Question #3718

A three-stage amplifier has a second stage power gain of 5 dB and noise figure of 4.5 dB.
The third stage has a power gain of 3 dB and a noise figure of 5.5 dB. The overall power
gain is 12 dB and the signal to noise ratio at the input of the first stage is 10 dB.
Calculate the power gain and noise figure of the first stage required to give a signal to
noise ratio at the output of the third stage of 4 dB

Expert's answer

Total Gain (dB) = Gain_{1} + Gain_{2} + Gain_{3}

therefore

Gain_{1}=12-5-3=4

Total NF (dB) = 4-10 = -6 and

Total NF (dB) = 10*log_{10}(nf_{1} + (nf_{2}-1)/gain_{1} + (nf_{3}-1)/(gain_{1}*gain_{2})=10*log_{10}(nf_{1}+(4.5-1)/4+(5.5-)/(4*5))

= 10*log_{10}(nf_{1}+1.1)= -6

Hence nf_{1 }= -0.8488

therefore

Gain

Total NF (dB) = 4-10 = -6 and

Total NF (dB) = 10*log

= 10*log

Hence nf

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