Abstract An approach to the analysis of composite amplifiers is presented. It is based on programmes written in MATLAB that take advantage of the plotting and graphic user interfaces capabilities of the software package. This approach provides better results than simulator-based ones, because the behaviour of the circuits is explored iteratively.

Keywords composite amplifiers; frequency response; graphical user interfaces; stability analysis

All textbooks devoted to analogue design at the undergraduate level present the frequency response through the well-known Bode diagrams. Usually, the theoretical background and the procedures for drawing the diagrams are well presented.1 When dealing with amplifying stages built with operational amplifiers (op-amps), however, a major drawback of most books is that only simple cases, with a single op-amp, are considered. At best, the behaviour of cascaded stages is illustrated.2,3 As a result, students are not well prepared to deal with more complex topics, such as stability and compensation, or with circuits that include more than one op-amp in the same feedback loop.

On the other hand, the SPICE simulator has been widely used as a teaching aid and is quite successful in tasks such as quickly testing a design.4 This simulator, however, is not the best choice when it is desired to gain a clear understanding of the behaviour of a circuit. A case in point is the response of an unstable amplifier: the resulting Bode diagram will be correct, but information related to the causes of the instability will be lacking. Also, when it is desired to explore the effect of varying the value of one element (say, a resistor in the feedback network), the best option available is the parametric analysis, but interpreting the results can be confusing and time consuming.
Mathematical programs have long been used in control theory. Their main advantage is the capability to draw complex graphs. Since classic control theory evolved from stability analysis in early amplifiers, it seems natural to use control-oriented programmes to illustrate analogue design topics. The purpose of the paper is to present an approach to the analysis of amplifying stages with complex feedback networks,5 using the MATLAB program, and taking advantage of the graphic user interface (GUI) capabilities. This approach provides a better understanding than those based on simulators.

Composite amplifier with two op-amps

Conclusions

The analysis of two complex amplifier configurations is presented. Since the behaviour of the circuits is not apparent from the transfer functions, two programmes written in MATLAB are used as a learning tool. They take advantage of the graphic user interface capabilities of the software package, and provide a clear understanding of the relation between the frequency response and the main circuit parameters. It has been found that this approach provides better results that using circuit simulators alone. The Bode diagrams are quickly updated when any of the sliders is moved and the behaviour of the circuits can be explored iteratively. The screen layout design can be complicated; however, a special design tool included with the software package simplifies the process. Once the theoretical principles involved are understood, the amplifiers can be tested in a circuit simulator or, better still, in the lab.