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Tham khảo tài liệu 'current trends and challenges in rfid part 3', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 50 Current Trends and Challenges in RFID Since the magnitude of ly should be 1 as per definition and considering physical frequencies s ja then gm Ũ Cgd 32 Therefore the unit gain frequency is gm 33 As can be observed the unit gain frequency is directly proportional to gm and inversely proportional to the internal capacitances. Therefore in terms of frequency response the transistor should have large gm and small capacitances. 4. RF CMOS noise model The two most important types of noise in MOS devices are the 1 f noise and the thermal noise. 4.1 Thermal noise The main source of thermal noise in a MOS transistor is due to the resistive channel in the active region and has a value of i2 4 kTỵgm 34 where k is the Boltzmann s constant about 1.38 x 10-23 J K T is the absolute temperature in kelvins and Y is a constant that is approximately 2 3 for long channel transistors and increase to the range 1-2 for short channel devices. The other source of thermal noise is the gate. Fluctuation in the channel potential couples capacitively into the gate terminal which in turn translates into a noise gate current. Noise gate current can also be produced by the resistive material of the gate. This total noise gate can be ignored at low frequencies but becomes significant at high frequencies as it is the case of RF circuits. It has been shown the gate noise may be expressed as ig 4 kTSgg 35 where 8 is approximately 4 3 for long channel transistors and increase to the range 2-4 for short channel devices and gg is given by gg 5gm 36 Mostly of the time instead of using a current source at the gate it is more convenient to consider an equivalent voltage source. The equivalent voltage source of expressions 31 and 32 is given by RF CMOS Background 51 where Tg is given by v 4 kT ổrg 1 r ----- g 5gm 37 38 4.2 1 f noise The 1 f noise also known as flicker noise or pink noise arises mainly due to the surface imperfections that can trap and release charges. Since MOS devices are naturally .
