Figure $19

Question

Figure $19 < 2$
-What change causes the circuit in Figure 19-2(b) to output the average of the three input voltages?
A) Change the input resistors to 3.3 $\mathrm { k } \Omega \text {. }$ 
B) Change the feedback resistor to 3.3 $\mathrm { k } \Omega \text {. }$ 
C) Change to a different type of op-amp. 
D) Change the feedback resistor to 10 $\mathrm { k } \Omega$

Quizplus · Accepted Answer

In order to output the average of the three input voltages, we need to divide the sum of the input voltages by 3. This can be achieved by changing the feedback resistor to 3.3 kΩ, which will provide a gain of 1/3. Therefore, the output voltage will be: 
Vout = ( Vin1 + Vin2 + Vin3)/ 3 
Vout = (Vin1/3 + Vin2/3 + Vin3/3 )
This can also be achieved by changing the input resistors to 3.3 kΩ, but that would require changing all three resistors. Changing the type of op-amp is not necessary for this circuit. Changing the feedback resistor to 10 kΩ will provide a gain of 1/10, which is not equal to 1/3 and will not output the average of the input voltages.

Figure 19<219 < 219<2 -What Change Causes the Circuit in Figure 19-2(b) to Output

Figure $19 < 2$ -What Change Causes the Circuit in Figure 19-2(b) to Output