There are situations in which it is desired to have IC2 , IC1 in a current mirror. A popular way to lower IC2 relative to IC1 is to reduce VBE2 by lifting Q2’s emitter off ground and inserting a...

There are situations in which it is desired to have IC2 , IC1 in a current mirror. A popular way to lower IC2 relative to IC1 is to reduce VBE2 by lifting Q2’s emitter off ground and inserting a suitable series resistance R to drop the required voltage difference, as shown. For instance, if we want to make IC2 5 IC1y2, R will have to drop 18 mV, by the familiar rule of thumb. The circuit is known as the Widlar current source for its inventor Bob Widlar, the designer of the fi rst monolithic op amp. In Fig. P2.53 let VCC 5 5 V and R1 5 4.3 kV. Assuming VBE1 5 700 mV, negligible base currents, and a small enough voltage drop across the load to ensure that Q2 is always in the FA region, fi nd R so that: (a) IC2 5 0.4 mA. (b) IC2 5 50 A. (c) IC2 5 123 A. Hint: use the rules of thumb when possible.