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:
Therefore, let┬┤s suppose:
At the output the level will be:
1.8 + 20 + 9 + 45 = 75.8 dBuV
vI min = 75.8 - gain = 75.8 - 20 = 55.8dBuV.
vI min = SNR output + Noise = SNR output + NF + thermal noise.
Because of this, when several amplifiers are cascaded, the equivalent NF gets higher.