ELEN-325. Introduction to Electronic Circuits: A Design Approach

III.1. Basic Model for the Operational Amplifier. The OPerational AMPlifier (OPAMP) is a key building block in analog integrated circuit design. The OPAMP is composed by several transistors and passive elements (resistors and capacitors) and arranged such that its low frequency voltage gain is very high; the dc gain of the OPAMP-741 is around 10 V/V (10 μV at the input give us 1 V at the output). The design of such complex circuit is discussed in chapter 6; here we will use a simplified linear macromodel to analyze the principles of OPAMP based circuits and their operation. Several circuits are studied such as basic amplifiers, first order and second order filters and some non-conventional applications. The versatility of the OPAMP will be evident at the end of this chapter. To define the fundamental parameters of a system, let us consider a linear two-port system with two terminals grounded, as the one shown in Figure 3.1. There are 4 variables vi, ii, vo and io to be studied. The interaction between the four variables can be defined in many different ways, depending on the definition of the dependent and independent variables; in real circuits these definitions depend on the input variable (current or voltage) and the most relevant output variable. Usually in voltage amplifiers the input signal is defined as vi while the output is vo.