Shown in Fig. P4.62 is a popular scheme for nulling the offset voltage. With perfectly matched halves and the wiper midway we have a perfectly balanced circuit, giving VC1 5 VC2 and thus VO 5 0....

Shown in Fig. P4.62 is a popular scheme for nulling the offset voltage. With perfectly matched halves and the wiper midway we have a perfectly balanced circuit, giving VC1 5 VC2 and thus VO 5 0. However, moving the wiper to the right or the left will imbalance the circuit, so by proper choice of the direction and extent of this deliberate imbalance we can cancel out the circuit’s inherent imbalance and thus null VO, creating the appearance of an offset-less circuit. (a) Let VCC 5 2VEE 5 12 V, IEE 5 1 mA, and REE 5 `. Moreover, suppose RC1 is 8% higher and RC2 is 5% lower than the intended nominal value of 10 kV, and suppose the BJTs are mismatched such that Is1 is 10.5% higher than Is2. If R is a 3-kV potentiometer, fi nd the wiper setting that will yield VO 5 0 (specify the wiper setting in terms of the portion of the 3-kV resistance assigned to the left of the wiper, and the portion to the right—for instance, 2.5 kV to the left and 0.5 kV to the right). (b) Repeat if the RC’s are interchanged with each other, so that the smaller one is now at the left and the larger at the right.