7-88. To see if this output (-50 V) is what the circuit will produce with the inputs (+2 V

and +3V), start by calculating the currents through the input resistors, R1 and R2

(remember that point A is at virtual ground):

E1

IR1 =

R1

2V

IR1 =

1 kΩ

IR1 = 2 mA

--------

E2

IR2 =

R2

3V

IR2 =

1 kΩ

IR2 = 3 mA

Next, calculate the current through the feedback resistor (R3):

IR3 = (IR1 + IR2)

IR3 = (2 mA + 3 mA)

IR3 = -5 mA

NOTE: The minus sign indicates current flow from left to right.

Finally, calculate the voltage dropped across R3 (which must equal the output voltage):

EOUT = *(*IR3 x R3)

EOUT = ( -5 mA x 10 kΩ)

EOUT = -50 V

As you can see, this circuit performs the function of adding the inputs together and

multiplying the result by the gain of the circuit.

7-89. One final type of summing amplifier is the scaling amplifier. This circuit multiplies

each input by a factor (the factor is determined by circuit design) and then adds these

values together. The factor that is used to multiply each input is determined by the ratio of

the feedback resistor to the input resistor. For example, you could design a circuit that

would produce the following output from three inputs (E1, E2, E3):

[(2 x E1) + (4 x E2) + (3 x E3)]

Using input resistors R1 for input number one (E1), R2 for input number two (E2), R3 for

input number three (E3), and R4 for the feedback resistor, you could calculate the values

for the resistors as follows: