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S. Boyd EE102

Lecture 3The Laplace transform

denition & examples

properties & formulas

linearity the inverse Laplace transform time scaling exponential scaling time delay derivative integral multiplication by t convolution

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Idea

the Laplace transform converts integral and differential equations intoalgebraic equations

this is like phasors, but

applies to general signals, not just sinusoids

allows us to analyze

LCCODEs

complicated circuits with sources, Ls, Rs, and Cs complicated systems with integrators, differentiators, gains

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Complex numbers

complex number in Cartesian form: z = x + jy

x = z, the real part of z

y = z, the imaginary part of z

j = 1 (engineering notation); i = 1 is polite term in mixedcompanycomplex number in polar form: z = re j

r is the modulus or magnitude of z is the angle or phase of z

exp( j) = cos + j sin

complex exponential of z = x + jy :

ez = ex + jy = ex ejy = ex (cos y + j sin y)

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The Laplace transform

well be interested in signals dened fort 0the Laplace transform of a signal (function) f is the function F = L(f )dened by

F (s) =

0f (t)e st dt

for those sC for which the integral makes sense

F is a complex-valued function of complex numbers s is called the (complex) frequency variable , with units sec

1; t is calledthe time variable (in sec); st is unitless

for now, we assume f contains no impulses at t = 0common notation convention: lower case letter denotes signal; capitalletter denotes its Laplace transform, e.g. , U denotes L(u), V in denotesL(vin ), etc.The Laplace transform 34

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Example

lets nd Laplace transform of f (t) = et :

F (s) =

0et e st dt =

0e(1 s ) t dt =

11 s

e(1 s ) t

0=

1s 1

provided we can say e(1 s ) t 0 as t , which is true for s > 1:e(1 s ) t = e j ( s ) t

=1e(1 s ) t = e(1 s ) t

the integral dening F makes sense for all sC with s > 1 (theregion of convergence of F )

but the resulting formula for F makes sense for all s

C except s = 1

well ignore these (sometimes important) details and just say that

L(et ) =

1

s 1The Laplace transform 35

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More examples

constant: (or unit step) f (t) = 1 (for t 0)

F (s) =

0e st dt =

1

se st

0=

1

s

provided we can say e st 0 as t , which is true for s > 0 since

e st = e j ( s ) t

=1e ( s ) t = e ( s ) t

the integral dening F makes sense for all s with s > 0

but the resulting formula for F makes sense for all s except s = 0

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sinusoid: rst express f (t) = cos t as

f (t) = (1 / 2)ejt + (1 / 2)e jt

now we can nd F as

F (s) =

0e st (1/ 2)ejt + (1 / 2)e jt dt

= (1 / 2)

0 e( s + j ) t

dt + (1 / 2)

0 e( s j ) t

dt

= (1 / 2)1

s j+ (1 / 2)

1s + j

=s

s2 + 2

(valid for s > 0; nal formula OK fors = j)

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powers of t : f (t) = tn (n 1)well integrate by parts, i.e. , use

ba u(t)v (t) dt = u(t)v(t)b

a b

av(t)u (t) dt

with u(t) = tn , v (t) = e st , a = 0 , b = F (s) =

0tn e st dt = tn e

st

s

0+

ns

0tn 1e st dt

= ns L(t

n 1)

provided tn e st 0 if t , which is true for s > 0applying the formula recusively, we obtain

F (s) =n!

sn +1

valid for s > 0; nal formula OK for alls = 0The Laplace transform 38

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Impulses at t = 0

if f contains impulses at t = 0 we choose to include them in the integraldening F :

F (s) =

0f (t)e st dt

(you can also choose to not include them, but this changes some formulaswell see & use)

example: impulse function, f =

F (s) =

0(t)e st dt = e st t =0 = 1

similarly for f = (k ) we have

F (s) =

0(k ) (t)e st dt = ( 1)k

dk

dtke st

t =0= sk e st t =0 = s

k

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Linearity

the Laplace transform is linear : if f and g are any signals, and a is anyscalar, we have

L(af ) = aF, L(f + g) = F + Gi.e. , homogeneity & superposition hold

example:

L 3(t) 2et = 3L((t)) 2L(et )

= 3 2

s 1=

3s 5s 1

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One-to-one property

the Laplace transform is one-to-one : if L(f ) = L(g) then f = g(well, almost; see below)

F determines f inverse Laplace transform L 1 is well dened(not easy to show)example (previous page):

L 1 3s 5

s 1= 3 (t) 2et

in other words, the only function f such that

F (s) =3s 5s 1

is f (t) = 3 (t)

2et

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what almost means: if f and g differ only at a nite number of points(where there arent impulses) then F = G

examples:

f dened as f (t) = 1 t = 20 t = 2has F = 0

f dened asf (t) = 1/ 2 t = 01 t > 0

has F = 1 /s (same as unit step)

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Inverse Laplace transform

in principle we can recoverf from F via

f (t) =1

2j + j

j F (s)est ds

where is large enough that F (s) is dened for s

surprisingly, this formula isnt really useful!

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Time scaling

dene signal g by g(t) = f (at ), where a > 0; then

G(s) = (1 /a )F (s/a )

makes sense: times are scaled by a, frequencies by 1/a

lets check:

G(s) =

0 f (at )e

st dt = (1 /a )

0 f ( )e

(s/a ) d = (1 /a )F (s/a )

where = at

example: L(et) = 1 / (s 1) so

L(eat ) = (1 /a )1

(s/a ) 1=

1s a

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Exponential scaling

let f be a signal and a a scalar, and dene g(t) = eat f (t); then

G(s) = F (s a)

lets check:

G(s) =

0 e

st eat f (t) dt =

0 e

( s

a ) t f (t) dt = F (s a)

example:

L(cos t) = s/ (s2 + 1) , and hence

L(e t cos t) =

s + 1(s + 1) 2 + 1

=s + 1

s2 + 2 s + 2

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Time delay

let f be a signal and T > 0; dene the signal g as

g(t) = 0 0 t < T f (t T ) t T (g is f , delayed by T seconds & zero-padded up to T )

ttt = T

f ( t ) g ( t )

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then we have G(s) = e sT F (s)

derivation:

G(s) =

0e st g(t) dt =

T e st f (t T ) dt

=

0e s ( + T ) f ( ) d

= e sT F (s)

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example: lets nd the Laplace transform of a rectangular pulse signal

f (t) = 1 if a t b0 otherwisewhere 0 < a < b

we can write f as f = f 1 f 2 wheref 1(t) =

1 t a0 t < a f 2(t) =1 t b0 t < b

i.e. , f is a unit step delayed a seconds, minus a unit step delayed b seconds

hence

F (s) = L(f 1) L(f 2)=

e as e bs

s

(can check by direct integration)

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Derivative

if signal f is continuous at t = 0 , then

L(f ) = sF (s) f (0)

time-domain differentiation becomes multiplication by frequencyvariable s (as with phasors) plus a term that includes initial condition ( i.e. , f (0) )

higher-order derivatives: applying derivative formula twice yields

L(f ) = sL(f ) f (0)= s(sF (s) f (0)) f (0)= s2F (s) sf (0) f (0)

similar formulas hold for

L(f (k ) )

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examples

f (t) = et , so f (t) = et and

L(f ) = L(f ) =1

s 1using the formula, L(f ) = s(

1s 1

) 1, which is the same

sin t =

1

ddt cos t, so

L(sin t) = 1

ss

s2 + 2 1 =

s2 + 2

f is unit ramp, so f is unit step

L(f ) = s1s2 0 = 1/s

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derivation of derivative formula: start from the dening integral

G(s) =

0f (t)e st dt

integration by parts yields

G(s) = e st f (t)

0

0f (t)(se

st ) dt

= limt f (t)e st

f (0) + sF (s)

for s large enough the limit is zero, and we recover the formula

G(s) = sF (s) f (0)

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