products:

Function:
A Low Noise Amplifier (LNA) discriminates the information carrying signal that is being received from background noise, and amplifies it sufficiently for further processing. The LNA sets the sensitivity of the receiver.

The noise performance of an LNA is determined by the input match. In most cases, the input is matched to Gamma opt for noise, _opt, and the output is matched for minimum reflection (best VSWR). Note that best noise performance usually does not correspond to a good input VSWR.

The noise figure of an LNA is largely determined by the noise properties of the transistor used as the basis for the amplifier. Circuit losses occuring before the transistor, including losses due to the Q of the input matching circuit, circuit board losses, and connector losses, will cause the noise figure of an LNA to be at least several tenths of a dB higher than the intrinsic NF_min of the active device used in the LNA. The use of feedback for matching will also reduce noise performance. In general Gallium Arsenide-based amplifiers have better noise performance than Silicon-based amplifiers.

In a cascade of two elements, the noise contribution of the second element is divided by the gain of the preceeding element. For this reason, it is advantageous for LNAs to have high gain.

Schematic symbol:

Key Specifications Noise Figure (F, NF) Range: tenths of dBs to 3 dB Associated Gain (Ga) Range: 6 to 25 dB (10 - 15 dB typ) Input Intercept Point (IP3 in) Range: -20 to +20 dB

 

Discrete Devices:

GaAs FETs as LNAs

   Noise Figure comparison for GaAs FETs

Bipolar Transistors as LNAs

   Noise Figure comparison for Bipolar Transistors

Integrated Circuits:

ICs for RF/uW LNAs

   Noise Figure comparison - RF/uW ICs

ICs for millimeter wave Receivers, Direct Tansmitters, or Upconverting Transmitters

   Noise Figure comparison - mmw ICs