Chapter 3 - Development of the Ideal Op Amp Equations

Abstract

This chapter discusses some ideal op amp equations. There is no such thing as an ideal op amp, but present day op amps come so close to ideal that Ideal Op Amp analysis approaches actual analysis. Op amps depart from the ideal in two ways. First, dc parameters such as input offset voltage are large enough to cause departure from the ideal, which assumes that input offset voltage is zero. Second, ac parameters such as gain are a function of frequency, so they go from large values at dc to small values at high frequencies. Although the ideal op amp analysis makes use of perfect parameters, the analysis is often valid because some op amps approach perfection. In addition, when working at low frequencies, several kHz, the ideal op amp analysis produces accurate answers. Several assumptions have to be made before the ideal op amp analysis can proceed. First, it has to be assumed that the current flow into the input leads of the op amp is zero. Second, the op amp gain is assumed to be infinite, hence it drives the output voltage to any value to satisfy the input conditions. Also, implicit in the infinite gain assumption is the need for zero input signal. Finally, one has to assume that the output impedance of the ideal op amp is zero.