Chapter 6 - Operational Amplifiers

Abstract

Operational amplifier (op amp) can be applied in several fields, including signal processing, filtering, switching circuitry, instrumentation, etc. An op amp is designed to handle two inputs at the same time. The output signal will be an amplified version of the difference between the two inputs. By applying negative feedback to the op amp, one can obtain a moderate-gain amplifier that is very stable and responds to frequencies much higher than a megahertz. There are two rules that govern the design of op amp circuits. The first is that the two input terminals of an op amp in any circuit are at the same voltage, and the second is that no current flows into either of the two input terminals. Op amp has finite bandwidth, which can be resolved to some extent by applying negative feedback–the more feedback, the larger the bandwidth, but the gain is proportionately reduced. Another limitation of op amps is the presence of a small voltage, called the input offset voltage, which due to imbalances in the internal op amp circuitry is always present. A further limitation is the slowed response of an op amp to a quickly changing input. A final limitation is the common-mode rejection ratio (CMRR) in differential amplifiers, which are amplifiers that provide a high degree of discrimination against common mode (interfering) signals while amplifying a differential signal.