In feedback amplifier terminology, what defines negative feedback in terms of the relationship between the returning feedback signal and the input signal?

Introduction:
Negative feedback is the foundation of linear amplifier stability, accuracy, and bandwidth extension. Recognizing how the feedback signal is combined with the input is critical to predict circuit behavior.

Given Data / Assumptions:

• Standard block-diagram convention with summing junction at the input.
• Feedback network samples a portion of the output and returns it to the input.
• Signals are considered at the same reference phase for comparison.

Concept / Approach:
Negative feedback means the returned signal is subtracted from the input. In other words, the feedback opposes the input error. This reduces distortion, desensitizes the closed-loop gain to device parameters, and improves linearity. In contrast, positive feedback adds to the input and can cause oscillation.

Step-by-Step Solution:
1) Denote input as Vin and feedback as Vf.2) In negative feedback, the error signal is Ve = Vin − Vf.3) The amplifier acts to reduce Ve toward zero, stabilizing the output.4) Therefore, the returning signal necessarily opposes the input signal.

Verification / Alternative check:
Write closed-loop gain: Av_cl = A / (1 + Aβ) for negative feedback. The plus sign in the denominator arises from subtraction at the summing junction, confirming opposition of Vf to Vin.

Why Other Options Are Wrong:
Proportional to output current: Feedback can sample voltage or current; proportionality alone does not define polarity.

Proportional to differential voltage gain: Not the defining feature of negative feedback.

Aids the input signal: That is positive feedback, risking instability or oscillation.

Proportional to input bias current: Irrelevant to feedback sign.

Common Pitfalls:
Assuming any feedback is stabilizing. Only negative feedback that subtracts the appropriate proportion of output from the input yields predictable closed-loop behavior.

Final Answer:
opposes the input signal