Operational Amplifiers and Comparators
Robert B. Northrop · 2012
The operational amplifier (op amp) had its origins back in the 1940-1960 era of electronics where its principal use was as an active element in analog computer systems. Early op amp circuits used vacuum tubes; op amps were used as summers, subtractors, integrators, filters, etc. in analog computer systems designed to model dynamic, electromechanical feedback systems such as autopilots, bombsights, and motor speed controls. With the rise of digital computers in the 1960s, analog computers were largely replaced for simulations by digital computers using software numerical methods such as IBM’s CSMP™, also Tutsim™, Simnon™, MATLAB™, etc. However, electrical engineers found that the basic linear and nonlinear op amp building blocks could be used for analog signal conditioning in other systems. With the advent of semiconductors, op amps were designed with transistors as gain elements, instead of bulky vacuum tubes. The next step, of course, was to design integrated circuit (IC) op amps, one of the first of which was the venerable (but now obsolete) LM741. (Why, you might ask, is the LM741 obsolete? Newer op amps with better gain-bandwidth products have lower DC bias currents (IB and IB′), lower DC offset voltages (VOS) and VOS tempco, lower noise, higher DC gains, higher CMRRs, higher Zins, etc., and are often less expensive.) At present, operational amplifiers (op amps) are used for a wide spectrum of linear biomedical signal conditioning applications, including differential instrumentation amplifiers, electrometer amplifiers, low-drift DC signal conditioning, active filters, summers, etc. Nonlinear op amp applications have expanded to include true-RMS to DC conversion, precision rectifiers, phase-sensitive rectifiers, peak detectors, log converters, etc.