(a) Use simple inspection to come up with an expression for Ri in the circuit of Fig. P2.48 (switch open). (b) Next, investigate the effect of closing the switch. Replace the BJT with its small-signal...

(a) Use simple inspection to come up with an expression for Ri in the circuit of Fig. P2.48 (switch open). (b) Next, investigate the effect of closing the switch. Replace the BJT with its small-signal model, and use the test-signal method to show that now Ri is increased to R i 5 R x 1 (1 1 g m R x) R y Rx 5 R1 //rRy 5 R2 //RE

(c) Calculate Ri for each of the two above cases if R1 5 R2 5 RE 5 10 kV, and the BJT has F 5 100 and VA 5 `, and is biased at IC 5 1 mA. Compare the two results, and comment! Note: this problem illustrates the so called bootstrapping technique, which is used to raise the input resistance of an emitter follower. In a practical situation, what is shown here as a switch is a capacitor, which acts as an open circuit at dc, but is designed to act as a short circuit at ac.