ABSTRACT
Amplifiers operate mostly in a linear mode, and their performance is improved by applying feedback. Basic circuit concepts such as complex transfer function, poles, frequency and transient responses, trade-off of gain for bandwidth and linearity, feed-forward zero, and stability criteria of feedback systems are essential to understand, design, and operate amplifier circuits properly. Stability is a concern only in feedback amplifiers. If the feedback loop gain is increased, poles may move into the right-half complex s-plane, thereby causing instability. An operational amplifier (OPAMP) can be considered a device or a circuit block that amplifies a differential input with a very high DC gain. Most low-frequency feedback amplifier designs using opamps start with an assumption that opamps are ideal, and only the stability and finite bandwidth effects are considered later. Therefore, three-pole opamps are conditionally stable only with low loop gain and would be unstable for most usable feedback conditions.
