1. Lecture 5 - RC/RL First-Order Circuits

2. Temporal Behavior of Circuit Responses
Till now we discussed static analysis of a circuit
Responses at a given time depend only on inputs at that time.
Circuit responds to input changes infinitely fast.
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3. Temporal Behavior of Circuit Responses
From now on we start to discuss dynamic circuit
Time-varying sources and responses
We have to introduce capacitors and inductors to explain such temporal behavior.
[Source: MIT]
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4. Parasitic Modeling of Interconnect
Two inverters communicating over a long interconnect
Interconnect has potential difference with surroundings
Electric field E, modeled as capacitor
Current flows through the interconnect loop
Magnetic flux density B linked by the loop, modeled as inductor
[Source: MIT]
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5. Outline Capacitors and inductors Response of a circuit
Natural response of RC/RL circuits
Step response of RC/RL circuits
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6. Passive element that stores energy in electric field
Capacitors
Passive element that stores energy in electric field
Parallel plate capacitor
[Source: Berkeley]
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7. Fulid-Flow Analogy A reservoir
Does DC current flow through a capacitor?
Does AC current flow through a capacitor?
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8. Example
[Source: Berkeley]
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9. V-I Relationship of Capacitors
[Source: Berkeley]
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10. Stored Energy The instantaneous power delivered to the capacitor is
The energy stored in a capacitor is:
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11. Capacitor Response
C =
P194
[Source: Berkeley]
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12. Practical (Imperfect) Capacitors
A real capacitor has parasitic effects, leading to a slow loss of the stored energy internally.
𝑅 𝑠 : resistivity of plate materials
𝐿 𝑠 :current through C creates magnetic field
𝑅 𝑝 : dielectric not perfect insulator (~100 MΩ)
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13. Capacitors in Series
[Source: Berkeley]
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14. Capacitors in Parallel
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15. Voltage Division
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16. Inductors A passive element that stores energy in magnetic field.
They have applications in power supplies, transformers, radios, TVs, radars, and electric motors.
Any conductor has inductance, but the effect is typically enhanced by coiling the wire up.
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17. V-I Relationship of Inductors
𝐿= Φ 𝑖
Faraday’s Law
[Source: Berkeley]
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18. Energy Stored in an Inductor
The power delivered to the inductor is:
The energy stored is:
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19. Inductor Response
[Source: Berkeley]
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20. Important Property of Inductors
[Source: Berkeley]
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21. Practical (Imperfect) Inductors
Like the ideal capacitor, the ideal inductor does not dissipate energy stored in it.
In reality, inductors do have internal resistance due to the wiring used to make them.
A winding resistance in series with it.
A small winding capacitance due to the closeness of the windings
These two characteristics are typically small, though at high frequencies, the capacitance may matter.
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22. Inductors in Series
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23. Inductors in Parallel
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24. Example
[Source: Berkeley]
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25. Summary of Capacitors and Inductors
[Source: Berkeley]
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26. Response of a Circuit Circuit (dynamic) response Transient response
the reaction of a certain voltage or current in the circuit to change, such as the adding of a new source, the elimination of a source, in the circuit configuration.
Transient response
Behavior when voltage or current source are suddenly applied to or removed from the circuit due to switching.
Temporary behavior
[Source: Berkeley]
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27. Example: Natural Response of a Circuit
Behavior (i.e., current and voltage) when stored energy in the inductor or capacitor is released to the resistive part of the network (containing no independent sources).
[Source: Berkeley]
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28. Response of a Circuit Steady-state response (aka. forced response)
Response that persists long after transient has decayed
[Source: Berkeley]
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29. RC and RL Circuits + – vs C R i
A circuit that contains only sources, resistors and a capacitor is called an RC circuit.
A circuit that contains only sources, resistors and an inductor is called an RL circuit. + – vs L R i
[Source: Berkeley]
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30. Natural Response Capacitor voltage cannot change instantaneously
RC Circuit
Capacitor voltage cannot change instantaneously
In steady state, a capacitor behaves like an open circuit
RL Circuit
Inductor current cannot change instantaneously
In steady state, an inductor behaves like a short circuit. + v – R C R i L
[Source: Berkeley]

31. Natural Response of a Charged Capacitor
(a) t = 0− is the instant just before the switch is moved from terminal 1 to terminal 2;
(b) t = 0 is the instant just after it was moved, t = 0 is synonymous with t = 0+.
[Source: Berkeley]
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32. Natural Response of RC Time constant: 𝜏=𝑅𝐶 Lecture 5
[Source: Berkeley]
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33. Time Constant 𝜏 (=𝑹𝑪)
A circuit with a small time constant has a fast response and vice versa.
[Source: Berkeley]
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34. Example
In the circuit below, let 𝑣 𝐶 0 =15V. Find 𝑣 𝐶 , 𝑣 𝑥 , and 𝑖 𝑥 for 𝑡>0.
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35. Natural Response of the RL Circuit
[Source: Berkeley]
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36. Example
The switch in the circuit below has been closed for a long time. At 𝑡=0, the switch is opened. Calculate 𝑖(𝑡) for 𝑡>0.
When 𝑡<0
When 𝑡>0
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37. Natural Response Summary
RC Circuit
Capacitor voltage cannot change instantaneously
time constant
RL Circuit
Inductor current cannot change instantaneously
time constant + v – R C R i L
[Source: Berkeley]

38. Step Response of RC Circuit
When a DC source is suddenly applied to a RC circuit, the source can be modeled as a step function.
The circuit response is known as the step response.
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39. The Unit Step u(t)
A step function is one that maintains a constant value before a certain time and then changes to another constant afterwards.
switching time may be shifted to 𝑡= 𝑡 0 by
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40. Equivalent Circuit of Unit Step
The unit step function has an equivalent circuit to represent when it is used to switch on a source.
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41. Example Express the voltage pulse below in terms of the unit step.
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42. Step Response of the RC Circuit
𝑣 0 − =𝑣 = 𝑣 0
Write down the Eq
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43. Step Response of the RC Circuit
This is known as the complete response, or total response.
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44. 𝑣 𝑡 = 𝑣 ∞ steady 𝑣 𝑠𝑠 + 𝑣 0 −𝑣(∞) 𝑒 −𝑡/𝜏 transient 𝑣 𝑡
Another Perspective
Another way to look at the response is to break it up into the transient response and the steady state response:
𝑣 𝑡 = 𝑣 ∞ steady 𝑣 𝑠𝑠 + 𝑣 0 −𝑣(∞) 𝑒 −𝑡/𝜏 transient 𝑣 𝑡
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45. Example
The switch has been in position A for a long time. At 𝑡=0, the switch moves to B. Find 𝑣(𝑡).
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46. Step Response of the RL Circuit
We will use the transient and steady state response approach.
We know that the transient response will be an exponential:
After a sufficiently long time, the current
will reach the steady state:
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47. Step Response of RL Circuit
This yields an overall response of:
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48. Example
Find 𝑖(𝑡) in the circuit for 𝑡>0. Assume that the switch has been closed for a long time.
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49. General Procedure for Finding RC/RL Response
Identify the variable of interest
For RL circuits, it is usually the inductor current iL(t).
For RC circuits, it is usually the capacitor voltage vc(t).
Determine the initial value (at t = t0- and t0+) of the variable
Recall that iL(t) and vc(t) are continuous variables:
iL(t0+) = iL(t0) and vc(t0+) = vc(t0)
Assuming that the circuit reached steady state before t0 , use the fact that an inductor behaves like a short circuit in steady state or that a capacitor behaves like an open circuit in steady state.
[Source: Berkeley]
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50. Procedure (cont’d)
Calculate the final value of the variable (its value as t  ∞)
Again, make use of the fact that an inductor behaves like a short circuit in steady state (t  ∞) or that a capacitor behaves like an open circuit in steady state (t  ∞).
Calculate the time constant for the circuit
t = L/R for an RL circuit, where R is the Thévenin equivalent resistance “seen” by the inductor.
t = RC for an RC circuit where R is the Thévenin equivalent resistance “seen” by the capacitor.
[Source: Berkeley]
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51. Response Form of Basic First-Order Circuits
[Source: Berkeley]
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52. New Content for the Discussion Session
Mutual inductance
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