range, and operating DC voltage/current. The performance capability of an LNA is largely dictated by the design, fabrication technology, and semiconductor type. For instance, a gallium arsenide (GaAs) and indium phosphide (InP) LNA may have lower added noise figure than a gallium arsenide (GaN) LNA, but generally a GaN LNA will be able to withstand higher input voltage levels (higher survivability), which may be desirable in some radar and space applications where high levels of noise/interference are common.
What is a low phase noise amplifier (LPNA)?
A low phase noise amplifier is very similar to a LNA, but includes added design features and considerations toward limiting added phase noise to a signal. Like with most electronics there is a trade-off between how well an amplifier can be built to limit added phase noise and added noise, so some LPNAs may not provide the same added noise level of dedicated LNAs. For applications, such as high-speed wireless communications, minimizing the phase noise in the signal chain may be more critical to ensuring the performance of the communication link than noise figure.