An RC-coupled amplifier has open-loop midband gain A = 200 and a lower cutoff frequency of 50 Hz. Negative feedback with feedback factor β = 0.1 is applied. Estimate the new lower cutoff frequency with feedback.

Introduction / Context:
Negative feedback reshapes an amplifier’s frequency response, typically trading gain for improved bandwidth and more controlled cutoff characteristics. In RC-coupled voltage amplifiers, applying negative feedback reduces the midband gain but extends both the low- and high-frequency bandwidth, often by approximately the desensitivity factor (1 + Aβ) for dominant single-pole regions.

Given Data / Assumptions:

• Open-loop midband gain A = 200.
• Lower cutoff (−3 dB) frequency f_L = 50 Hz before feedback.
• Feedback factor β = 0.1, negative (regenerative effects are excluded).
• Single-pole dominant behavior around f_L so textbook bandwidth scaling applies.

Concept / Approach:
For a simple first-order low-frequency pole, negative feedback reduces the effective time constant by approximately the factor (1 + Aβ). Thus the lower cutoff shifts downward by the same factor, improving low-frequency response. Closed-loop gain also reduces to A_cl ≈ A / (1 + Aβ).

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