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Review of Second Order Electrical Circuits: Series RLC Circuit, Parallel RLC Circuit

This lecture reviews second order electrical circuits, specifically series RLC circuits and parallel RLC circuits. It covers topics such as the natural response of second order circuits, sinusoidal signals, and complex exponentials. Relevant educational materials are provided in Chapter 8.2 and 8.3.

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Review of Second Order Electrical Circuits: Series RLC Circuit, Parallel RLC Circuit

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  1. Lecture 21 Review: Second order electrical circuits Series RLC circuit Parallel RLC circuit Second order circuit natural response Sinusoidal signals and complex exponentials Related educational materials: Chapter 8.2, 8.3

  2. Summary: Series & parallel RLC circuits • Series RLC circuit: • Parallel RLC circuit

  3. Second order input-output equations • In general, the governing equation for a second order system can be written in the form: • Where •  is the damping ratio (  0) • n is the natural frequency (n  0)

  4. Solution of second order differential equations • The solution of the input-output equation is (still) the sum of the homogeneous and particular solutions: • We will consider the homogeneous solution first:

  5. Homogeneous solution (Natural response) • Assume form of solution: • Substituting into homogeneous differential equation: • We obtain two solutions:

  6. Homogeneous solution – continued • Natural response is a combination of the solutions: • So that: • We need two initial conditions to determine the two unknown constants: • ,

  7. Natural response – discussion •  and n are both non-negative numbers •   1  solution composed of decaying exponentials •  < 1  solution contains complex exponentials

  8. Sinusoidal functions • General form of sinusoidal function: • Where: • VP = zero-to-peak value (amplitude) •  = angular (or radian) frequency (radians/second) •  = phase angle (degrees or radians)

  9. Sinusoidal functions – graphical representation • T = period • f = frequency • cycles/sec (Hertz, Hz) •  = phase • Negative phase shifts sinusoid right

  10. Complex numbers • Complex numbers have real and imaginary parts: • Where:

  11. Complex numbers – Polar coordinates • Our previous plot was in rectangular coordinates • In polar coordinates: • Where:

  12. Complex exponentials • Polar coordinates are often expressed as complex exponentials • Where

  13. Sinusoids and complex exponentials • Euler’s Identity:

  14. Sinusoids and complex exponentials – continued • Unit vector rotating in complex plane: • So

  15. Complex exponentials – summary • Complex exponentials can be used to represent sinusoidal signals • Analysis is (nearly always) simpler with complex exponentials than with sines, cosines • Alternate form of Euler’s identity: • Cosines, sines can be represented by complex exponentials

  16. Second order system natural response • Now we can interpret our previous result

  17. Classifying second order system responses • Second order systems are classified by their damping ratio: •  > 1  System is overdamped (the response consists of decaying exponentials, may decay slowly if  is large) •  < 1  System is underdamped (the response will oscillate) •  = 1  System is critically damped (the response consists of decaying exponentials, but is “faster” than any overdamped response)

  18. Note on underdamped system response • The frequency of the oscillations is set by the damped natural frequency, d

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