When operated as a current amplifier, an npn BJT will only source emitter current, while a pnp BJT will only sink emitter current. In ac situations, current must be sourced during the positive...

When operated as a current amplifier, an npn BJT will only source emitter current, while a pnp BJT will only sink emitter current. In ac situations, current must be sourced during the positive alternations of the load voltage, but must be sunk during the negative alternations. To serve these situations, the configuration of Fig. P2.8 is used. During positive alternations Q1 conducts and Q2 is off; during negative alternations Q2 conducts and Q1 is off. This popular transistor team, known as a push-pull stage, is at the basis of a variety of output stages, including those found in op amps and audio power amplifiers. Assume a 100-V load and 612-V power supplies, and let the BJTs have, respectively, Is1 5 10 fA and F1 5 150, Is2 5 20 fA and F2 5 100. Using vBE1 5 VT ln (iC1yIs1) for Q1 and vEB2 5 VT ln (iC2yIs2) for Q2, with iC > iE and iB 5 iEy(F 1 1) for both BJTs, fi nd the input voltage vI as well as the input current (either iI1 or iI2) needed to achieve: (a) vO 5 11 V, (b) vO 5 21 V, (c) vO 5 15 V, and (d) vO 5 28 V.