The characteristics of an emitter follower can be made much closer to ideal through the use of negative feedback, a subject that will be explored in greater detail in Chapter 7. The circuit of...

The characteristics of an emitter follower can be made much closer to ideal through the use of negative feedback, a subject that will be explored in greater detail in Chapter 7. The circuit of Fig. P4.12, known as super emitter follower, uses Q1 as the emitter follower proper, and Q2 to provide negative feedback around Q1.

(a)Draw the ac equivalent of the circuit by replacing each BJT with its small-signal equivalent consisting of gm, r, and ro (ignore r for simplicity). Hence, use the test-signal method to fi nd an expression for the output resistance Ro.

(b) Assuming both BJTs have 0 5 100 and VA 5 50 V, calculate Ro, compare with the value provided by a single-BJT emitter follower operating at IC 5 1 mA, and comment on your result.