Electronics Engineering (ELEX) Board Practice Exam

For a 741 op-amp with r₀ = 75 Ω and A = 200 k Ω, what is Z₀ if R_f = 360 k Ω and R₁ = 12 k Ω?

1. 0.011 Ω

2. 1.2 Ω

3. 0.1 Ω

4. 2.5 Ω

The correct answer is: 0.011 Ω

To determine the output impedance (Z₀) of an op-amp in a feedback configuration, one needs to consider the open-loop gain (A), the feedback resistors (R_f and R₁), and the output resistance of the op-amp (r₀). In this scenario, we have the following values: - Open-loop gain (A) = 200,000 (200 kΩ) - Output resistance (r₀) = 75 Ω - Feedback resistor (R_f) = 360 kΩ - Input resistor (R₁) = 12 kΩ The relationship for calculating the output impedance in a feedback configuration is: Z₀ = r₀ / (1 + Aβ) Where β is the feedback factor, which can be calculated as: β = R₁ / (R₁ + R_f) Substituting the given values to find β: β = 12 kΩ / (12 kΩ + 360 kΩ) = 12 kΩ / 372 kΩ = 0.03225806. Now we can calculate the term (1 + Aβ): 1 + Aβ = 1 + 200,000 *