There are many cases in RF circuit design where there is a need to increase the power of a signal while avoiding adding any signal degrading characteristics. An example of this is when a mixer needs to be driven at a certain input power beyond what is available from the baseband transceiver components. Another example is increasing the signal strength of a relatively weak signal received by an antenna. In these circumstances Low Noise Amplifiers (LNAs) are the go to signal conditioning devices used for providing a marginal amount of signal gain while minimally contributing to the noise floor or distortion of a signal. There are also High Gain LNAs, which are specifically designed to substantially increase the gain of a signal in a single device.
LNAs are similar to many other types of amplifiers with the key difference that these devices are optimized for very low added noise figures and often also good linearity. There are many components that contribute to the overall noise in a device and in a signal. The issue faced with amplifying low power signals is that all of the noise and interference in a signal is also amplified. Hence, having an amplifier contributing additional noise in a signal is often problematic. In a similar way, having an amplifier contribute nonlinear distortion to a low power input signal can also have an exaggerated undesirable impact on the signal quality.
Key LNA Electrical Specifications & Features
- Frequency range [Hz]
- Interconnect impedance [Ohms]
- Small signal gain [dB]
- Noise figure [dB]
- Gain flatness [dB]
- Gain variance at base plate operating temperature (OTR) [dB]
- Input & output VSWR
- 1 dB compression point output power [dBm]
- Output 3rd order intercept point [dBm]
- Saturated output power [dBm]
- Spurious [dBc]
- RF power handling (maximum RF input power) [dBm]
- Reverse isolation [dB]
- Operating DC voltage [V]
- Operating DC current [A]
- Operating temperature range [degree C]
- Interconnect type [waveguide, coaxial connectorized, SMT, etc]
Most coaxial connectorized amplifier designs have a relatively low noise figure (NF), or added noise figure, of 0.5 dB to 1.5 dB, and gain figures between 10 dB and 20 dB. High gain LNAs are typically multistage amplifier devices and may contain a typical low noise amplifier stage followed by a higher gain amplifier with a slightly worse noise figure as the original LNA. This combination is viable as the second gain stage will contribute less to the noise floor than the first gain stage.
There are also many other variations of LNAs, such as GaN Input Protected Low Noise Amplifiers and low phase noise amplifiers (LPNAs), which are designed with high input power level operation/survivability and low phase noise, respectively.