What is the primary purpose of feedback in an amplifier?

• A. To increase bandwidth only
• B. To reduce noise entirely
• C. To realize a stable gain without regard to device variations
• D. To shape the closed-loop response and stabilize gain, input/output impedance

Answer: (D)

Feedback uses a portion of the output to influence the input, creating a closed-loop that makes the amplifier’s overall behavior depend more on the feedback network than on the exact parameters of the active device. The big benefit is a gain that stays predictable and stable even when transistors or resistors drift with temperature or manufacturing variations. Along with this stable gain, the feedback network also shapes how the amplifier responds across frequency and helps set the input and output impedances to convenient values. So the primary purpose is to shape the closed-loop response and stabilize the gain (and impedance), rather than merely increasing bandwidth, eliminating noise completely, or ignoring device variations.