3-30Figure 3-20.—Two-input adder.If the signal on input number one (E1) is +3 volts and the signal on input number two (E2) is +4volts, the output signal (E_{out}) should be -7 volts [(+3 V) + (+4 V) = +7 V and change the polarity to get-7 V].With +3 volts at E1 and 0 volts at point A (which is at virtual ground), the current through R1 mustbe 3 milliamperes.Mathematically:(The + sign indicates a current flow from right to left.)By the same sort of calculation, with +4 volts at E2 and 0 volts at point A the current through R2must be 4 milliamps.This means that a total of 7 milliamps is flowing from point A through R1 and R2. If 7 milliamps isflowing from point A, then 7 milliamps must be flowing into point A. The 7 milliamps flowing into pointA flows through R3 causing 7 volts to be developed across R3. With point A at 0 volts and 7 voltsdeveloped across R3, the voltage potential at E_{out} must be a -7 volts. Figure 3-21 shows these voltagesand currents.