D 2.39 An ideal op amp is connected in the weighted summer configuration of Fig. 2.10. The feedback resistor Rf =100 k, and six 100-k resistors are connected to the inverting input terminal of the op amp. Show, by sketching the various circuit configurations, how this basic circuit can be used to implement the following functions: (a) vO = –(v1 +2v2 +3v3) (b) vO = –(v1 +v2 +2v3 +2v4) (c) vO = –(v1 +5v2) (d) vO = –6v1 In each case find the input resistance seen by each of the signal sources supplying v1, v2, v3, and v4. Suggest at least two additional summing functions that you can realize with this circuit. How would you realize a summing coefficient that is 0.5?