Noise resistance in cascaded amplifiers

Q12. For a two stage amplifier, first amplifier has Voltage gain = 20, Input Resistance Rin1=700Ω, equivalent Resistance Req1=1800Ω and Output Resistor Ro1=30KΩ. The corresponding values of second amplifier are : 25, 80 KΩ, 12 KΩ, 1.2 MΩ respectively. What is the value of equivalent input noise resistance of the given two stage amplifier?

Options:

a) 2609.1Ω
b) 2607.1Ω
c) 107.1Ω
d) 2107.1Ω

Correct Answer: b) 2607.1Ω


Explanation :

R1= Rin1+ Req1 = 700+ 1800 =2500Ω
R2= (Ro1 Rin2)/ (Ro1+ Rin2)+ Req2 = 30*80/(30+80) + 12 = 40.92KΩ
R3 = Ro2 = 1.2MΩ
Equivalent input noise resistance of a two stage amplifier is given by
Req= R1 + R2/ A21 +R3/ (A21 A22)
= 2500 + 40.92*103/(20)2 + 1.2*106/(20)2(25)2
= 2607.1Ω
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    Discussion

  • RE: Noise resistance in cascaded amplifiers -Athena (10/04/16)
  • R2= (Ro1 Rin2)/ (Ro1+ Rin2)+ Req2 = 30*80/(30+80) + 12 = 40.92KO?????

    I calculated 33.82kO... where did you get 40.92k??