As we know, the function of the source-bypass capacitor in the CS confi guration is to establish an ac ground at the source. The circuit of Fig. P3.59 eliminates the need for such a capacitance by...

As we know, the function of the source-bypass capacitor in the CS confi guration is to establish an ac ground at the source. The circuit of Fig. P3.59 eliminates the need for such a capacitance by utilizing the diode-connected FET M2 instead. Even though M2 does not provide a true ac ground at M1’s source, its ac resistance (Rs2) is relatively low, and the small amount of source degeneration that it introduces for M1 is a price well worth the elimination of the bypass capacitor. This technique is widely used in IC implementations, where M1 and M2 are matched devices. With a signal source having a dc component of 0 V, the two FETs experience the same VGS drop and thus carry the same current ID. Consequently, RS has to be specifi ed to carry a current of 2ID.

(a) Regarding M1 as a CS-SD amplifi er with a total source-degeneration resistance RS1 5 RS //Rs2), derive expressions for the smallsignal parameters Ri , Ro, and voyvi .

(b) Assuming matched FETs with Vt 5 1 V, k 5 1 mA/V2 , and 5 0.02 V21 , and VDD 5 2VSS 5 12 V, RD 5 15 KV, and RS 5 7.5 KV, fi nd the dc operating points of the two FETs. (c) Calculate Ri , Ro, and voyvi numerically.