A4 Operational Amplifiers and Feedback

Topic

An ideal operational amplifier would possess infinite gain, infinite input resistance, zero output resistance, and infinite bandwidth. Real-world amplifiers deviate from this ideal in several ways: their gain-bandwidth product is a fixed value specific to each model, meaning that demanding higher gain leaves less usable bandwidth available, and vice versa; they exhibit a small voltage offset between their inputs even in a state of perfect balance; they draw a small amount of input current; and they cannot change their output instantaneously, but rather at a limited maximum rate.

Negative feedback—feeding a fraction of the output back to the input and subtracting it—is the fundamental technique in operational amplifier design; it reduces the amplifier's raw gain but vastly improves linearity, stabilizes effective bandwidth, and ensures that the final gain depends almost exclusively on precise external passive components rather than the more variable characteristics of the amplifier itself.

The stability of a feedback circuit is assessed using phase margin: if this margin is insufficient, the circuit may undergo unwanted oscillation instead of clean amplification, whereas a wide margin guarantees a stable response free from overshoot. To ensure stability across any configuration, operational amplifiers incorporate internal compensation that deliberately limits their frequency response, accepting this trade-off to prevent unexpected oscillation. Precision models, characterized by extremely low input offsets, are used in instrumentation and sensors, while ultra-high-bandwidth models are reserved for video and radio-frequency applications. That is why this module—which opens with an ideal amplifier that no real-world component can match—concludes by showing that negative feedback does not attempt to bring the actual amplifier closer to that ideal on a component-by-component basis; instead, it achieves the same practical result through a different approach: sacrificing raw gain so that the final behavior depends on precise external resistors rather than on an imperfect amplifier.

ID:4062