Figure 1 shows an operational amplifier used as an inverting amplifier - AQA - A-Level Physics - Question 1 - 2017 - Paper 8

The input pin shown by a (+) on the operational amplifier is referred to as the non-inverting input.

The relationship between the output voltage $V_{out}$ and input voltage $V_{in}$ for an inverting amplifier is given by the formula:

$V_{out} = -\frac{R_f}{R_{in}} V_{in}$ This indicates that the output voltage is inversely proportional to the input voltage.

For channel 1, the voltage gain $A_{v}$ can be calculated as:

$A_{v} = \frac{R_f}{R_{in}} = \frac{150 k\Omega}{7.5 k\Omega} = 20$ Thus, the voltage gain produced by channel 1 is 20.

Given the input signals, the output voltage $V_{out}$ can be calculated using:

$V_{out} = R_{v} \cdot \left( \frac{V_{1}}{R_{1}} + \frac{V_{2}}{R_{2}} \right)$

Where:

• $V_{1} = 15 mV$
• $V_{2} = -100 mV$

Substituting in the values, we find:

By using variable resistors, the gain can easily be altered to accommodate changes in input signals. This allows for better control over the relative levels of the two channels, enabling the user to adjust the mix according to preference and ensuring optimal sound balance.