*2.68 (a) Find Ad and Acm for the difference amplifier circuit shown in Fig. P2.68. (b) If the op amp is specified to operate properly as long as the common-mode voltage at its positive and negative inputs falls in the range ±2.5 V, what is the corresponding limitation on the range of the input common-mode signal vIcm? (This is known as the common-mode range of the differential amplifier.) (c) The circuit is modified by connecting a 10-k resistor between node A and ground, and another 10-k resistor between node B and ground. What will now be the values of A d, Acm, and the input common-mode range?