tiếng ồn điện trở có thể trở nên chiếm ưu thế. Phổ tiếng ồn hiện tại của các điện trở phản hồi là màu trắng (tần số độc lập) và được đưa ra bởi các phương trình nổi tiếng nhiệt tiếng ồn: (5,37) Tiếng ồn này hiện đóng góp trực tiếp đầu vào được gọi tiếng ồn TIA trong Eq. | MA SPECIFICATIONS 169 to subtract it out. Then we divide this result by four to obtain the available input-referred mean-square noise voltage due to the MA only. Again dividing this result by the squared transimpedance Rị is giving us the desired input-referred mean-square noise current. Finally we can express the input-referred noise voltage in terms of the noise figure using Eq. 2 - vls 4R2 4r2 Combining Eqs. and we find the power penalty due to the MA noise to be 1 Ị 4Ft pp Solving this equation for F yields the maximum permissible noise figure A n2 2 _ F 1 PP1 - 1 -------. From this equation we can conclude that a high transimpedance helps to relax the noise-figure requirements for the MA. Furthermore a low-noise TIA also requires a low-noise MA to keep the power penalty low. Typical Values. Next we calculate some typical numbers based on a power penalty PP and a noise bandwidth BW . For a typical s system Rt 3 kQ i TIA 380 nA we obtain the numerical value F 1 4 2 nA 17-1 dB- mV 2 And for a typical 10-Gb s system Rr 1 kii i 1A 1 400 nA we get 4- IkQ 2 . l 400nA 2 __ F 1 - ------ . mV 2 From a wireless designer s perspective these noise figures are very large compared with what s typical for a low-noise amplifier LNA . However low noise figures are harder to achieve for a broadband amplifier and care must be taken to meet these numbers. Input Dynamic Range Definition. The input dynamic range of the MA describes the minimum and maximum input signal for which the MA performs a useful function for example for which the BER is sufficiently low. The minimum input signal lower end of the dynamic 170 MAIN AMPLIFIERS range is given by the MA s sensitivity. Similar to the receiver sensitivity definition in Section the MA sensitivity is the minimum peak-to-peak signal voltage at the input of the MA necessary to achieve a specified BER vfens v
