12.27 Figure P12.27 shows a class AB output stage with a common-emitter transistor added to increase the voltage gain and reduce the current that vI has to supply. Neglecting the small-signal resistances of D1 and D2, find the small-signal voltage gain vo/vi. (Hint: Use the expressions for voltage gain and input resistance of the class AB stage without Q3, given in the statement for Problem 12.26.)