What is a Low Noise Amplifier?

A low noise amplifier (LNA) is an electronic device that amplifies weak signals at its input without adding significant noise. These amplifiers are typically used in receivers. For LNAs the most important specifications are:

  • Gain
  • Noise Figure
  • Linearity

Gain

The LNA’s gain refers to its ability to increase the value of the input signal. It is most often specified in dB. Typical gain values for an LNA are between 10 and 30 dB.

Noise Figure

Noise figure provides a measure of the noise contribution due to the LNA itself. In the picture below, the LNA amplifies both the signal and noise present at its input equally. In addition, the output includes noise due to the LNA. This in turn reduces the signal-to-noise ratio. A good LNA contributes very little noise to the overall picture.

picture1

Linearity

The linearity of an LNA is a measure of its ability to amplify the signal without distortion. When an LNA is operating linearly, the output power in dB is the sum of the input signal and the gain.  However, as the input signal level increases beyond a certain point, the output starts to level off and the LNA is no longer linear.

In the picture below, the LNA is operating linearly when the signal present at its input has an amplitude of -60 dBm. However, as the input signal is increased to 0 dBm, the device is no longer linear, the output signal is distorted, the gain is no longer 20 dB and the output signal amplitude is only +5 dBm.

picture2

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6 thoughts on “What is a Low Noise Amplifier?

  1. Hi there,

    Can you please tell me what’s the best LNA optimized just for Airband listening you can deliver?

    Thank you

      1. Thank you.
        Can you please provide me a table (showing the typical Attenuation) with more values ranging from 85MHz to 140MHz?

      2. Yes, the gain at 85 MHz is -12 dB; 90 MHz is -9 dB; 100 MHz is +3dB; 110 MHz is +17 dB; 120 MHz is +20 dB; 130 MHz is +20 dB and 140 MHz is +18 dB. If you want higher attenuation in the FM band, you can Cascade with a FM notch filter (after this filter LNA)

      3. Many thanks for the data provided. Guess I’ll buy your LNA when I have the funds. So the best configuration would be: Antenna -> FM notch filter -> LNA. Correct?

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