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NOT 447-2002-en



Why is the number of cascaded amplifiers limited?

This factor is closely related to the minimum input level:

VMin Input Level= C/N Output + Noise = C/N Output + NF +10 log (ktb)

The minimum input level for an amplifier depends on the desired C/N at the output.

Two parameters are essential in computing it:

  • The noise figure of the amplifier: NF
  • and the thermal noise which is a function of both the temperature and the signal bandwidth (1.8 dBμV for B/G standard signals at 20┬║c room temperature)

Therefore, let┬┤s suppose:

  • The desired C/N for the output: 45 dB (very good ratio)
  • Amplifier NF: 9 dB
  • Amplifier gain: 20 dB

At the output the level will be:

1.8 + 20 + 9 + 45 = 75.8 dBuV

Then the minimum input level is:

vI min = 75.8 - gain = 75.8 - 20 = 55.8dBuV.

This is equal to

vI min = SNR output + Noise = SNR output + NF + thermal noise.

Because of this, when several amplifiers are cascaded, the equivalent NF gets higher.

Therefore, the minimum input level gets bigger and bigger until this minimum input level will be equal to the maximum input level accepted by the amplifier to maintain a given intermodulation level.

Once this point has been reached no more amplifiers can be cascaded.

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