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Questions and Answers
In the context of cascaded amplifiers, what does the denominator in the equation for $NF_{tot}$ (total noise figure) represent?
In the context of cascaded amplifiers, what does the denominator in the equation for $NF_{tot}$ (total noise figure) represent?
- The ratio of input to output impedance of the first stage.
- The 'available power gain' of the first stage. (correct)
- The current gain of the first stage.
- The voltage gain of the first stage.
How is the 'available power gain' of a stage defined?
How is the 'available power gain' of a stage defined?
- The maximum power that the stage can deliver, regardless of load.
- The ratio of output voltage to input voltage.
- The power delivered to a specific load impedance divided by the input power.
- The 'available power' at its output divided by the available source power. (correct)
In Friis' equation, what is the impact on the total noise figure ($NF_{tot}$) as the gain of the preceding stages increases?
In Friis' equation, what is the impact on the total noise figure ($NF_{tot}$) as the gain of the preceding stages increases?
- The noise contribution of each subsequent stage remains constant.
- The noise contribution of each subsequent stage decreases. (correct)
- The noise contribution of each subsequent stage oscillates.
- The noise contribution of each subsequent stage increases linearly.
According to Friis' equation, which stages in a cascaded system are the most critical in terms of noise contribution?
According to Friis' equation, which stages in a cascaded system are the most critical in terms of noise contribution?
If a stage in a cascade suffers from attenuation (loss), what effect does this have on the noise figure of the following stages when referred back to the input of the stage with loss?
If a stage in a cascade suffers from attenuation (loss), what effect does this have on the noise figure of the following stages when referred back to the input of the stage with loss?
What parameters are required to calculate $NF_2$ (noise figure of the second stage)?
What parameters are required to calculate $NF_2$ (noise figure of the second stage)?
Assuming a two-stage amplifier, what parameters of the first stage influence the overall noise figure ($NF_{tot}$)?
Assuming a two-stage amplifier, what parameters of the first stage influence the overall noise figure ($NF_{tot}$)?
How does the available power gain, $A_{P1}$, relate the available output power, $P_{out,av}$, and the available source power, $P_{S,av}$?
How does the available power gain, $A_{P1}$, relate the available output power, $P_{out,av}$, and the available source power, $P_{S,av}$?
What is the significance of computing $NF_2$ (noise figure of the second stage) with respect to the output impedance of the first stage?
What is the significance of computing $NF_2$ (noise figure of the second stage) with respect to the output impedance of the first stage?
In the equation (NF_{tot} = NF_1 + (NF_2 - 1) / A_{P1}), if the available power gain of the first stage ($A_{P1}$) decreases, what happens to the contribution of the second stage's noise figure ($NF_2$) to the total noise figure ($NF_{tot}$)?
In the equation (NF_{tot} = NF_1 + (NF_2 - 1) / A_{P1}), if the available power gain of the first stage ($A_{P1}$) decreases, what happens to the contribution of the second stage's noise figure ($NF_2$) to the total noise figure ($NF_{tot}$)?
Flashcards
NF_2 Equation
NF_2 Equation
Noise figure of the second stage (NF_2) with respect to a source impedance of R_out1.
NF_tot Equation
NF_tot Equation
The total noise figure (NF_tot) of a two-stage system, considering the noise figures and gains of both stages.
Available Power Gain (A_P1)
Available Power Gain (A_P1)
The ratio of the available power at the output of the first stage to the available power from the source. It indicates how much the first stage amplifies the signal power.
Friis' Equation
Friis' Equation
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Noise Contribution by Stage
Noise Contribution by Stage
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Effect of Attenuation on NF
Effect of Attenuation on NF
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Study Notes
- The noise figure of the second stage (NF_2) with respect to a source impedance of R_out1 is given by: NF_2=1+(V_n2^2 ) Ì…/((R_("in " 2)^2)/(R_("in " 2)+R_("out " 1) )^2 A_v2^2 ) 1/(4kTR_("out " 1) )
- The total noise figure (NF_tot) is: NF_tot=NF_1+(NF_2-1)/((R_in1^2)/(R_in1+R_S )^2 A_v1^2 R_S/R_vut1 )
- The denominator in the NF_tot equation represents the "available power gain" of the first stage.
- Available power gain is the ratio of available output power (P_out,av) to available source power (P_S,av).
- The available output power of the first stage (P_out,av) is: P_(out,av)=V_in^2 (R_in1^2)/(R_S+R_("in " 1) )^2 A_v1^2â‹…1/(4R_("out " 1) )
- The available source power (P_S,av) is: P_(S,av)=(V_in^2)/(4R_S )
- NF_tot can also be written as: NF_tot=NF_1+(NF_2-1)/A_P1, where A_P1 is the available power gain of the first stage.
- NF_2 is computed with respect to the output impedance of the first stage.
- For 'm' stages, the total noise figure is: NF_tot=1+(NF_1-1)+(NF_2-1)/A_P1 +⋯+(NF_m-1)/(A_P1⋯A_(P(m-1)) )
- This equation is called "Friis' equation".
- Friis' equation suggests that the noise contribution of each stage decreases as the total gain preceding it increases.
- The first few stages in a cascade are the most critical in terms of noise contribution.
- If a stage has attenuation (loss), the noise figure of subsequent circuits is amplified when referred back to the input of that stage.
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