1. EENG 2610: Circuit Analysis Class 12: First-Order Circuits
Oluwayomi Adamo
Department of Electrical Engineering
College of Engineering, University of North Texas

2. Transient Analysis Circuits in transition
Transient analysis: study of circuit behavior in transition phase.
Transition phase is caused by a sudden change in circuit
Suddenly apply or remove voltage or current source,
Open or close a switch in the circuit.
Transition is affected by capacitor or inductor in circuit
No transient analysis is needed for resistive circuit network.
Because capacitor and inductor can store energy, the circuit response to a sudden change will go through a transition period before settling down to a steady-state value.

3. First-Order and Second-Order Circuits
First-order circuits contain only a single capacitor or inductor
Second-order circuits contain both a capacitor and an inductor
Two techniques for transient analysis that we will learn:
Differential equation approach
Step-by-step approach
Laplace transform method is a much simpler method for transient analysis – you will learn it in another course.

4. First-order differential equation
Solution of transient analysis requires to solve a first-order differential equation:
General solution to the first-order differential equation:
: particular integral solution (or forced response)
: complementary solution (or natural response)
xc(t) is the solution to the homogeneous equation:
Now we only consider f (t) is constant, that is, f (t) = A :
Solution:

5. A drop of 63.2%

6. The Differential Equation Approach
State-variable approach
Step 1: Find state-variables
Voltage across the capacitor and current through inductor are called state-variables.
State-variables cannot change instantaneously.
Step 2: Find initial value of state variables at
Step 3: Write KCL equation for the voltage across the capacitor and/or KVL equation for the current through the inductor for
Step 4: Solve first-order differential equation

7. Example 7.1: Calculate the current
Assume the switch has been in position 1 for a long time.

8. Example 7.2: Find output voltage

10. Step-by-Step Approach for First-Order Circuits
Step1: We assume a solution for the variable x(t) (either voltage or current) of the form
Step 2: Assuming that the original circuit has reached steady state before switch actions, draw this previous circuit with the capacitor replaced by an open circuit or the inductor replaced by a short circuit. Solve for the voltage across the capacitor, vC(0-), or the current through the inductor, iL(0-), prior to switch action.
Step 3: Voltage across a capacitor and the current through an inductor cannot change in zero time: vC(0+)=vC(0-), iL(0+)=iL(0-). Therefore, draw the circuit valid for t=0+ with the switches in their new positions. Replace a capacitor with a voltage source of value vC(0+) or an inductor with a current source of value iL(0+). Solve for the initial value of the variable x(0+).

11. Step 4: Assuming that steady state has been reached after the switches are thrown, draw the equivalent circuit, valid for , by replacing the capacitor by an open circuit or the inductor by a short circuit. Solve for the steady-state value of the variable
Step 5: Find Thevenin equivalent resistance at the terminals of the storage element by looking into the circuit from the terminals of the storage element. The time constant for a circuit containing a capacitor is , and for a circuit containing an inductor the time constant is
Step 6: Evaluate the constants K1, K2 in step 1 using the relations: Then, we can find that the solution is
Important: This solution form applies only to a first-order circuit having constant, DC sources.

12. If switch action occurs at any time t0, the step-by-step analysis yields the following equations:
The function is essentially time-shifted by t0 seconds.

13. Example 7.3: The circuit is in steady state prior to time t=0, when the switch is closed. Calculate the current i(t) for t>0.