1. Week 5a

2. Lecture 5a Review: Types of Circuit Excitation
Why Sinusoidal Excitation?
Phasors
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3. Types of Circuit Excitation
Linear Time-
Invariant
Circuit
Linear Time-
Invariant
Circuit
Steady-State Excitation OR
Linear Time-
Invariant
Circuit
Linear Time-
Invariant
Circuit
Digital
Pulse
Source
Sinusoidal (Single-
Frequency) Excitation
Transient Excitation
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4. Why is Sinusoidal Single-Frequency Excitation Important?
Some circuits are driven by a single-frequency
sinusoidal source.
Example: The electric power system at frequency of
60+/-0.1 Hz in U. S. Voltage is a sinusoidal function of time because it is produced by huge rotating generators powered by mechanical energy source such as steam (produced by heat from natural gas, fuel oil, coal or nuclear fission) or by falling water from a dam (hydroelectric).
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5. Bonneville Dam (Columbia River) Where Much
of California’s Electric Power Comes From
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6. Turbine-generator sets at Bonneville Dam
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7. Where 3-Phase Electricity Comes From
Generator
driven by
falling
water has
3 separate
coils
Direct current
in the rotor
(rotating coil)
produces a
magnetic field
that generates
currents in
stationary coils
A, B and C
Output voltages
from the 3 coils
(they leave the
generating plant
on 3 separate
cables)
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8. Why Sinusoidal Excitation? (continued)
Some circuits are driven by sinusoidal sources whose frequency changes slowly over time.
Example: Music reproduction system (different notes).
And, you can express any periodic electrical signal as a
sum of single-frequency sinusoids – so you can
analyze the response of the (linear, time-invariant)
circuit to each individual frequency component and
then sum the responses to get the total response.
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9. Representing a Square Wave as a Sum of Sinusoids
Square wave with 1-second period. (b) Fundamental compo-
nent (dotted) with 1-second period, third-harmonic (solid black)
with1/3-second period, and their sum (blue). (c) Sum of first ten
components. (d) Spectrum with 20 terms.
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10. Single-frequency sinusoidal-excitation AC circuit problems
The technique we’ll show works on circuits composed of linear
elements (R, C, L) that don’t change with time  “linear
time-invariant circuits”.
The circuit is driven with independent voltage and/or current
sources whose voltages or currents vary at a single frequency, f,
measured in Hertz (abbreviated Hz) this is the number of cycles the
voltages or currents execute per second. We can represent the
source voltages or currents as functions of time as
v(t) = V0cos(wt) or i(t) = I0cos(wt),
where w = 2pf is the angular frequency in radians per second.
Example: In the U. S. the AC power frequency, f, is 60 Hz and the peak voltage V0 is 170 V, so w = 377 radians/s and
v(t) = 170cos(377t) V. More generally, we might have sources
v(t) = V0sin(wt) or i(t) = I0cos(wt = f), where f is a phase angle.
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11. We could solve our circuit equations using such functions of time, but we’d have to do a lot of tedious trigonometric transformations. Instead we use a mathematical trick to eliminate time dependence from our equations!
The trick is based on a fundamental fact about linear, time-invariant circuits excited with sinusoidal sources: the frequencies of all the voltages and currents in the circuit are identical.
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12. RULE: “Sinusoid in”-- “Same-frequency sinusoid out” is true for linear time-invariant circuits. (The term “sinusoid” is intended to include both sine and cosine functions of time.)
Intuition: Think of sinusoidal excitation (vibration) of a linear mechanical system – every part vibrates at the same frequency, even though perhaps at different phases.
Circuit of linear elements
(R, L, C)
Excitation:
Output:
vS(t) = VScos(wt + f)
Iout(t) = I0cost(wt + a)
Given ?
SAME
SAME w
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13. PHASORS You can solve AC circuit analysis problems that involve
PHASORS You can solve AC circuit analysis problems that involve Circuits with linear elements (R, C, L) plus independent and dependent voltage and/or current sources operating at a single angular frequency w = 2pf (radians/s) such as v(t) = V0cos(wt) or i(t) = I0cos(wt) By using any of Ohm’s Law, KVL and KCL equations, doing superposition analysis, nodal analysis or mesh analysis, AND Using instead of the terms below on the left (general excitation), the terms below on the right (sinusoidal excitation):
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14. Resistor I-V relationship General excitation Sinusoidal excitation
vR = iRR VR = IRR where R is the resistance in ohms,
  VR = phasor voltage across the resistor,
IR = phasor current through the resistor,
and boldface indicates complex quantity.
Capacitor I-V relationship
iC = CdvC/dt IC = VC / ZC where IC = phasor current
through the capacitor, VC = phasor voltage
across the capacitor, the capacitive
impedance ZC in ohms is ZC = 1/jwC ,
j = (-1)1/2, and boldface capital letters are
complex quantities.
(Note: EE’s use j for (-1)1/2 instead of i,
since i might suggest current)
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15. Inductor I-V relationship General excitation Sinusoidal excitation
vL = LdiL/dt VL = IL ZL where VL is the phasor voltage
across the inductor, IL is the phasor
current through the inductor, the
inductive impedance in ohms ZL is
ZL = jwL , j = (-1)1/2 and boldface capital
letters are complex quantities.
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16. Example 1
We’ll explain what phasor currents and voltages are shortly, but first let’s look at an example of using them:
Here’s a circuit containing an AC voltage source with angular frequency w, and a capacitor C. We represent the voltage source and the current that flows (in boldface print) as phasors VS and I -- whatever they are! + V S I C -
We can obtain a formal solution for the unknown current in this circuit by writing KVL: VS + IZC = 0
We can solve symbolically for I: I = VS/ZC = jwCVS
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17. for introducing phasor voltages and currents, and impedances!
Note that so far we haven’t had to include the variable of time in our equations -- no sin(wt), no cos(wt), etc. -- so our algebraic task has been almost trivial. This is the reason
for introducing phasor voltages and currents, and impedances!
In order to “reconstitute” our phasor currents and voltages to see what functions of time they represent, we use the rules below. Note that often (for example, when dealing with the gain of amplifiers or the frequency characteristics of filters), we may not even need to go back from the phasor domain to the time domain – just finding how the magnitudes of voltages and currents vary with frequency w may be the only information we want.
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18. Rules for “reconstituting” phasors (returning to the time domain)
Rule 1: Use the Euler relation for complex numbers:
ejf = cos(f) + jsin(f), where j = (-1)1/2
Rule 2: To obtain the actual current or voltage i(t) or v(t)
as a function of time
1. Multiply the phasor I or V by ejwt, and
2. Take the real part of the product
For example, if I = 3 amps, a real quantity, then
i(t) = Re[Iejwt] = Re[3ejwt] = 3cos(wt) amps where Re
means “take the real part of”
imaginary
real j 1 f
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19. Rule 3: If a phasor current or voltage I or V is not purely real but is complex, then multiply it by ejwt and take the real part of the product.
For example, if V = V0ejf, then v(t) = Re[Vejwt] = Re[V0ejfejwt] = Re[V0ej(wt + f)] = V0cos(wt + f)
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20. Finishing Example 1 +
Apply this approach to the capacitor circuit above, where the voltage source has the value vS(t) = 4 cos(wt) volts. The phasor voltage VS is then purely real: VS = 4. The phasor current is I = VS/ZC = jwCVS = (wC)VSejp/2, where we use the fact that j = (-1)1/2 = ejp/2; thus, the current in a capacitor leads the capacitor voltage by p/2 radians (90o). Note: Often (especially in this class) we may not care about the phase angle, and will focus just on the amplitude of the voltage or current that we obtain. This will be particularly true of filters and amplifiers.
vS(t) = 4 cos(wt)
i(t) C -
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21. The actual current that flows as a function of time, i(t),
is obtained by substituting VS = 4 into the equation for I above,
multiplying by ejwt, and taking the real part of the product.
i(t) = Re[j (wC) x 4ejwt] = Re[4(wC)ej(wt + p/2)]
i(t) = 4(wC)cos(wt + p/2) amperes
Note: We obtained the current as a function of time (the
current waveform) without ever having to work with
trigonometric identities!
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22. Analysis of an RC Filter
Consider the circuit shown below. We want to use phasors and complex impedances to find how the ratio |Vout/Vin| varies as the frequency of the input sinusoidal source changes. This circuit is a filter; how does it treat the low frequencies and the high frequencies? + + R V V C
out in I - -
Assume the input voltage is vin(t) = Vincos(wt) and represent
it by the phasor Vin. A phasor current I flows clockwise in the
circuit.
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23. Write KVL: -Vin + IR +IZC = 0 = -Vin + I(R + ZC)
The phasor current is thus I = Vin/(R + ZC)
The phasor output voltage is Vout = I ZC.
Thus Vout = Vin[ZC /(R + ZC)]
If we are only interested in the dependence upon frequency
of the magnitude of (Vout / Vin) we can write
| Vout / Vin | = |ZC/(R + ZC)| = 1/|1 + R/ ZC |
Substituting for ZC, we have 1 + R/ ZC = 1 + jwRC,
whose magnitude is Thus,
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24. Explore the Result
If wRC << 1 (low frequency) then | Vout / Vin | = 1
If wRC >> 1 (high frequency) then | Vout / Vin | ~ 1/wRC
If we plot | Vout / Vin | vs. wRC we obtain roughly the plot
below, which was plotted on a log-log plot:
The plot shows that this is a low-pass filter. Its cutoff frequency is at the frequency w for which wRC = 1.
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25. Notice that we’ve obtained a lot of information about how this
particular RC circuit performs just by looking at the magnitude
of the ratio of phasor output voltage to phasor input voltage
(i.e., we haven’t had to study the phase angles associted with
those phasor voltages. (In more detailed studies the phase angles
can be important – but not in this course.)
Does this behavior make sense from what we know about
capacitors? YES!
At low frequency a capacitor is like an open circuit –
so the output voltage would equal the input voltage
At high frequency a capacitor is like a short circuit –
so the output voltage would be very small.
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26. Here is a useful web site to explore:
You’ll find some demonstrations dealing with phasors
and impedances there.
And Appendix A in Hambley is a review of complex numbers.
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27. Why Does the Phasor Approach Work?
Phasors are discussed at length in your text (Hambley 3rd Ed., pp ) with an interpretation that sinusoids can be visualized as the real axis projection of vectors rotating in the complex plane, as in Fig This is the most basic connection between sinusoids and phasors.
We present phasors as a convenient tool for analysis of linear time-invariant circuits with a sinusoidal excitation. The basic reason for using them is that they eliminate the time dependence in such circuits, greatly simplifying the analysis.
Your text discusses complex impedances in Sec. 5.3, and circuit analysis with phasors and complex impedances in Sec Skim over this LIGHTLY.
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28. Motivations for Including Phasors in EECS 40
It enables us to include a lab where you measure the behavior of RC filters as a function of frequency, and use LabVIEW to automate that measurement.
It enables us to (probably) include a nice operational amplifier lab project near the end of the course to make an “active” filter (the RC filter is passive).
It enables you to find out what impedances are and use them as real EEs do.
The subject was also supposedly included (in a way) in EECS 20 which some of you may have taken.
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