Đầu vào-Referred Spectrum ồn hiện tại. Các đầu vào được gọi phổ tiếng ồn hiện nay của TIA có thể được chia thành hai thành phần chính: tiếng ồn từ các điện trở phản hồi (hoặc điện trở, trong thực hiện sự khác biệt giữa) và tiếng ồn từ các bộ khuếch đại front-end. | TIA CIRCUIT CONCEPTS 123 its contribution to the input-referred TIA noise. A straightforward way to do this transformation is to calculate the transfer function from z to the output of the TIA and divide that by the ttansfer function from to the output. Equivalently but easier we can calculate the implicit transfer function from in D to A under the condition that the TIA output signal is zero. The implicit transfer function from the input current to the drain current has a low-pass characteristics therefore the inverse function which refers the drain current back to the input has high-pass characteristics. It can be shown that this high-pass transfer function is 57 1 4- sRf- Cr - - 8m Rf where Ct Cf C and C is the input capacitance of the FET stage at zero output signal that is C Cgs Cgd- Now using this high-pass to refer the white channel noise D 4kTVgm back to the input yields z2 n _ l 27r . FCr 2 Aỉ front ữ n 2 4kTFgm I . 2jtCt 2 4 7T------ AkTV f2 Sm R ỊT Srn And here for the first time we encounter an y2-noise component. We now understand that it arises from a white-noise source which became emphasized because of a low-pass transfer function from the input to the source location. Figure illustrates the channel-noise component of Eq. and the feedback-resistor noise component of Eq. graphically. It is interesting to observe that the input-referred channel noise starts to rise at the frequency 1 2rr RfCt given by the zero in Eq. . This frequency is lower than the 3-dB bandwidth of the TIA which is ự2A A 1 2tt -RfCt cf. Eq. . As a result the output-referred noise spectrum has a hump as shown in Fig. . ill Response of TIA Noise from Feedback Resistor 0 1 2A HfCt RpCT Fig. Noise spectrum components of a T1A with FET front-end. To summarize we can write the input-referred noise current spectrum of an FET front-end as _ __ ItĩCtỹ lhmm .f 2qIG 4kTr- l .f gm 124 TRANSIMPEDANCE AMPLIFIERS where we have
