Analysis of the circuit in Fig. 2.20(b), assuming ideal op amps, is straightforward, as is
illustrated in Fig. 2.20(c). The key point is that the virtual short circuits at the inputs of op
amps A1 and A2 cause the input voltages vI1 and vI2 to appear at the two terminals of resistor
(2R1). Thus the differential input voltage vI2 − vI1 ≡ vId appears across 2R1 and causes a current