One of the most important functions in electronic instrumentation is that of amplification (ideal op amp characteristics ).The need to amplify low-level electric signals arises frequently in a number of applications. Perhaps the most familiar use of amplifiers arises in converting the low-voltage signal from a cassette tape player, a radio receiver, or a compact disk player to a level suitable for driving a pair of speakers. below Figure depicts a typical arrangement. Amplifiers have a number of applications of interest to the non–electrical engineer, such as the amplification of low-power signals from transducers (e.g., bioelectrodes, strain gauges, thermistors, and accelerometers) and other, less obvious functions that will be reviewed in this chapter, for example, filtering and impedance isolation.We turn first to the general features and characteristics of amplifiers, before delving into the analysis of the operational amplifier.
Ideal Amplifier Characteristics
The simplest model for an ideal op amp characteristics is depicted in above Figure , where a signal vS (t) is shown being amplified by a constant factor A, called the gain of the amplifier. Ideally, the load voltage should be given by the expression
Note that the source has been modeled as a Thévenin equivalent, and the load as an equivalent resistance. Thévenin’s theorem guarantees that this picture can be representative of more complex circuits. Hence, the equivalent source circuit is the circuit the amplifier “sees” from its input port; and RL, the load, is the equivalent resistance seen from the output port of the amplifier.
What would happen if the roles were reversed? That is, what does the source see when it “looks” into the input port of the amplifier, and what does the load see when it “looks” into the output port of the amplifier? While it is not clear at this point how one might characterize the internal circuitry of an amplifier (which is rather complex), it can be presumed that the amplifier will act as an equivalent load with respect to the source and as an equivalent source with respect to the load. After all, this is a direct application of Thévenin’s theorem. above Figure provides a pictorial representation of this simplified characterization of an amplifier. so that the input voltage vin is given by
The equivalent input voltage seen by the amplifier is then amplified by a constant factor A. This is represented by the controlled voltage source Avin. The controlled source appears in series with an internal resistor Rout, denoting the internal (output) resistance of the amplifier. Thus, the voltage presented to the load is
or, by substituting the equation for vin,
In other words, the load voltage is an amplified version of the source voltage.
Unfortunately, the amplification factor is now dependent on both the source and load impedances, and on the input and output resistance of the amplifier. Thus, a given amplifier would perform differently with different loads or sources. What are the desirable characteristics for a voltage amplifier that would make its performance relatively independent of source and load impedances? Consider, once again, the expression for vin. If the input resistance of the amplifier Rin were very large, the source voltage vS and the input voltage vin would be approximately equal:
By an analogous argument, it can also be seen that the desired output resistance for the amplifier Rout should be very small, since for an amplifier with Rout = 0, the load voltage would be