4 - Signal processing with operational amplifiers

Abstract

Many of the processes of signal processing are based on the use of an op amp. These processes include addition of two or more signals, multiplication, integration, precise rectification, and active filtering. This chapter deals with instrumentation amplifiers, inverting and non-inverting summing amplifiers, integrators, charge amplifiers, and precision rectifiers. The specification of an instrumentation amplifier includes a high input impedance at both inverting and non-inverting inputs, with a well-defined gain set by negative feedback. This is impossible to achieve with one op amp, because the virtual earth at the inverting input of the op amp makes the input impedance of the circuit equal to the value of the input resistor. The circuit of an integrator performs the mathematical function of integration, with respect to time, of its input voltage. The ideal circuit would use just R1 and the feedback capacitor C. In a practical circuit, Rf has to be included to provide dc negative feedback to set the output voltage to zero in the absence of an input signal. RF is usually made very large, and one can ignore it initially to keep the analysis simple. The voltages vi and vo represent the instantaneous values of the input and output voltages.