Figure 4 shows an operational amplifier circuit used in an audio mixing desk - AQA - A-Level Physics - Question 2 - 2019 - Paper 8

To calculate the output voltage, we will use the formula for a difference amplifier:

$V_{out} = (V_{+} - V_{-}) \times \left(\frac{R_f}{R_{in}}\right)$

Given:

• Input A (V+) = 150 mV
• Input B (V-) = 0 V
• For resistor selection, using Rf = 1 MΩ and Rin = 100 kΩ:

Using the formula:

$V_{out} = (0 V - 150 mV) \times \left(\frac{1 MΩ}{100 kΩ}\right)$

Calculating:

$V_{out} = (-150 mV) \times 10 = -1.5 V$

Thus, the amplitude of the output voltage is 1.5 V, with the correct sign being negative.

The operational amplifier circuit effectively removes common noise present in both signals. Since signal 1 and signal 2 are combined in a difference amplifier, any noise that affects both signals equally will be subtracted out and thus minimized in the output.

Additionally, the signals are processed in such a way that their strengths can reinforce each other, provided they are in phase. When signal 2 is subtracted from signal 1, the noise reduction and the different phases will contribute to the output signal's clarity and strength, leading to a more accurate representation of the original audio input.