1. Sinusoidal Functions, Complex Numbers, and Phasors
Discussion D9.2
Sections 4-2, 4-3
Appendix A
3/26/2006

2. Sinusoids
A sinusoid is a signal that has the form of the sine or cosine function. XM XM
x(t)
x(t)

3. Sinusoids
In general:
Note trig identities:

4. A sinusoidal current or voltage is usually referred to as an alternating current (or AC) or voltage and circuits excited by sinusoids are called ac circuits. Sinusoids are important for several reasons:
Nature itself is characteristically sinusoidal (the pendulum, waves, etc.).
A sinusoidal current or voltage is easy to generate.
Through Fourier Analysis, any practical periodic signal (one that repeats itself with a period T) can be represented by a sum of sinusoids.
A sinusoid is easy to handle mathematically; its derivative and integral are also sinusoids.
When a sinusoidal source is applied to a linear circuit, the steady-state response is also sinusoidal, and we call the response the sinusoidal steady-state response.

5. Complex Numbers What is the solution of X2 = -1 imaginary
Complex Plane
Note: j
real -1 1 -j

6. Complex Numbers Complex imag Plane A j real Euler's equation:
j measured positive
counter-clockwise
Note:

7. Sinusoidal Functions and Phasors

8. Relationship between sin and cos
Note that or

9. Relationship between sin and cos

10. Comparing Sinusoids
Note: positive angles are counter-clockwise

11. KVL - This is a differential equation we must solve for i(t). How?VR VL + -
This is a differential equation we must solve for i(t).
How?
Guess a solution and try it!
Note that
It turns out to be easier to use as the forcing function rather than and then take the real part of the solution.
This is because
which will allow us to convert the differential equation to an algebraic equation. Let's see how.

12. Solve the differential equation for i(t).VR VL + -
Instead, solve
(1)
and take the real part of the solution
Guess that
and substitute in (1)
Divide by

13. Solve the differential equation for i(t).VR VL + -
(1)
(2)
(3)
(4)
Rearrange (3)
Recall that
Therefore (4) can be written as
(5)

14. Solve the differential equation for i(t).VR VL + -
(1)
(2)
(5)
Therefore, from (5)
(6)
Substituting (6) in (2) and taking the real part

15. Phasors
A phasor is a complex number that represents the amplitude and phase of a sinusoid. XM j
Recall that when we substituted
in the differential equations, the
cancelled out.
We are therefore left with just the phasors

16. Solve the differential equation for i(t) using phasors.VR VL + -
(1)
(2)
(3)
Substitute (2) and (3) in (1)
(4)
Divide by and solve for I
(5)

17. By using phasors we have transformed the problem from solving a set of differential equations in the time domain to solving a set of algebraic equations in the frequency domain.
The phasor solutions are then transformed back to the time domain.

18. Impedance Impedance Units = ohms -VR VL + -
Note that impedance is a complex number containing a real, or resistive component, and an imaginary, or reactive, component.

19. Admittance Admittance Units = siemens - conductance susceptance + VRVL + -
conductance
susceptance

20. Capacitor Circuit (1) - (2) (3) Substitute (2) and (3) in (1) (4)VR VC + -
(1)
(2)
(3)
Substitute (2) and (3) in (1)
(4)
Divide by and solve for I
(5)

21. Impedance
Impedance VR VC + -

22. I in phase with V I lags V I leads V Im V V I I Re Im V I V Re I Im V

23. Expressing Kirchoff’s Laws in the Frequency Domain
KVL: Let v1, v2,…vn be the voltages around a closed loop.
KVL tells us that:
Assuming the circuit is operating in sinusoidal steady-state at frequency  we have: or
Phasor
Since
Which demonstrates that KVL holds for phasor voltages.

24. KCL: Following the same approach as for KVL, we can show that
Where Ik is the phasor associated with the kth current entering a closed surface in the circuit.
Thus, both KVL and KCL hold when working with phasors in circuits operating in sinusoidal steady-state. This implies that all of the circuit analysis methods (mesh and nodal analysis, source transformations, voltage & current division, Thevenin equivalent, combining elements, etc,) work in the same way we found for resistive circuits. The only difference is that we must work with phasor currents & voltages and the impedances &/or admittances of the elements.

25. Find Zin

27. Impedances in series add like resistors in series
We see that if we replace Z by R the impedances add like resistances.
Impedances in series add like resistors in series
Impedances in parallel add like resistors in parallel

28. Voltage Division
But
Therefore