tailieunhanh - Electric Circuits, 9th Edition P56

Electric Circuits, 9th Edition P56. Designed for use in a one or two-semester Introductory Circuit Analysis or Circuit Theory Course taught in Electrical or Computer Engineering Departments. Electric Circuits 9/e is the most widely used introductory circuits textbook of the past 25 years. As this book has evolved over the years to meet the changing learning styles of students, importantly, the underlying teaching approaches and philosophies remain unchanged. | 526 Introduction to Frequency Selective Circuits elements resistors capacitors and inductors. The largest output amplitude such filters can achieve is usually 1 and placing an impedance in series with the source or in parallel with the load will decrease this amplitude. Because many practical filter applications require increasing the amplitude of the output passive filters have some significant disadvantages. The only passive filter described in this chapter that can amplify its output is the series RLC resonant filter. A much greater selection of amplifying filters is found among the active filter circuits the subject of Chapter 15. Low-Pass Filters Here we examine two circuits that behave as low-pass filters the series RL circuit and the series RC circuit and discover what characteristics of these circuits determine the cutoff frequency. Figure a A series RL low-pass filter b The equivalent circuit at w 0. and c The equivalent circuit at w oo. The Series RL Circuit Qualitative Analysis A scries RL circuit is shown in Fig. a . The circuit s input is a sinusoidal voltage source with varying frequency. The circuit s output is defined as the voltage across the resistor. Suppose the frequency of the source starts very low and increases gradually. We know that the behavior of the ideal resistor will not change because its impedance is independent of frequency. But consider how the behavior of the inductor changes. Recall that the impedance of an inductor is ja L. At low frequencies the inductor s impedance is very small compared with the resistor s impedance and the inductor effectively functions as a short circuit. The term low frequencies thus refers to any frequencies for which ojL R. The equivalent circuit for a 0 is shown in Fig. b . In this equivalent circuit the output voltage and the input voltage are equal both in magnitude and in phase angle. As the frequency increases the impedance of the inductor increases relative to that of the .