Summing Amplifier and Difference Amplifier

Summing Amplifier

In this experiment, we wire a summing amplifier (OP 27) as shown above. The summing amplifier takes two voltage inputs V_a and V_b and and sums up a multiple of the signals according to the values of the feedback resistor to the input resistors. Thus V_out = -(R3/R1)*(V_a + V_b), where R3 is the feedback resistor and R1 = R2 are the input resistors. We use a 1.8 kΩ feedback resistor and two 3.6 kΩ input resistors (measured values shown above). The resistor ratio R3/R1 is theoretically 0.5, therefore according to the formula we expect our output voltage to be -0.5 the sum of the input voltages.

The circuit is wired according to the diagram with a multimeter connected to the output of the op and and ground to measure V_out (not labeled in previous diagram). We provide a constant 1 V for input V_b and vary the other input voltage V_a from - 4 V to 5 V.

Here we see the data values for the measured input and output voltages as well as the theoretical output voltage calculated as V_out_theo = -(1.74 kΩ/ 3.5 kΩ)*(V_a + V_b) =  -0.50*(V_a + V_b). The actual measured output values are close to the theoretical values with no more that 5.46 % error.

Difference Amplifier

In this experiment, we wire a difference amplifier as shown above using the same OP 27 op amp as in the previous experiment. The resistors used should be equal and are all 3.6 kΩ (measured values on diagram). The difference amplifier then behaves according to the following formula V_out =  V_b - V_a. V_b is kept constant at 1 V throughout this experiment and the other input is varied from -4 V to 5 V.

We wire our circuit according to the diagram above and add +/- 5 V rail voltages. This will limit the output voltages to +/- 5 V, although it will usually be even less.

The following data shows the input voltages V_a and V_b and the measured output voltage V_out as well as the theoretical voltage V_out_theo. A percent error column shows the accuracy of our experiment. We see that the percent error is low with exception to the first and last measurements. In these measurements, the output voltages approach the rail voltages of +/- 5 V. 

This plot shows the linear region (in orange) with the two saturation points (in blue). If we were to extend the orange line, it would not go through the blue points. Omitting the saturation points makes our data very accurate at a 3.0% maximum error.