14 Issues in RFIC Design: Noise, Linearity, and Signals2.2.5 The Noise Figure of an Amplifier CircuitWe can now make use of the definition of noise figure just developed and apply it to an amplifier circuit . For the purposes of developing (2.14) into a more useful form, it is assumed that all practical amplifiers can be characterized by an input-referred noise model, such as the one shown in Figure 2.4 where the amplifier is characterized with current gain Ai. (Later chapters will detail how to take a practi-cal amplifier and make it fit this model.) In this model, all noise sources in the circuit are lumped into a series noise voltage source vn and a parallel current noise source ins placed in front of a noiseless transfer function.If the amplifier has finite input impedance, then the input current will be split by some ratio a between the amplifier and the source admittance Ys: αα=22inin22SNRnsii (2.16)Assuming that the input referred noise sources are correlated, the output signal-to-noise ratio is αα=++222inout2222SNR(|| )iinsnn sA iA iiv Y (2.17)Thus, the noise factor can now be written in terms of (2.16) and (2.17): 22(total)2(source)||onsnn sonsNiiv YFNi++== (2.18)This can also be interpreted as the ratio of the total output noise to the total output noise due to the source admittance.In (2.17), it was assumed that the two input noise sources were correlated with each other. In general, they will not be correlated with each other, but rather the current in will be partially correlated with vn and partially uncorrelated. We can expand both the current and voltage into these two explicit parts: ncuiii= + (2.19) ncuvvv=+ (2.20)Figure 2.4 Input referred noise model for a device. Current due to vn is not shown.