The Laplace Transform M. J. Roberts - All Rights Reserved. Edited by Dr. Robert Akl 1 Generalizing the Fourier Transform M. J. Roberts - All Rights Reserved. Edited by Dr. Robert Akl 2 Generalizing the Fourier Transform M. J. Roberts - All Rights Reserved. Edited by Dr. Robert Akl 3 Generalizing the Fourier Transform The extra factor e- s t is sometimes called a convergence factor because, when chosen properly, it makes the integral converge for some signals for which it would not otherwise converge. For example, strictly speaking, the signal A u ( t ) does not have a CTFT because the integral does not converge. But if it is multiplied by the convergence factor, and the real part of s is chosen appropriately, the CTFT integral will converge. ¥ ò -¥ ¥ ò -¥ ¥ A u ( t ) e- jw t dt = A ò e- jw t dt ¬ Does not converge 0 ¥ Ae- s t u ( t ) e- jw t dt = A ò e-(s + jw )t dt ¬ Converges (if s > 0) 0 M. J. Roberts - All Rights Reserved. Edited by Dr. Robert Akl 4 Complex Exponential Excitation M. J. Roberts - All Rights Reserved. Edited by Dr. Robert Akl 5 Complex Exponential Excitation M. J. Roberts - All Rights Reserved. Edited by Dr. Robert Akl 6 Pierre-Simon Laplace 3/23/1749 - 3/2/1827 M. J. Roberts - All Rights Reserved. Edited by Dr. Robert Akl 7 The Transfer Function () () Let x t be the excitation and let y t be the response of a () system with impulse response h t . The Laplace transform of () y t is ¥ ¥ ( ) ò y(t ) e Y s = -¥ - st dt = ò () () -¥ é h t * x t ù e- st dt û ë ö - st æ¥ Y s = ò ç ò h t x t - t dt ÷ e dt ø -¥ è -¥ () ¥ () ( ¥ ) ¥ ( ) ò h (t ) dt ò x (t - t ) e Y s = -¥ - st dt -¥ M. J. Roberts - All Rights Reserved. Edited by Dr. Robert Akl 8 The Transfer Function () () () () () Let x t = u t and let h t = e-4t u t . Find y t . ¥ () () () ò ( ) ( y t = x t *h t = -¥ ) x t h t - t dt = ¥ () ( ) ( ) u t e u t - t dt ò -¥ -4 t -t t 4t -4t ì t -4( t -t ) e 1 1e d t = e-4t ò e4t d t = e-4t = , t >0 ïò e y t = í0 4 4 0 ï , t <0 î0 y t = 1 / 4 1- e-4t u t () ( ) ( )( ) () 1 1 1 1/ 4 1/ 4 X ( s) = 1 / s , H ( s) = Þ Y ( s) = ´ = s+4 s s+4 s s+4 x ( t ) = (1 / 4 ) (1- e ) u ( t ) -4t M. J. Roberts - All Rights Reserved. Edited by Dr. Robert Akl 9 Cascade-Connected Systems If two systems are cascade connected the transfer function of the overall system is the product of the transfer functions of the two individual systems. M. J. Roberts - All Rights Reserved. Edited by Dr. Robert Akl 10 Direct Form II Realization M. J. Roberts - All Rights Reserved. Edited by Dr. Robert Akl 11 Direct Form II Realization M. J. Roberts - All Rights Reserved. Edited by Dr. Robert Akl 12 Direct Form II Realization M. J. Roberts - All Rights Reserved. Edited by Dr. Robert Akl 13 Direct Form II Realization M. J. Roberts - All Rights Reserved. Edited by Dr. Robert Akl 14 Direct Form II Realization M. J. Roberts - All Rights Reserved. Edited by Dr. Robert Akl 15 Direct Form II Realization A system is defined by y¢¢ ( t ) + 3 y¢ ( t ) + 7 y ( t ) = x¢ ( t ) - 5 x ( t ) . s-5 H ( s) = 2 s + 3s + 7 M. J. Roberts - All Rights Reserved. Edited by Dr. Robert Akl 16 Inverse Laplace Transform There is an inversion integral s + j¥ () () 1 st y t = Y s e ds , s = s + jw ò j2p s - j¥ () () for finding y t from Y s , but it is rarely used in practice. Usually inverse Laplace transforms are found by using tables of standard functions and the properties of the Laplace transform. M. J. Roberts - All Rights Reserved. Edited by Dr. Robert Akl 17 Existence of the Laplace Transform Time Limited Signals () x ( t ) is also bounded for all t, the Laplace transform integral If x t = 0 for t < t0 and t > t1 it is a time limited signal. If converges and the Laplace transform exists for all s. M. J. Roberts - All Rights Reserved. Edited by Dr. Robert Akl 18 Existence of the Laplace Transform () () ( ) ( ) Let x t = rect t = u t + 1 / 2 - u t - 1/ 2 . ¥ - s/2 s/2 s/2 - s/2 e e e e X s = ò rect t e- st dt = ò e- st dt = = , All s -s s -¥ -1/2 () () 1/2 M. J. Roberts - All Rights Reserved. Edited by Dr. Robert Akl 19 Existence of the Laplace Transform Right- and Left-Sided Signals Right-Sided Left-Sided M. J. Roberts - All Rights Reserved. Edited by Dr. Robert Akl 20 Existence of the Laplace Transform Right- and Left-Sided Exponentials Right-Sided x (t ) = e u (t - t0 ) , a Î at Left-Sided x ( t ) = eb t u ( t 0 - t ) , b Î M. J. Roberts - All Rights Reserved. Edited by Dr. Robert Akl 21 Existence of the Laplace Transform Right-Sided Exponential x ( t ) = ea t u ( t - t 0 ) , a Î ¥ ¥ t0 t0 X ( s ) = ò ea t e- st dt = ò e(a - s ) t e- jw t dt If Re ( s ) = s > a the asymptotic behavior of e(a - s ) t e- jw t as t ® ¥ is to approach zero and the Laplace transform integral converges. M. J. Roberts - All Rights Reserved. Edited by Dr. Robert Akl 22 Existence of the Laplace Transform Left-Sided Exponential x ( t ) = eb t u ( t 0 - t ) , b Î X( s) = t0 t0 -¥ -¥ b t - st e ò e dt = ( b -s )t - jw t e dt e ò If s < b the asymptotic behavior of e( b -s )t e- jw t as t ® -¥ is to approach zero and the Laplace transform integral converges. M. J. Roberts - All Rights Reserved. Edited by Dr. Robert Akl 23 Existence of the Laplace Transform The two conditions s > a and s < b define the region of convergence (ROC) for the Laplace transform of right- and left-sided signals. M. J. Roberts - All Rights Reserved. Edited by Dr. Robert Akl 24 Existence of the Laplace Transform Any right-sided signal that grows no faster than an exponential in positive time and any left-sided signal that grows no faster than an exponential in negative time has a Laplace transform. If x ( t ) = x r ( t ) + x l ( t ) where x r ( t ) is the right-sided part and x l ( t ) is the left-sided part and if x r ( t ) < K r ea t and x l ( t ) < K l eb t and a and b are as small as possible, then the Laplace-transform integral converges and the Laplace transform exists for a < s < b . Therefore if a < b the ROC is the region a < b . If a > b , there is no ROC and the Laplace transform does not exist. M. J. Roberts - All Rights Reserved. Edited by Dr. Robert Akl 25 Laplace Transform Pairs The Laplace transform of g1 ( t ) = Aea t u ( t ) is G1 ( s ) = ¥ ò Ae u ( t ) e at - st -¥ ¥ dt = A ò e - ( s-a )t 0 ¥ (a - s ) t dt = A ò e e - jw t 0 A dt = s -a This function has a pole at s = a and the ROC is the region to the right of that point. The Laplace transform of g 2 ( t ) = Aebt u ( -t ) is G2 ( s) = ¥ ò Ae u ( -t ) e -¥ bt 0 - st ( b - s )t dt = A ò e -¥ 0 ( b - s )t dt = A ò e -¥ e - jw t A dt = s-b This function has a pole at s = b and the ROC is the region to the left of that point. M. J. Roberts - All Rights Reserved. Edited by Dr. Robert Akl 26 Region of Convergence M. J. Roberts - All Rights Reserved. Edited by Dr. Robert Akl 27 Region of Convergence Some of the most common Laplace transform pairs (There is more extensive table in the book.) M. J. Roberts - All Rights Reserved. Edited by Dr. Robert Akl 28 Laplace Transform Example M. J. Roberts - All Rights Reserved. Edited by Dr. Robert Akl 29 Laplace Transform Example Find the inverse Laplace transform of 4 10 X( s) = , - 3<s < 6 s+3 s-6 4 The ROC tells us that must inverse transform into a s+3 10 right-sided signal and that must inverse transform into s-6 a left-sided signal. x ( t ) = 4e-3t u ( t ) + 10e6t u ( -t ) M. J. Roberts - All Rights Reserved. Edited by Dr. Robert Akl 30 Laplace Transform Example Find the inverse Laplace transform of 4 10 X( s) = ,s >6 s+3 s-6 The ROC tells us that both terms must inverse transform into a right-sided signal. x ( t ) = 4e-3t u ( t ) - 10e6t u ( t ) M. J. Roberts - All Rights Reserved. Edited by Dr. Robert Akl 31 Laplace Transform Example Find the inverse Laplace transform of 4 10 X( s) = , s < -3 s+3 s-6 The ROC tells us that both terms must inverse transform into a left-sided signal. x ( t ) = -4e-3t u ( -t ) + 10e6t u ( -t ) M. J. Roberts - All Rights Reserved. Edited by Dr. Robert Akl 32 MATLAB System Objects M. J. Roberts - All Rights Reserved. Edited by Dr. Robert Akl 33 MATLAB System Objects M. J. Roberts - All Rights Reserved. Edited by Dr. Robert Akl 34 Partial-Fraction Expansion M. J. Roberts - All Rights Reserved. Edited by Dr. Robert Akl 35 Partial-Fraction Expansion M. J. Roberts - All Rights Reserved. Edited by Dr. Robert Akl 36 Partial-Fraction Expansion M. J. Roberts - All Rights Reserved. Edited by Dr. Robert Akl 37 Partial-Fraction Expansion 10s K1 K2 H ( s) = = + , s > -4 ( s + 4 )( s + 9) s + 4 s + 9 é ù 10s -40 é 10s ù K1 = ê ( s + 4 ) =ê = = -8 ú ú ( s + 4 ) ( s + 9 ) úû s=-4 ë s + 9 û s=-4 5 êë é ù 10s -90 é 10s ù K2 = ê ( s + 9) =ê = = 18 ú ú ( s + 4 ) ( s + 9 ) úû s=-9 ë s + 4 û s=-9 -5 êë -8 18 -8s - 72 + 18s + 72 10s H ( s) = + = = . Check. s+4 s+9 ( s + 4 )( s + 9) ( s + 4 )( s + 9) ¯¯¯¯¯ ( ) h ( t ) = -8e-4t + 18e-9t u ( t ) M. J. Roberts - All Rights Reserved. Edited by Dr. Robert Akl 38 Partial-Fraction Expansion M. J. Roberts - All Rights Reserved. Edited by Dr. Robert Akl 39 Partial-Fraction Expansion M. J. Roberts - All Rights Reserved. Edited by Dr. Robert Akl 40 Partial-Fraction Expansion 10s K12 K11 K2 H ( s) = = + , s >4 2 2 + ( s + 4 ) ( s + 9) ( s + 4 ) s + 4 s + 9 ­ Repeated Pole é 2 K12 = ê ( s + 4 ) ê ë Using ù -40 ú = = -8 ( s + 9 ) úû s=-4 5 10s (s + 4) 2 1 d m- k é K qk = s - pq m-k ë ( m - k )! ds ( ) m H ( s)ù , k = 1, 2, , m û s® pq 1 d 2-1 d é 10s ù 2 é ù K11 = s + 4 H s = ) ( ) û s®-4 2-1 ë( ds êë s + 9 úû s®-4 ( 2 - 1)! ds M. J. Roberts - All Rights Reserved. Edited by Dr. Robert Akl 41 Partial-Fraction Expansion é ( s + 9 )10 - 10s ù 18 K11 = ê = ú 2 ( s + 9) ë û s=-4 5 18 -8 18 / 5 -18 / 5 K2 = - Þ H ( s) = + , s > -4 2 + 5 (s + 4) s + 4 s + 9 ( ) ( 18 2 18 2 -8s - 72 + s + 13s + 36 s + 8s + 16 5 5 H ( s) = ( s + 4 )2 ( s + 9 ) ) , s > -4 10s H ( s) = , s > -4 2 ( s + 4 ) ( s + 9) 18 -4t 18 -9t ö æ -4t h ( t ) = ç -8te + e - e ÷ u ( t ) è ø 5 5 M. J. Roberts - All Rights Reserved. Edited by Dr. Robert Akl 42 Partial-Fraction Expansion 10s 2 H ( s) = , s > -4 ¬ Improper in s ( s + 4 )( s + 9) 10s 2 H ( s) = 2 , s > -4 s + 13s + 36 10 Synthetic Division ® s 2 + 13s + 36 10s 2 10s 2 + 130s + 360 - 130s - 360 130s + 360 é -32 162 ù H ( s ) = 10 = 10 - ê + , s > -4 ú ( s + 4 )( s + 9) ës+ 4 s + 9û h ( t ) = 10d ( t ) - éë162e-9t - 32e-4t ùû u ( t ) M. J. Roberts - All Rights Reserved. Edited by Dr. Robert Akl 43 Inverse Laplace Transform Example Method 1 G ( s) = ( s - 3) ( s s 2 - 4s + 5 ) , s <2 s , s <2 G ( s) = ( s - 3) ( s - 2 + j ) ( s - 2 - j ) 3 / 2 (3 + j) / 4 (3 - j) / 4 , s <2 G ( s) = s-2- j s-3 s-2+ j æ 3 3t 3 + j ( 2- j )t 3 - j ( 2+ j )t ö e + e g (t ) = ç - e + ÷ø u ( -t ) è 2 4 4 M. J. Roberts - All Rights Reserved. Edited by Dr. Robert Akl 44 Inverse Laplace Transform Example 3 + j ( 2- j )t 3 - j ( 2+ j )t ö æ 3 g ( t ) = ç - e 3t + e + e u ( -t ) ÷ è 2 ø 4 4 This looks like a function of time that is complex-valued. But, with the use of some trigonometric identities it can be put into the form { } g ( t ) = ( 3 / 2 ) e2t éë cos ( t ) + (1 / 3) sin ( t ) ùû - e3t u ( -t ) which has only real values. M. J. Roberts - All Rights Reserved. Edited by Dr. Robert Akl 45 Inverse Laplace Transform Example Method 2 s G ( s) = ( s - 3) s 2 - 4s + 5 ( ) , s <2 s G ( s) = , s <2 ( s - 3) ( s - 2 + j ) ( s - 2 - j ) 3 / 2 (3 + j) / 4 (3 - j) / 4 G ( s) = , s <2 s-3 s-2+ j s-2- j Getting a common denominator and simplifying 3 / 2 1 6s - 10 3/2 6 s- 5 / 3 G ( s) = = , s <2 2 2 s - 3 4 s - 4s + 5 s - 3 4 ( s - 2 ) + 1 M. J. Roberts - All Rights Reserved. Edited by Dr. Robert Akl 46 Inverse Laplace Transform Example M ethod 2 3/2 6 s - 5 / 3 G ( s) = , s <2 2 s - 3 4 ( s - 2) + 1 The denominator of the second term has the form of the Laplace transform of a damped cosine or damped sine but the numerator is not yet in the correct form. But by adding and subtracting the correct expression from that term and factoring we can put it into the form 3/2 3é s - 2 1/ 3 ù G ( s) = - ê + ú , s <2 2 2 s - 3 2 ë ( s - 2) + 1 ( s - 2) + 1û M. J. Roberts - All Rights Reserved. Edited by Dr. Robert Akl 47 Inverse Laplace Transform Example M ethod 2 3/2 3é s - 2 1/ 3 ù G ( s) = - ê + ú , s <2 2 2 s - 3 2 ë ( s - 2) + 1 ( s - 2) + 1 û This can now be directly inverse Laplace transformed into { } g ( t ) = ( 3 / 2 ) e2t éë cos ( t ) + (1 / 3) sin ( t ) ùû - e 3t u ( -t ) which is the same as the previous result. M. J. Roberts - All Rights Reserved. Edited by Dr. Robert Akl 48 Inverse Laplace Transform Example M ethod 3 When we have a pair of poles p2 and p3 that are complex conjugates A K2 K3 we can convert the form G ( s ) = + + into the s - 3 s - p2 s - p3 s ( K 2 + K 3 ) - K 3 p2 - K 2 p3 A A Bs + C form G ( s ) = + = + 2 2 s-3 s - ( p1 + p2 ) s + p1 p2 s - 3 s - ( p1 + p2 ) s + p1 p2 In this example we can find the constants A, B and C by realizing that s A Bs + C G ( s) = º + 2 , s <2 2 ( s - 3) s - 4s + 5 s - 3 s - 4s + 5 ( ) is not just an equation, it is an identity. That means it must be an equality for any value of s. M. J. Roberts - All Rights Reserved. Edited by Dr. Robert Akl 49 Inverse Laplace Transform Example M ethod 3 A can be found as before to be 3 / 2. Letting s = 0, the 3/2 C identity becomes 0 º + and C = 5 / 2. Then, letting 3 5 s = 1, and solving we get B = -3 / 2. Now 3 / 2 ( -3 / 2 ) s + 5 / 2 G ( s) = + , s <2 2 s-3 s - 4s + 5 or 3/2 3 s - 5 / 3 , s <2 2 s - 3 2 s - 4s + 5 This is the same as a result in Method 2 and the rest of the solution G ( s) = is also the same. The advantage of this method is that all the numbers are real. M. J. Roberts - All Rights Reserved. Edited by Dr. Robert Akl 50 Use of MATLAB in Partial Fraction Expansion M. J. Roberts - All Rights Reserved. Edited by Dr. Robert Akl 51 Laplace Transform Properties M. J. Roberts - All Rights Reserved. Edited by Dr. Robert Akl 52 Laplace Transform Properties M. J. Roberts - All Rights Reserved. Edited by Dr. Robert Akl 53 Laplace Transform Properties M. J. Roberts - All Rights Reserved. Edited by Dr. Robert Akl 54 Laplace Transform Properties M. J. Roberts - All Rights Reserved. Edited by Dr. Robert Akl 55 Laplace Transform Properties Final Value Theorem The final value theorem applies to a function G ( s ) if all the poles of sG ( s ) lie in the open left half of the s plane. Be sure to notice that this does not say that all the poles of G ( s ) must lie in the open left half of the s plane. G ( s ) could have a single pole at s = 0 and the final value theorem would still apply. M. J. Roberts - All Rights Reserved. Edited by Dr. Robert Akl 56 Use of Laplace Transform Properties M. J. Roberts - All Rights Reserved. Edited by Dr. Robert Akl 57 Use of Laplace Transform Properties M. J. Roberts - All Rights Reserved. Edited by Dr. Robert Akl 58 The Unilateral Laplace Transform In most practical signal and system analysis using the Laplace transform a modified form of the transform, called the unilateral Laplace transform, is used. The unilateral Laplace transform is ¥ defined by G ( s ) = ò - g ( t ) e- st dt . The only difference between 0 this version and the previous definition is the change of the lower integration limit from - ¥ to 0 -. With this definition, all the Laplace transforms of causal functions are the same as before with the same ROC, the region of the s plane to the right of all the finite poles. M. J. Roberts - All Rights Reserved. Edited by Dr. Robert Akl 59 The Unilateral Laplace Transform The unilateral Laplace transform integral excludes negative time. If a function has non-zero behavior in negative time its unilateral and bilateral transforms will be different. Also functions with the same positive time behavior but different negative time behavior will have the same unilateral Laplace transform. Therefore, to avoid ambiguity and confusion, the unilateral Laplace transform should only be used in analysis of causal signals and systems. This is a limitation but in most practical analysis this limitation is not significant and the unilateral Laplace transform actually has advantages. M. J. Roberts - All Rights Reserved. Edited by Dr. Robert Akl 60 The Unilateral Laplace Transform The main advantage of the unilateral Laplace transform is that the ROC is simpler than for the bilateral Laplace transform and, in most practical analysis, involved consideration of the ROC is unnecessary. The inverse Laplace transform is unchanged. It is s + j¥ 1 + st g (t ) = G s e ds ( ) ò j2p s - j¥ M. J. Roberts - All Rights Reserved. Edited by Dr. Robert Akl 61 The Unilateral Laplace Transform M. J. Roberts - All Rights Reserved. Edited by Dr. Robert Akl 62 The Unilateral Laplace Transform The time shifting property applies only for shifts to the right because a shift to the left could cause a signal to become non-causal. For the same reason scaling in time must only be done with positive scaling coefficients so that time is not reversed producing an anti-causal function. The derivative property must now take into account the initial value of the function at time t = 0 - and the integral property applies only to functional behavior after time t = 0. Since the unilateral and bilateral Laplace transforms are the same for causal functions, the bilateral table of transform pairs can be used for causal functions. M. J. Roberts - All Rights Reserved. Edited by Dr. Robert Akl 63 The Unilateral Laplace Transform The Laplace transform was developed for the solution of differential equations and the unilateral form is especially well suited for solving differential equations with initial conditions. For example, d2 d x t + 7 ( ) é ù éë x ( t ) ùû + 12 x ( t ) = 0 û 2 ë dt dt d with initial conditions x 0 = 2 and x ( t ) )t = 0- = -4. ( dt Laplace transforming both sides of the equation, using the new derivative property for unilateral Laplace transforms, d 2 s X ( s ) - s x 0 - ( x ( t ) )t = 0- + 7 éë s X ( s ) - x 0 - ùû + 12 X ( s ) = 0 dt ( ) ( ) ( ) M. J. Roberts - All Rights Reserved. Edited by Dr. Robert Akl 64 The Unilateral Laplace Transform M. J. Roberts - All Rights Reserved. Edited by Dr. Robert Akl 65 Pole-Zero Diagrams and Frequency Response M. J. Roberts - All Rights Reserved. Edited by Dr. Robert Akl 66 Pole-Zero Diagrams and Frequency Response M. J. Roberts - All Rights Reserved. Edited by Dr. Robert Akl 67 Pole-Zero Diagrams and Frequency Response jw lim- H ( jw ) = lim- 3 =0 w ®0 w ®0 jw + 3 jw lim+ H ( jw ) = lim+ 3 =0 w ®0 w ®0 jw + 3 jw lim H ( jw ) = lim 3 =3 w ®-¥ w ®-¥ jw + 3 jw lim H ( jw ) = lim 3 =3 w ®+¥ w ®+¥ jw + 3 M. J. Roberts - All Rights Reserved. Edited by Dr. Robert Akl 68 Pole-Zero Diagrams and Frequency Response lim- w ®0 lim w ®-¥ H ( jw ) = - p 2 p -0=- p 2 æ pö H ( jw ) = - - ç - ÷ = 0 2 è 2ø lim+ w ®0 lim w ®+¥ M. J. Roberts - All Rights Reserved. Edited by Dr. Robert Akl H ( jw ) = p H ( jw ) = p 2 2 -0= - p 2 p 2 =0 69 Pole-Zero Diagrams and Frequency Response M. J. Roberts - All Rights Reserved. Edited by Dr. Robert Akl 70 Pole-Zero Diagrams and Frequency Response M. J. Roberts - All Rights Reserved. Edited by Dr. Robert Akl 71 Pole-Zero Diagrams and Frequency Response M. J. Roberts - All Rights Reserved. Edited by Dr. Robert Akl 72 Pole-Zero Diagrams and Frequency Response M. J. Roberts - All Rights Reserved. Edited by Dr. Robert Akl 73 Pole-Zero Diagrams and Frequency Response M. J. Roberts - All Rights Reserved. Edited by Dr. Robert Akl 74 Pole-Zero Diagrams and Frequency Response M. J. Roberts - All Rights Reserved. Edited by Dr. Robert Akl 75 Pole-Zero Diagrams and Frequency Response M. J. Roberts - All Rights Reserved. Edited by Dr. Robert Akl 76 Pole-Zero Diagrams and Frequency Response M. J. Roberts - All Rights Reserved. Edited by Dr. Robert Akl 77