(a) In the Darlington confi guration of Fig. P4.23 replace each BJT with its small-signal model, and use the test-signal method to obtain an expression for the resistance Rc seen looking into the...

(a) In the Darlington confi guration of Fig. P4.23 replace each BJT with its small-signal model, and use the test-signal method to obtain an expression for the resistance Rc seen looking into the collector of the composite device. (b) Discuss the limiting cases I → 0 and I → IC2.