Ch. 4 – Circuit Theorems Linearity - Linear relationship between cause and effect Linear Circuit - output is linearly related (proportional to) the input. Ex. = Ri Homogeneity (scaling) B. Additivity
Find the current in resistor R2.
Superposition The response of a circuit due to multiple sources can be taken as the sum of the effects of each source acting alone (all others off) Turning Off Sources: Vs replace with short circuit Is replace with open circuit Note: Only independent sources can be turned off. Dependent sources should be left on.
Example 1. Apply superposition to find the current in the 3- resistor. Ans: 5/6 A
Example 2. Find the terminal voltage Vab using superposition. Ans: 6 V
Source Transformation - Should be equivalent for any load attached to a-b. Extreme cases:
For an arbitrary RL:
For Independent Sources: For Dependent Sources:
Example 3. Find the current in RL=8 using source transformation.
Example 4. Find vx using source transformation.
Given the circuit. Find the v-i characteristic of the circuit as seen by RL. a) Remove RL (replace by open terminals a-b)
b) Determine (“measure”) the open-circuit voltage vab.
c) Connect a voltage source Vnew equal to vab at b. Vnew = 16 V Vab = 16 V Vaa’ = 0 V
d. Reconnect RL. What is the current through RL? Vaa’ = 0 V IRL = 0 A
Use superposition: IRL = Iorig + IVnew only = 0 Iorig = -IVnew only = I(-Vnew) Current in the original circuit = current produced by Voc = -Vnew
Thevenin’s Theorem A linear two-terminal circuit can be replaced by an equivalent circuit consisting of a voltage source VTh in series with a resistor RTh, where: VTh = Voc = open-circuit voltage at the terminals RTh = equivalent resistance at the terminals with all the independent sources turned off.
Example 5. Find the Thevenin equivalent circuit with respect to terminals a-b. Find the current through RL = 6, 16, and 36 . Ans: 30V, 4
Thevenin Equiv. Circuit: with dependent sources VTh = Voc = open-circuit voltage at the terminals RTh = equivalent resistance at the terminals with all the independent sources turned off. Finding RTh: Turn off all independent sources. Apply either: A test voltage source Vo Find Io. A test current source Io Find Vo. c) RTh = Vo/Io Note: Easier to use test voltage or test current of value ONE.
Example 6. Find the Thevenin equivalent circuit.
Norton’s Theorem - A corollary of to Thevenin’s Theorem RN = RTh, and IN = VTh/Rth Finding Norton Current:
Norton Equivalent Circuit A linear two-terminal circuit can be replaced by an equivalent circuit consisting of a current source IN in parallel with a resistor RN, where: IN = Isc = short-circuit current through the terminals RN = equivalent resistance at the terminals with all the independent sources turned off.
Example 7. Find the Norton equivalent circuit. Answer: 1A, 4 ohms
Maximum Power Transfer Find the value of the load resistor that will receive maximum power from the circuit.