Question

In a negative feedback amplifier \( A = 100 \), \( \beta = 0.04 \) and \( V_s = 50\,\text{mV} \), then feedback will be _______.

• A. 4
• B. 8
• C. 10
• D. 2

Hint:

Use \( V_f = \beta \cdot V_{out} \) for feedback. Always calculate output from closed-loop gain.

Answer 1

The Correct Option is A

Step 1: Negative feedback causes some fraction of output to be returned to the input.
Step 2: Feedback voltage = \( \beta \cdot V_{out} \)
Step 3: Closed-loop gain of amplifier is:
\[ A_f = \frac{A}{1 + A\beta} = \frac{100}{1 + 100 \cdot 0.04} = \frac{100}{5} = 20 \] Step 4: Output voltage \( V_{out} = A_f \cdot V_s = 20 \cdot 50\,\text{mV} = 1000\,\text{mV} = 1\,\text{V} \)
Step 5: Feedback voltage \( = \beta \cdot V_{out} = 0.04 \cdot 1 = 0.04\,\text{V} = 40\,\text{mV} \)
Step 6: Convert to number: \[ \frac{40\,\text{mV}}{10\,\text{mV}} = 4 \]