The circuit of Fig. P4.100, known as a VBE multiplier, provides an alternative method for the generation of the voltage drop VBB necessary to bias the push-pull BJTs. Its advantage is that VBB can be...

The circuit of Fig. P4.100, known as a VBE multiplier, provides an alternative method for the generation of the voltage drop VBB necessary to bias the push-pull BJTs. Its advantage is that VBB can be adjusted by playing with the ratio R1yR2. (a) Show that VBB 5 (1 1 R1yR2) 3 [VBE 1(R1// R2)IB]. Usually the base current is small enough to meet the condition (R1//R2)IB ! VBE, in which case the above expression simplifi es as VBB 5 MVBE, with M 5 1 1 R1yR2 as a multiplier term. (b) If I1 5 I2 5 200 A and the BJT has F 5 250 and Is 5 1 fA, specify suitable values for R1 and R2 to achieve VBB 5 1250 mV under the constraint IC 5 150 A. (c) Replace the BJT with its small-signal model and fi nd the dynamic resistance Rbb between the collector and emitter nodes.