There are situations in which the source follower’s output resistance Ro, which is dominated by 1y(gm 1 gmb), is not suffi ciently low due to the notoriously poor FET transconductance. A clever way to...

There are situations in which the source follower’s output resistance Ro, which is dominated by 1y(gm 1 gmb), is not suffi ciently low due to the notoriously poor FET transconductance. A clever way to lower Ro is via negative feedback, a subject that will be explored in greater detail in Chapter 7. The circuit of Fig. P4.21, known as a super source follower, uses M1 as the source follower proper, and M2 to provide negative feedback around M1. (Note that for the circuit to function we must have I2 . I1.) (a) Draw the ac equivalent of the circuit by replacing each FET with its small-signal model, consisting of gm, ro, and gmb (when appropriate). Hence, develop an expression for the gain voyvi . (b) Use the test-signal method to develop an expression for the output resistance Ro, and verify that negative feedback reduces the output resistance of M1 by about gm2ro1. (c) Assuming 5 0.1, calculate voyvi and Ro if both FETs have gm 5 1 mA/V and ro 5 20 kV.