EGR 2201 Unit 5
Linearity, Superposition, & Source
Transformation
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Read Alexander & Sadiku, Sections 4.1 to 4.4.
Homework #5 and Lab #5 due next week.
Quiz next week.

Techniques That Can Simplify
Circuit Analysis
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Nodal analysis and mesh analysis are powerful tools for analyzing circuits.
But you should also know other techniques that may simplify the equations that you get when you perform nodal or mesh analysis, or may save you from having to perform nodal or mesh analysis at all.

Example of a Simplifying
Technique
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One such technique is our method of finding the equivalent resistance of several series-parallel resistors.
Example: In this circuit, if you want to find i, using this technique will get you the answer much faster than doing a full nodal or mesh analysis of the circuit.

Other Useful Techniques
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The current-divider rule and voltagedivider rule are two other useful tools that can save you work when analyzing circuits.
Chapter 4 presents several new techniques:
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Linearity
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Superposition
Source transformation
Thevenin’s theorem
Norton’s theorem
Maximum-power-transfer theorem

Linear Circuits
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A linear circuit is a circuit whose output is linearly related (or directly proportional) to its input.
Typically the input is a voltage source or current source, and the output is a resistor’s voltage or current.
So, in a linear circuit, if the input voltage or current increases by a factor of k, then the output voltage or current must increase by that same factor.

Why Linearity is Useful
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Linearity is a useful property because it lets us easily answer certain “whatif” questions about linear circuits.
Example: Suppose we know that in the linear circuit shown, i = 2 A when v s
= 10 V.
Then we know that doubling v s to 20 V will also double
i to 4 A.

How to Tell if a Circuit is Linear
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Any circuit that contains only the following elements is a linear circuit:
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Resistors
Capacitors
Inductors
Independent sources
Linear dependent sources
All of the circuits that we will study in this course are linear circuits.
In other courses you will study nonlinear circuits.

Caution: Power Is Not Related
Linearly to Voltage and Current
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Recall that a resistor’s power is non-
linearly related to its current and voltage by the equations
p = i 2  R p = v 2  R
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Therefore, for example, doubling v the circuit
R’s power.
s shown will
not double in

Techniques That Can Simplify
Circuit Analysis

Chapter 4 presents several new techniques:






Linearity
Superposition
Source transformation
Thevenin’s theorem
Norton’s theorem
Maximum-power-transfer theorem

(1 of 2)
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The superposition principle says that to find the total effect of two or more independent sources in a linear circuit, you can find the effect of each source acting alone, and then combine those effects.

(2 of 2)
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To find the effect of an independent source acting alone, you must turn off all the other independent sources.
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To turn off a voltage source, replace it by a short circuit.
To turn off a current source, replace it by an open circuit.

Steps in Using the Superposition
Principle
1.
Select one independent source and turn off all the others, as explained above. Find the desired voltage or current due to this source using techniques from earlier chapters.
2.
Repeat step 1 for each of the other independent sources.
3.
Add the values obtained from the steps above, paying careful attention to the signs (+ or  ) of each value.

Example: Using Superposition
(1 of 2)
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Suppose we wish to find v in the circuit shown.
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With the voltage source acting alone:
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With the current source acting alone:

Example: Using Superposition
(2 of 2)
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In this circuit, use voltage-divider rule to find v
1
= 2 V.
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In this circuit, use current-divider rule to find i
3
= 2 A, Ohm’s law to find v
2
= 8 V.
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Therefore, in the original circuit,
v = 2 V + 8 V = 10 V.

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For this circuit, you might argue that nodal (or mesh) analysis is easier than superposition.
And you might be right.
But looking ahead to the second half of this course, if one source were a
DC source and the other were an AC source, we would have no choice but to use superposition.

Techniques That Can Simplify
Circuit Analysis

Chapter 4 presents several new techniques:


Linearity
Superposition




Source transformation
Thevenin’s theorem
Norton’s theorem
Maximum-power-transfer theorem

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Before we look at source transformation, note the following:
Just as we can combine series or parallel resistors into a single equivalent resistor, we can sometimes combine sources into a single equivalent source.
In particular:
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We can combine series voltage sources into an equivalent voltage source.
We can combine parallel current
sources into an equivalent current source.

Series-Aiding Voltage Sources
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If two series-connected voltage sources produce current in the same direction, they are said to be series-aiding.
The net effect on the circuit is the same as that of a single source whose voltage equals the sum of the two voltage sources.
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Example: Most flashlights have two 1.5-V batteries in series to give 3 V.

Series-Opposing Voltage Sources
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If two series-connected sources produce current in opposite directions, they’re said to be series-
opposing.
The effect is the same as that of a single source equal in magnitude to the difference between the source voltages and having the same polarity as the larger of the two.

Parallel Voltage Sources?
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Although there are exceptions, in general we don’t connect voltage sources in parallel with each other.
To see why not, think about this: what is the voltage between points a and b in the circuit below?

Parallel Current Sources
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Current sources can be connected in parallel. The parallel-connected sources can be replaced by a single equivalent current source that produces a current equal to the algebraic sum of the sources.
Case 1: Both sources push current in same direction through rest of circuit.
Case 2: Sources push current in opposite directions through rest of circuit.

Series Current Sources?
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In general we don’t connect current sources in series with each other.
To see why not, think about this: how much current flows around the loop in the circuit below?

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Sometimes when you're analyzing a circuit, it can be useful to substitute a voltage source (and a resistor in series with it) by a current source (and a resistor in parallel with it), or vice versa.
Either substitution is called a source
transformation.
Two questions:
1.
2.
Why would this be useful?
Are we allowed to do this?

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Suppose we wish to find v o in the circuit to the right.
This is not difficult, but it requires a bit of work.
We may get the answer more easily if we can first replace the current-source-andparallel-resistor with a voltage-source-and-series-resistor.

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Yes, we’re allowed to make this substitution, as long as we use the right values for the elements.
The resistor R in series with the voltage source must be the same size as the one in parallel with the current source.
The current source’s and voltage source’s values must satisfy the following: 𝑣 𝑠 𝑣 𝑠
= 𝑖 𝑠
𝑅 or 𝑖 𝑠
=
𝑅

(1 of 2)
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In some cases we can use source transformation repeatedly to convert a complicated problem into a much simpler one.
The book’s Example 4.6 shows such a case:
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Original circuit (Our goal is to find v o
):
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After transforming both sources:
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Continued on next slide…

(2 of 2)
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…From previous slide:
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After combining
4 with 2 and transforming 12-V source:
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After combining
6 with 3 and combining
2-A with 4-A: