04/05 - Lab 14: Summing Amplifier

Lab 14: Summing Amplifier

In this lab, we used the OP-27 as a summing amplifier. Op amps can be used to perform various mathematical operations. In the previous lab, we used it for multiplying the input voltage by a constant. In this lab, we will use them to sum multiple input voltages.

For this summing amplifier setup, the output voltage should follow the equation derived below (but with two terms instead of three):

In this picture are the actual resistance values recorded, along with a diagram of the setup we chose for this lab.

Actual circuit setup is shown below:

We set V_b to 1 V, and used the wavegen from the analog discovery to vary the voltage of V_a. We chose R_1 = R_2 = 3.6 kOhm, and Rf to be half that value, or R_f = 1.8 kOhm, so that the sum of the two voltages is cut in half in order to prevent the op amp from saturating. In mathematical terms, this circuit should yield the following voltage: V_o = -1/2(V_a +1). The values of V_a and V_o are recorded in the table and corresponding graph below.

In Class Examples:

1. Predict: The OP27 is powered at 9V with V- connected to ground. A -100 – 100mV square wave is input. R1 ~2.5K and R2~7.5K. Make a quantitative sketch of the output.

2. For the circuit shown, Calculate vo if vs = 0:

3. Design a difference amplifier to have a gain of 2 and a common mode input resistance of 10 k Ω at each input.

4.