1. CSE245: Computer-Aided Circuit Simulation and Verification Lecture Note 2: State Equations Spring 2010 Prof. Chung-Kuan Cheng

2. State Equations 1.Motivation 2.Formulation 3.Analytical Solution 4.Frequency Domain Analysis 5.Concept of Moments

3. Motivation Why –Whole Circuit Analysis –Interconnect Dominance Wires smaller  R increase Separation smaller  C increase What –Power Net, Clock, Interconnect Coupling, Parallel Processing Where –Matrix Solvers, Integration For Dynamic System –RLC Reduction, Transmission Lines, S Parameters –Whole Chip Analysis –Thermal, Mechanical, Biological Analysis

4. Formulation General Equation (a.k.a. state equations) Equation Formulation –Conservation Laws KCL (Kirchhoff’s Current Law) –n-1 equations, n is number of nodes in the circuit KVL (Kirchhoff’s Voltage Law) –m-(n-1) equations, m is number of branches in the circuit. –Branch Constitutive Equations m equations

5. Formulation State Equations (Modified Nodal Analysis): Desired variables 1.Capacitors: voltage variables 2.Inductors: current variables 3.Current controlled sources: control currents 4.Controlled voltage sources: currents of controlled voltage sources. Freedom of the choices 1.Tree trunks: voltage variables 2.Branches: current variables

6. Conservation Laws KCL KVL n-1 independent cutsets m-(n-1) independent loops

7. Branch Constitutive Laws Each branch has a circuit element –Resistor –Capacitor Forward Euler (FE) Approximation Backward Euler (BE) Approximation Trapezoidal (TR) Approximation –Inductor Similar approximation (FE, BE or TR) can be used for inductor. v=R(i)i i=dq/dt=C(v)dv/dt

8. Branch Constitutive Laws Inductors v=L(i)di/dt Mutual inductance V 12 =M 12,34 di 34 /dt

9. Formulation - Cutset and Loop Analysis find a cutset for each trunk –write a KCL for each cutset find a loop for each link –write a KVL for each loop Select tree trunks and links cutset matrix loop matrix

10. Formulation - Cutset and Loop Analysis Or we can re-write the equations as: In general, the cutset and loop matrices can be written as

11. Formulation – State Equations From the cutset and loop matrices, we have In general, one should –Select capacitive branches as tree trunks no capacitive loops for each node, there is at least one capacitor (every node actually should have a shunt capacitor) –Select inductive branches as tree links no inductive cutsets Combine above two equations, we have the state equation

12. Formulation – An Example State Equation Output Equation (suppose v 3 is desired output)

13. Responses in Time Domain State Equation The solution to the above differential equation is the time domain response Where

14. Exponential of a Matrix Properties of e A k! can be approximated by Stirling Approximation That is, higher order terms of e A will approach 0 because k! is much larger than A k for large k’s. Calculation of e A is hard if A is large

15. Responses in Frequency Domain: Laplace Transform Definition: Simple Transform Pairs Laplace Transform Property - Derivatives

16. Responses in Frequency Domain Time Domain State Equation Laplace Transform to Frequency Domain Re-write the first equation Solve for X, we have the frequency domain solution

17. Serial Expansion of Matrix Inversion For the case s  0, assuming initial condition x 0 =0, we can express the state response function as For the case s   assuming initial condition x 0 =0, we can express the state response function as

18. Concept of Moments The moments are the coefficients of the Taylor’s expansion about s=0, or Maclaurin Expansion Recall the definition of Laplace Transform Re-Write as Moments

19. Concept of Moments Re-write Maclaurin Expansion of the state response function Moments are

20. Moments Calculation: An Example

21. For the state response function, we have A voltage or current can be approximated by

22. Moments Calculation: An Example (Cont’d) (1) Set V s (0) =1 (suppose voltage source is an impulse function) (2) Short all inductors, open all capacitors, derive V c (0), I L (0) (3) Use V c (i), I L (i) as sources, i.e. I c (i+1) =CV c (i) and V L (i+1) =LI L (i), derive V c (i+1), I L (i+1) (4) i++, repeat (3)