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 ($V_{out}$), we use the formula: $V_{out} = (V_{+} - V_{-}) \times \left(\frac{R_f}{R_{in}}\right)$

For this configuration, substituting the values:

• $V_{+} = 150 mV$ (input A)
• $V_{-} = 0 V$ (input B)
• $R_f = 1 M\Omega$, $R_{in} = 100 k\Omega$

This gives: $V_{out} = (0 V - 150 mV) \times \left(\frac{1 M\Omega}{100 k\Omega}\right)$

Calculating this results in: $V_{out} = -1.5 V$

The operational amplifier circuit, specifically a difference amplifier, effectively reduces electrical noise. This is achieved because the noise present in both signals (signal 1 and signal 2) will be subtracted when processed by the amplifier. Since the noise is common to both signals, it will be eliminated, enhancing the signal-to-noise ratio.

Moreover, due to the phase relationship between signal 1 and signal 2, which are inversions of each other, any noise that appears in both signals will cancel each other out. As a result, the relationship between the input signals will be preserved, leading to a stronger output signal.