22专业外语2(从35页开始有5分).ppt

Single-Stage Amplifiers Thus in (Vgs-Vth)2 the first term increases and the second decreases, resulting in a greater change in the drain current and hence a lower output impedance. Design of Analog COMS Integrated Circuit Single-Stage Amplifiers Source followers exhibit a high input impedance and a moderate output impedance, but at the cost of two drawbacks: nonlinearity and voltage headroom limitation. Design of Analog COMS Integrated Circuit Single-Stage Amplifiers We consider these issues in detail. As mentioned in relation to Fig. 3.27(a), even if a source follower is biased by an ideal current source, its input-output characteristic displays some nonlinearity due to the nonlinear dependence of Vth upon the source potential. Design of Analog COMS Integrated Circuit Single-Stage Amplifiers In submicron technologies, ro of the transistor also changes substantially with VDS, thus introducing additional variation in the small-signal gain of the circuit. Design of Analog COMS Integrated Circuit Single-Stage Amplifiers For this reason, typical source followers suffer from several percent of nonlinearity. The nonlinearity due to body effect can be eliminated if the bulk is tied to the source. eliminate [英] [i?limineit] vt. 排除； 消除 Design of Analog COMS Integrated Circuit Single-Stage Amplifiers This is usually possible only for PFETs because all NFETs share the same substrate. Fig. 3.36 shows a PMOS source follower employing two separate n-wells so as to eliminate the body effect of M1. Design of Analog COMS Integrated Circuit Single-Stage Amplifiers Design of Analog COMS Integrated Circuit Single-Stage Amplifiers The lower mobility of PFETs, however, yields a higher output impedance in this case than that available in an NMOS counterpart. Design of Analog COMS Integrated Circuit Single-Stage Amplifiers Source followers also shift the dc level of the signal by Vgs, thereby consuming voltage headroom a