Fundamentals of Electric Circuits Review

This document is intended for the reviewing of the course Fundamentals of Electric Circuits.

Preface

Current

Current is the rate of charge flow past a given point in a given direction.

i=dqdt

Voltage

Voltage is the energy required to move 1 C of charge through an element.

v=dwdq

Power

Power is the energy supplied or absorbed per unit time.

p=dwdt=vi

Passive Sign Convention

Passive sign convention is satisfied if the direction of current is selected such that current enters through the terminal that is more positively biased.

Law of Energy Conservation

p = 0

Sources

  • An ideal voltage source has zero internal resistance and is capable of producing any amount of current.
  • An ideal current source has infinite resistance. It is able to generate any voltage to establish the desired current through it.
  • Ideal dependent source

Basic Laws

Ohm’s Law

v = Ri

i = Gv

p=vi=i2R=v2G=v2R=i2G

R=ρlA

  • open circuit and short circuit

Topology

  • A branch represents any single element in a circuit.
  • A node is the point of connection between two or more branches.
  • A loop is any closed path in a circuit.
  • A set of loops is said independent when any of them contains at least one new branch in addition to all others.
  • A mesh is a loop which does not contain any other loops within it.

b = m + n − 1

Kirchhoff’s Law

  • Principle of the Conservation of Charge

Kirchhoff’s Current Law (KCL)

The algebraic sum of currents entering a node (or a closed boundary) is zero. In other words, the sum of the currents entering a node is equal to that leaving the node.

0=dqdt=n=1Nin

Kirchhoff’s Voltage Law (KVL)

  • The algebraic sum of all voltages around a loop is zero.
  • Sum of voltage drops = Sum of voltage rises.

m=1Mwm=0=m=1Mqvmm=1Mvm=0

Equivalent Circuits/Sources

Two circuits (or sources) are said to be equivalent if they have the same V-I relationship at their terminals.

Resistance

Req=R1R2R1+R2,Geq=G1+G2

The Principle of Voltage Division

Resistors in series

vn=Rnn=1NRnv

The Principle of Current Division

Resistors in parallel

in=Gnn=1NGn

Wye-Delta Transformation

  • Delta to Wye

{R1=RbRcRa+Rb+RcR2=RcRaRa+Rb+RcR3=RaRbRa+Rb+Rc

  • Wye to Delta

{Ra=R2+R3+R2R3R1Rb=R1+R3+R1R3R2Rc=R1+R2+R1R2R3

Y-networks is more like a serial connection, and Δ network is more like a parallel connection.

RY < RΔ

When the Y and Δ network is balanced

{R1=R2=R3=RYRa=Rb=Rc=RΔ{RY=RΔ3RΔ=3RY

Systematic Methods for Circuit Analysis

Nodal Voltage Analysis

  1. Choose one node as the reference node, then label the voltage on the remaining nodes
  2. Apply KCL to each non-reference nodes, branch currents represented in terms of node voltages according to Ohm’s Law
  3. Solve the n − 1 simultaneous equations
  • Nodal analysis with voltage sources

  • Case 1 - the V-source connects to the reference node

  • Case 2 - the V-source connects to two non-reference nodes

    • Treat these two nodes as one supernode

Nodal analysis is good for

  • The network contains many elements connected in parallel
  • The network contains many current sources
  • The circuit has fewer nodes than meshes
  • Node voltages are what being solved for

Mesh Current Analysis

  1. Label the mesh currents
  2. Apply KVL to each of the n meshes, write the voltages across each elements in terms of the mesh currents according to the Ohm’s Law
  3. Solve the n simultaneous equations

Always consider the circuit in clockwise direction

  • Mesh analysis with current sources
  • Case 1 - the current source is NOT shared by multiple meshes
  • Case 2 - the current source IS shared by multiple meshes
    • Treat the two sharing meshes as one supermesh

Mesh analysis is applicable for planar circuits Mesh analysis is good for

  • The network contains many serially connected elements
  • The network contains many voltage sources
  • The circuit has fewer meshes than nodes
  • Branch or mesh currents are what being solved for

Circuit Theorems

Linear Circuits

  • HomogeneityProperty

f(αx) = αf(x) = αy

  • Additivity Property

{y1=f(x1)y2=f(x2)f(x1+x2)=y1+y2

  • Linearity

f(αx1+βx2) = αy1 + βy2

Superposition Theorem

In a linear system, the voltage across (or current through) an element is the algebraic sum of those caused by each independent source acting alone

  1. Turn off all independent source except one, find the output
  2. Repeat Step 1 for other sources
  3. Adding up

Source Transformation

two circuits are equivalent if their voltage versus current (v-i) relations at port are identical

Thevenin’s Theorem

A linear two terminal circuit can be replaced by an equivalent circuit consisting of a voltage source VTh in series with a resistor RTh .

  1. Open the circuit, to find out the voltage between terminals
  2. Add an assumed source (generally voltage source), then calculate the equivalent resistance

Maximum Power Transfer

RL=RTh,pmax=VTh24RTh

Norton’s Theorem

A linear two terminal circuit can be replaced by an equivalent circuit consisting of a current source IN and a parallel > resistor RN

  1. Find the open-circuit voltage
  2. Find the short-circuit current
  3. The equivalent or input resistance Rin at terminals when all independent sources are turned off

Operational Amplifiers

Basics of Op-Amp

Op amp is an active electric element

  • input resistance
  • output resistance
  • open loop voltage gain

Ideal op amp

  1. Ro ≈ 0
  2. Ri = ∞ ⇒ i1 ≈ 0, i2 ≈ 0
  3. A ≈ ∞ ⇒ v1 ≈ v2

virtual opening and virtual shorting

Inverting Amplifier

v0=RfR1vi

Trans-Resistance Amplifier

vois=R

Summing Amplifier/Summer

v0=(RfR1v1+RfR2v2+RfR3v3)

Non-Inverting Amplifier

Av=vovi=1+RfR1

Voltage Follower

Also named unity gain amplifier

Difference Amplifier

vo=R2(1+R1/R2)R1(1+R3/R4)v2R2R1v1

If R1 = R2, R3 = R4, this becomes a subtractor

Capacitors and Inductors

Capacitor

q = Cv

For planar capacitor

C=ϵAd

v(t)=1Cti(τ)dτ+v(t0)

i=dqdt=Cdvdt

w=12Cv2=q22C

  • Capacitor is open circuit to dc
  • Voltage on a capacitor must be continuous
  • In parallel: Ceq = C1 + C2 + ⋯ + CN
  • In series: 1Ceq=1C1+1C2++1CN

Inductor

v=Ldidt

For solenoid

L=N2μAl

i(t)=1Ltv(τ)dτ+i(t0)

  • Inductor is short circuit to dc
  • Current on a inductor must be continuous
  • In parallel: 1Leq=1L1+1L2++1LN
  • In series: Leq = L1 + L2 + ⋯ + LN

Singularity Functions

Step Function

Impulse/Delta Function

00+δ(t)dt  = 1

Unit Ramp Function

Relationship

  • In form of differentiation

δ(t)=du(t)dt,u(t)=dr(t)dt

  • In form of integration

u(t) = ∫ − ∞tδ(λ)dλ ,  r(t) = ∫ − ∞tu(λ)dλ 

First Order Circuits

Source-Free RC Circuit

v(t) = V0e − t/RC = V0e − t/τ

  • natural response
  • decay rate

wR(t)=12CV02(1e2t/τ)

  • initial voltage
  • time constant

Source-Free RL Circuit

i(t)=i(0)eRt/L=i(0)et/τ,τ=LR

wR(t)=12LI02(1e2t/τ)

  • initial current
  • time constant

Step Response of RC Circuit

  • Complete/Total Response

v(t)={V0,t<0Vs+(V0Vs)et/τ,t>0,τ=RC

  • natural response
  • forced response

or

  • transient response
  • steady-state response

Step Response of RL Circuit

i(t)={I0,t<0VsR+(I0VsR)et/τ,t>0,τ=LR

Instantaneous value = Final + (Initial−Final)e − (tt0)/τ


Second Order Circuits

Source-Free Series RLC Circuits

Ri+Ldidt+1Cti(τ)dτ=0d2idt2+RLdidt+iLC=0

The solution is in the form of

i(t) = A1es1t + A2es2t

In which

s2+RLs+1LC=0s1=α+α2ω02,s2=αα2ω02

Where α = R/2L is damping factor, ω0=1/LC is resonant frequency.

  • over damped, α > ω0
  • critically damped, α = ω0
  • under damped, α < ω0
  • undamped, α = 0

Source-Free Parallel RLC Circuits

vR+1Ltv(τ)dτ+Cdvdt=0d2vdt2+1RCdvdt+1LCv=0

The general solution is

v(t) = A1es1t + A2es2t

Where

α=12RC,ω0=1LC

Step Response of Series RLC Circuit

  • Over damped - v(t) = Vs + A1es1t + A2es2t
  • Critically damped - v(t) = Vs + (A1+A2t)r − αt
  • Under damped - v(t) = Vs + (A1cosωdt+A2sinωdt)e − αt

α=R2L

Step Response pf Parallel RLC Circuit

  • Over damped - i(t) = Is + A1es1t + A2es2t
  • Critically damped - i(t) = Is + (A1+A2t)r − αt
  • Under damped - i(t) = Is + (A1cosωdt+A2sinωdt)e − αt

α=12RC


Sinusoids and Phasors

Sinusoids

v(t) = Vmsin (ωt+ϕ)

  • amplitude
  • angular frequency
  • argument
  • phase
  • period
  • leads
  • lags
  • out of phase
  • in phase
  • anti phase

Complex Number

  • rectangular form z = x + jy
  • polar form z = rϕ
  • exponential form z = rejϕ

Where r=x2+y2 is the modulus, ϕ=tan1yx is the argument.

  • multiplication

z1z2 = r1r2∠(ϕ1+ϕ2)

  • division

z1z2=r1r2(ϕ1ϕ2)

  • reciprocal

1z=1r(ϕ)

  • square root

z=r(ϕ/2)

  • complex conjugate

z* = x − jy = r∠(−ϕ) = re − jϕ

Phasors

v(t) = Vmcos (ωt+ϕ) = ℜ[Vmej(ωt+ϕ)]

Where V = Vmejϕ = Vmϕ is the phasor

Time Domain Phasor Domain
dv /dt  jωV
vdt  V/jω

Generalized Ohm’s Law

V = RI

V = jωLI

I = jωCV

  • resistance
  • reactance
  • impedance

Z + R + jX

  • admittance
Element Impedance
R Z = R
L Z = jωL
C Z=1jωC

Ohm’s Law

V = ZI

Sinusoid Steady State Analysis

To obtain superposition theorem, the sources in the circuit must have the same frequency


AC Power Analysis

Instantaneous Power

p(t)=v(t)i(t)=VmImcos(ωt+θv)cos(ωt+vi)=12VmImcos(2ωt+θv+θi)

Average Power

  • effective value / RMS value

P=12VmImcos(θvθi)

  • apparent power S
  • power factor
  • power factor angle
  • complex power

S=12VI=VrmsIrms

  • real power
  • reactive / quadrature power

P = ℜ(S),  Q = ℑ(S)

Maximum Average Power Transfer

Pmax=|VTh|28RTh


Magnetically Coupled Circuits

Faraday’s Law pf electromagnetic induction

e=Ndϕdt

Self Inductance

self inductance

v=Ndϕdt=Ndϕdididt=Ldidt

L=NdϕdiL

Mutual Inductance

Dot Convention

If a current enters the dotted terminal of one coil, the reference polarity of the mutual voltage in the second coil is positive at the dotted terminal of the second coil.

Cascade Connection

  • Series aiding connection

L = L1 + L2 + 2M

Else opposing connected, should be minus 2M

Coefficient of Coupling

k=ϕ12ϕ1=ϕ12ϕ11+ϕ12=ML1L2

M=kL1L2

Transformers

Linear / Air-Core Transformers

  • input impedance
  • reflected / coupled impedance

ZR=ω2M2R2+jωL2+ZL

  • port V-I property

[V1V2]=[jωL1jωMjωMjωL2]=[I1I2]

Ideal Transformers

V2V1=N2N1

Zin=ZLn2


Frequency Response

Y(ω) = H(ω)X(ω)


Laplace Transform

  • one-side / unilateral Laplace Transform

ℒ[f(t)] = F(s) = ∫0f(t)e − stdt 

Where

s = σ + jω

L1[F(s)]=f(t)=12πjσjσ+jF(s)estds

Properties

  • Linearity

ℒ[a1f1(t)+a2f2(t)] = a1F1(t) + a2F2(t)

  • Scale changing

L[f(at)]=1aF(sa),a>0

  • Time shift

ℒ[f(ta)u(ta)] = e − asF(s),  a > 0

  • Frequency shift

ℒ[e − atf(t)] = F(s+a)

  • Differentiation in time domain

L[df(t)dt]=sF(s)f(0)

  • Initial value theorem

f(0+) = lims → ∞sF(s)

  • Final value theorem

f(∞) = lims → 0sF(s)

  • Higher order differentiation in time domain

L[dnf(t)dtn]=snF(s)sn1f(0)sn2df(0)dtdn1f(0)dn1

  • Integration in time domain

L[0tf(x)dx]=F(s)s

  • Differentiation in frequency domain

L[tnf(t)]=(1)ndnF(s)dsn

Method of Partial Expansion

Residue Method

  • Case 1: Simple distinct poles

F(s)=N(s)(s+p1)(s+p2)(s+pn)

F(s)=k1s+p1+k1s+p2++k1s+pn

Where ki are known as residuals

By Heaviside’s Theorem

ki = (s+pi)F(s)|s =  − pi

  • Case 2: Repeated poles

Method of Algebra

Solving Equations at Specific Values

Inverse Laplace Transform

Convolution Integral

x(t) = ∫ − ∞x(λ)δ(tλ)dλ  = x(t) * h(t)

Circuit Analysis in s-Domain

  1. Transform the circuit from the time domain to the s-domain
  2. Solve the circuit using nodal analysis, mesh analysis, etc.
  3. Take the inverse transform of the solution and thus obtain the solution in the time domain
  • inductor

V(s)=sLILi(0),I(s)=1sLV(s)+i(0)s

  • capacitor

V(s)=1sCI(s)+v(0)s,I(s)=C[sVv(0)]

  • impedance
Resistor Inductor Capacitor
Z(s) = R Z(s) = sL Z(s) = 1/sC