In the OP AMP circuit shown below, what is the input voltage if output voltage V_out = -0.62 V?

Concept:

An inverting amplifier has the configuration shown below. The voltage gain of an inverting OP-AMP is given by: \( A_v = \frac{V_{out}}{V_{in}} = -\frac{R_f}{R_{in}} \)

Given:

Feedback resistor, \(R_f = 1.2~M\Omega\), Input resistor, \(R_{in} = 40~k\Omega\), Output voltage, \(V_{out} = -0.62~V\)

Calculation:

Gain of the amplifier: \( A_v = -\frac{1.2 \times 10^6}{40 \times 10^3} = -30 \)

Using the gain formula: \( V_{in} = \frac{V_{out}}{A_v} = \frac{-0.62}{-30} = 0.0207~V = 20.7~mV \)

Since it is an inverting amplifier, input voltage must be negative: \( V_{in} \approx -20~mV \)

Answer:

Option 1) -20 mV