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

The input pin marked by a (+) is called the non-inverting input.

The gain of the inverting amplifier can be derived as follows:

The relation is given by:

$V_{out} = - \frac{R_f}{R_{in}} V_{in}$

Where:

• $V_{out}$ is the output voltage.
• $V_{in}$ is the input voltage.
• $R_f$ is the feedback resistor.
• $R_{in}$ is the input resistor.

The voltage gain for channel 1 can be calculated using:

$V_{gain} = \frac{R_f}{R_{in}} = \frac{150k \Omega}{7.5k \Omega} = 20$

Thus, the voltage gain produced by channel 1 is 20.

For channel 1 with an input signal of 15 mV:

Using the gain calculated, we find:

$V_{out} = V_{in} \times V_{gain} = 15mV \times 20 = 0.3V$

For channel 2 with an input of -100 mV, we calculate:

$V_{out} = -100mV \times V_{gain} = -100mV \times 20 = -2V$

Thus, the output voltage V_out can range based on the input signals.

By using variable resistors for the input resistors, the gain can easily be adjusted. This allows for:

• Changing the relative levels of the two channels easily.
• Setting the required balance between the two signals, enhancing flexibility in mixing.