3-30
Figure 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 +4
volts, the output signal (Eout) 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 must
be 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 R2
must be 4 milliamps.
This means that a total of 7 milliamps is flowing from point A through R1 and R2. If 7 milliamps is
flowing from point A, then 7 milliamps must be flowing into point A. The 7 milliamps flowing into point
A flows through R3 causing 7 volts to be developed across R3. With point A at 0 volts and 7 volts
developed across R3, the voltage potential at Eout must be a -7 volts. Figure 3-21 shows these voltages
and currents.
