Analysis Methods Overview
• Solving Linear Equations
• Nodal Analysis
• Supernodes (Nodal Analysis with Voltage Sources)
• Mesh Analysis
• Supermeshes (Mesh Analysis with Current Sources)
• Introduction to BJT Transistors
This is a very important chapter.
Portland State University ECE 221 Analysis Methods Ver. 1.66 1
Review of Basic Concepts: Current
i4
i5
i3
i2
i1
• What goes in, has to come out
• Kirchhoff’s current law
• Similar to conservation of mass
• Conservation of electrons
Portland State University ECE 221 Analysis Methods Ver. 1.66 2
Review of Basic Concepts: Voltage
10 V-
+
-
++ - + -
2 kΩ2 kΩ
5 kΩ 7 kΩv1 v2
v3 v4
• The voltage drop from one node to another is the same, nomatter what path is chosen
• Kirchhoff’s voltage law
Portland State University ECE 221 Analysis Methods Ver. 1.66 3
Resistors in Parallel with Voltage Sources
CircuitRVs vo
-
+
CircuitVs vo
-
+
• What is vo in each case?
• What effect does the resistor have on the current pumped into thecircuit?
Portland State University ECE 221 Analysis Methods Ver. 1.66 4
Resistors in Series with Current Sources
CircuitIs CircuitIs
Rio io
• What is io in each case?
• What effect does the resistor have on the voltage seen by thecircuit?
Portland State University ECE 221 Analysis Methods Ver. 1.66 5
Network Terminology
Planar Circuit A circuit that can be drawn on a plane with nocrossing branches
Node Point or portion of a circuit where 2 or more elements arejoined
Essential Node Point or portion of a circuit where 3 or moreelements are joined
Branch Path that connects 2 nodes
Essential Branch Path that connects 2 essential nodes w/o passingthrough an essential node
Loop Path with last node same as starting node that does not crossitself
Mesh Loop that does not enclose any other loops
Note: this isn’t in the text.
Portland State University ECE 221 Analysis Methods Ver. 1.66 6
Example 1: Terminology
20 V 2 A
R1 R2
R3 R4 R4
R6 R7 R8
35ip
ip
Identify the following informationNodes: Essential Nodes:Branches: Essential Branches:EB’s with Unknown Current: Meshes:
Portland State University ECE 221 Analysis Methods Ver. 1.66 7
Example 2: Circuit Analysis The Hard Way
10 V
i1 i3
i2 i4
2 mA
1 kΩ 2 kΩ
5 kΩ 10 kΩ
Can solve with KCL & KVL. Four unknowns.
Portland State University ECE 221 Analysis Methods Ver. 1.66 8
Solving Linear Equations
• Much of our circuit analysis will focus on finding a set of linearequations and solving these equations
• Need as many equations as there are unknowns
• Three possible approaches
– Algebra (elimination, substitution, etc.)
– Cramer’s rule
– Linear algebra
• Last is easiest and least susceptible to errors
• Requires use your scientific calculators
Portland State University ECE 221 Analysis Methods Ver. 1.66 9
Example 2: Solving Linear Equations
i1 = i2 + i3
i4 = i3 + 2m
10 = 1k i1 + 5k i2
5k i2 = 2k i3 + 10k i4
Rewrite so variables are in consistent order on left side and constantsare on the right side
i1 − i2 − i3 = 0− i3 + i4 = 2m
1k i1 + 5k i2 = 10+ 5k i2 − 2k i3 − 10k i4 = 0
Portland State University ECE 221 Analysis Methods Ver. 1.66 10
Example 2: Continued (1)
i1 − i2 − i3 = 0− i3 + i4 = 2m
1k i1 + 5k i2 = 10+ 5k i2 − 2k i3 − 10k i4 = 0
In Matrix form this becomes⎡⎢⎢⎣
1 −1 −1 00 0 −1 1
1k 5k 0 00 5k −2k −10k
⎤⎥⎥⎦
⎡⎢⎢⎣
i1i2i3i4
⎤⎥⎥⎦ =
⎡⎢⎢⎣
02m100
⎤⎥⎥⎦
or
Ai = b
Portland State University ECE 221 Analysis Methods Ver. 1.66 11
Example 2: Continued (2)
Ai = b where
A =
⎡⎢⎢⎣
1 −1 −1 00 0 −1 1
1k 5k 0 00 +5k −2k −10k
⎤⎥⎥⎦ i =
⎡⎢⎢⎣
i1i2i3i4
⎤⎥⎥⎦ b =
⎡⎢⎢⎣
02m100
⎤⎥⎥⎦
• Your calculator should be able to solve this directly
• You should only need to enter A and b
• Your calculator will return a vector i
• Simultaneously solves for all the unknown variables
• Much faster than Cramer’s rule or brute-force algrebra
• Read the manuals for your calculators
• This will save you time (homework & exams) and reduce errors
Portland State University ECE 221 Analysis Methods Ver. 1.66 12
Example 2: Continued (3)
Linear Equations:⎡⎢⎢⎣
1 −1 −1 00 0 −1 11k 5k 0 00 5k −2k −10k
⎤⎥⎥⎦
⎡⎢⎢⎣
i1i2i3i4
⎤⎥⎥⎦ =
⎡⎢⎢⎣
02m100
⎤⎥⎥⎦
Calculator should return:⎡⎢⎢⎣
i1i2i3i4
⎤⎥⎥⎦ =
⎡⎢⎢⎣
+0.909+1.818−0.909+1.091
⎤⎥⎥⎦ mA
Portland State University ECE 221 Analysis Methods Ver. 1.66 13
Nodal Analysis: Introduction
• There is an another way to solve for currents and voltages
– Easier
– More methodical
– Still based on Ohm’s law, KVL, & KCL
• Nodal analysis is one of two key methods
• Mesh analysis is the other
• We will discuss nodal analysis first
• Based on KCL
• Must understand terminology introduced earlier
• Use to solve for voltages
• All voltages have a common reference point
Portland State University ECE 221 Analysis Methods Ver. 1.66 14
Nodal Analysis: Step 1 – Identify Essential Nodes
10 V 2 mA
1 kΩ 2 kΩ
5 kΩ 10 kΩ
• Some essential nodes may include portions of the circuit (pieces ofwire)
• Circle the entire node to prevent errors
Portland State University ECE 221 Analysis Methods Ver. 1.66 15
Nodal Analysis: Step 2 – Pick a Reference
10 V 2 mA
1 kΩ 2 kΩ
5 kΩ 10 kΩ
• Second step is to pick a reference node
• Is often easiest to choose the node that interconnects the mostbranches
• Must be an essential node
• Usually is at bottom of circuit
• Label with the same symbol used for ground
Portland State University ECE 221 Analysis Methods Ver. 1.66 16
Nodal Analysis: Step 3 – Label Other Essential Nodes
10 V 2 mA
1 kΩ 2 kΩ
5 kΩ 10 kΩ
• Also a bit easier if voltages are labeled
• All voltages are measured relative to the reference node
Portland State University ECE 221 Analysis Methods Ver. 1.66 17
Nodal Analysis: Step 4 – Apply KCL All Labeled Nodes
10 V 2 mA
1 2
-
+v2
-
+v1
1 kΩ 2 kΩ
5 kΩ 10 kΩ
Portland State University ECE 221 Analysis Methods Ver. 1.66 18
Nodal Analysis: Step 5 – Solve Linear Equations
Linear Equations:
Solution (from calculator):
Portland State University ECE 221 Analysis Methods Ver. 1.66 19
Nodal Analysis: Step 6 – Solve for Variables of Interest
10 V 2 mA
1 2
-
+v2
-
+v1
i1 i3
i2 i4
1 kΩ 2 kΩ
5 kΩ 10 kΩ
i1 =i2 =i3 =i4 =
Portland State University ECE 221 Analysis Methods Ver. 1.66 20
Nodal Analysis: Review of Steps
• Step 1: Identify essential nodes
• Step 2: Pick a reference
– Must be an essential node
– Always label with the ground symbol
– Best to pick essential node with most branches
– Often at the bottom of the circuit diagram
• Step 3: Label other essential nodes
• Step 4: Apply KCL to all labelled nodes except reference node
• Step 5: Solve linear equations
– Generates voltage at each node (relative to reference node)
• Step 6: Solve for variables of interest
– Usually easy after Step 5
Portland State University ECE 221 Analysis Methods Ver. 1.66 21
Nodal Analysis: Use of Laws
• All three laws are used
• KCL is applied at each labelled node except the reference node
• Ohm’s law is used to determine the current in branches thatcontain resistors
• KVL is used to determine the voltage drop across the resistors
Portland State University ECE 221 Analysis Methods Ver. 1.66 22
Example 3: Nodal Analysis
144 V
-
+v2
-
+v1 3 A
4 Ω
5 Ω10 Ω
80 Ω
Solve for v1 and v2.
Portland State University ECE 221 Analysis Methods Ver. 1.66 23
Example 3: Workspace
Portland State University ECE 221 Analysis Methods Ver. 1.66 24
Example 4: Nodal Analysis
20 mA
-
+v2
-
+v1
-
+v3 5 V2 kΩ
2.7 kΩ2.7 kΩ
3.3 kΩ
4.7 kΩ
10 kΩ
Solve for v1, v2, and v3.
Portland State University ECE 221 Analysis Methods Ver. 1.66 25
Example 4: Workspace
Portland State University ECE 221 Analysis Methods Ver. 1.66 26
Example 5: Dependent Voltage Source
50 V-
+
10 Ω
10 Ω
30 Ω 39 Ω 78 Ω
v/5
v
Solve for v.
• What effect does the 10 Ω resistor have on the circuit?
• What is the current flowing through the dependent source?
• How can we apply KCL at the essential nodes without thisinformation?
• Ans: One extra variable
• Implies we need an extra equation
Portland State University ECE 221 Analysis Methods Ver. 1.66 27
Example 5: Continued
50 V-
+
10 Ω
10 Ω
30 Ω 39 Ω 78 Ω
v/5
v
Solve for v.
Portland State University ECE 221 Analysis Methods Ver. 1.66 28
Example 5: Workspace
Portland State University ECE 221 Analysis Methods Ver. 1.66 29
Nodal Analysis and Supernodes
• Supernodes eliminate the need to introduce an extra variable(unknown current)
• Necessary when a voltage source is between two labeled nodes(excluding reference node)
• Still need to use voltage source to generate one of the equations
Portland State University ECE 221 Analysis Methods Ver. 1.66 30
Example 6: Dependent Source Continued
50 V-
+
10 Ω
10 Ω
30 Ω 39 Ω 78 Ω
v/5
v
Solve for v. Use a supernode.
Portland State University ECE 221 Analysis Methods Ver. 1.66 31
Example 6: Workspace
Portland State University ECE 221 Analysis Methods Ver. 1.66 32
Example 7: Dependent Voltage Source
20 V
+ -
1 Ω2 Ω 4 Ω
20 Ω 40 Ω 80 Ω 3.125v
v
35iφ
iφ
Find the power developed by the 20 V source.
Portland State University ECE 221 Analysis Methods Ver. 1.66 33
Example 7: Workspace
Portland State University ECE 221 Analysis Methods Ver. 1.66 34
Example 8: Nodal Analysis
11 mA
i1
20 Vi2
10 Vi3
250 Ω
500 Ω
1 kΩ
25 kΩ
Solve for i1, i2, and i3.
Portland State University ECE 221 Analysis Methods Ver. 1.66 35
Example 8: Workspace
Portland State University ECE 221 Analysis Methods Ver. 1.66 36
Example 9: Nodal Analysis
1 A
3i
i
-
+v
1 Ω
1 Ω
2 Ω
2 Ω4 Ω
Solve for v.
Portland State University ECE 221 Analysis Methods Ver. 1.66 37
Example 9: Workspace
Portland State University ECE 221 Analysis Methods Ver. 1.66 38
Mesh Analysis: Introduction
• Recall: There is an easier way to solve for currents and voltagesthan applying KVL and KCL directly
• Nodal analysis is one of two key methods
• Mesh analysis is the other
– Applies KVL to solve for currents
– More abstract
– Work with imaginary currents
– Only applies to planar circuits
Portland State University ECE 221 Analysis Methods Ver. 1.66 39
Mesh Analysis: Step 1 – Label Meshes
40 V 64 V
ia icib
1.5 Ω2 Ω
3 Ω 4 Ω
45 Ω
Find the branch currents ia, ib, and ic.
• Recall: A mesh is a loop that does not enclose any other loops
Portland State University ECE 221 Analysis Methods Ver. 1.66 40
Mesh Analysis: Step 2 – Apply KVL to Each Mesh
40 V 64 V
ia icib
1.5 Ω2 Ω
3 Ω 4 Ω
45 Ω
Portland State University ECE 221 Analysis Methods Ver. 1.66 41
Mesh Analysis: Step 3 – Solve Linear Equations
[50 −45−45 50.5
] [i1i2
]=
[4064
]
i1 = 9.8 A
i2 = 10 A
Portland State University ECE 221 Analysis Methods Ver. 1.66 42
Mesh Analysis: Step 4 – Solve for Variables of Interest
40 V 64 V
ia icib
1.5 Ω2 Ω
3 Ω 4 Ω
45 Ω
ia =ib =ic =
Portland State University ECE 221 Analysis Methods Ver. 1.66 43
Mesh Analysis: Review of Steps
• Step 1 – Label Meshes
• Step 2 – Apply KVL to Each Mesh
• Step 3 – Solve Linear Equations
• Step 4 – Solve for Variables of Interest
– Usually easy after Step 3
• Limitation: Only works with planar circuits
Portland State University ECE 221 Analysis Methods Ver. 1.66 44
Example 10: Mesh Analysis
12 V
110 V 70V
2 Ω
3 Ω
4 Ω
6 Ω
10 Ω 12 Ω
Find the total power developed in the circuit.
Portland State University ECE 221 Analysis Methods Ver. 1.66 45
Example 10: Workspace
Portland State University ECE 221 Analysis Methods Ver. 1.66 46
Example 11: Mesh Analysis
18 V 15 V3 A
2 Ω
3 Ω
6 Ω
9 Ω
Find the total power dissipated.
• Problem: What is the voltage across the 3 A source?
• Solutions
1 Add it as a variable
2 Use a supermesh
• Second option requires less work
Portland State University ECE 221 Analysis Methods Ver. 1.66 47
Example 11: Mesh Analysis
18 V 15 V3 A
2 Ω
3 Ω
6 Ω
9 Ω
Find the total power dissipated. Add a variable.
Portland State University ECE 221 Analysis Methods Ver. 1.66 48
Example 11: Workspace
Portland State University ECE 221 Analysis Methods Ver. 1.66 49
Example 12: Mesh Analysis
18 V 15 V3 A
2 Ω
3 Ω
6 Ω
9 Ω
Find the total power dissipated. Use a supermesh.
Portland State University ECE 221 Analysis Methods Ver. 1.66 50
Example 12: Workspace
Portland State University ECE 221 Analysis Methods Ver. 1.66 51
Example 13: Mesh Analysis
200 V
4.3 id
ie
ib
id
ia
ic
10 Ω
10 Ω
25 Ω
50 Ω
100 Ω
Find the branch currents ia – ie.
Portland State University ECE 221 Analysis Methods Ver. 1.66 52
Example 13: Workspace
Portland State University ECE 221 Analysis Methods Ver. 1.66 53
Example 14: Mesh Analysis
1.5 mA
8 V
2 kΩ
3 kΩ
4 kΩ
4 kΩ 4 kΩ
5 kΩ
7 kΩ
3iα
iα
Solve for iα
Portland State University ECE 221 Analysis Methods Ver. 1.66 54
Example 14: Workspace
Portland State University ECE 221 Analysis Methods Ver. 1.66 55
Nodal versus Mesh Analysis
• You should know how to do both
• Which is more efficient depends on the problem
• Will learn which to use with experience
• Nodal analysis used more often
• On exams, I will specify which method to use
Concise Summary:
Nodal Analysis Mesh AnalysisMethod KCL KVLSolve For Node Voltages Mesh Currents“Super” Conditions Voltage Sources Current Sources
Portland State University ECE 221 Analysis Methods Ver. 1.66 56