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Published byBenedict Rich Modified over 8 years ago
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1 Module 10 Thévenin Equivalent Circuits with Dependent Sources Up until now… + _ Port Resistors and Independent Sources Only
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2 Review: Find the Thévenin Equivalent Find Open Circuit Voltage V Th + _ V Th
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3 Set sources to zero R Th Find net resistance R Th seen at the port + _
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4 What happens when circuit has a Dependent Source? iBiB iB iB + _ RBRB v1v1 RERE Port Set to zero (short ckt) Do NOT set to zero Example
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5 V Th Dependent Source: Must Use “Test Source” Method to find R Th Rationale: + _ Set to zero (short ckt) + _ V test I test R Th V test R Th = Test source
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6 In the Test Source method: V test I test = R Th + _ V test = V test / R Th R Th Method B I test Method A + _ V test = I test R Th I test
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7 + _ V test Apply to Our Circuit iBiB iB iB + _ RBRB v1v1 RERE Circuit with independent source set to zero I test = ? Solve the circuit for I test Compute V test / I test to find R Th
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8 iBiB iB iB + _ RBRB v1v1 RERE Apply KVL: + _ V test I test = ? V test + _ + _ R B – i B =
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9 + _ RBRB RERE + _ V test I test = ? V test R B V test R B Apply KCL: KCL I test = V test R B V test R B + V test R E + V test R E 1 R B RB RB + 1 R E + = V test
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10 Solve for R Th = V test / I test + _ RBRB RERE + _ V test I test V test R B V test R B V test R E I test = 1 R B RB RB + 1 R E + V test I test 1 R B RB RB + 1 R E + V test =
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11 1 R B RB RB + 1 R E + 1 Let’s look at the denominator… 1 R E + 1 R B + 1 RERE + 1 = Back to the main problem… Resistors in Parallel R 1 ||R 2 = R1R2R1R2 R 1 + R 2 = 1R11R1 1R21R2 + 1
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12 Solve for R Th = V test / I test + _ RBRB RERE + _ V test I test V test R B V test R B V test R E I test 1 R B RB RB + 1 R E + V test = = I test RERE R B + 1 = V test I test RERE R B + 1 R Th =
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13 What happens if we erroneously set Dependent Source to zero? + _ RBRB RERE + _ V test I test V test R B V test R E = V test I test RERE R B R Th = Wrong! X But why?
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14 + _ RBRB RERE + _ V test I test V test R B V test R E V test + _ iB iB V test sets the voltage across R B V test across R B sets the value of i B i B sets the value of the dependent source
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15 Another Example iBiB iB iB RBRB RERE + _ V1V1 RCRC Find the Thévenin Equivalent Resistance of this circuit: Set the independent source to zero Apply a test current source Port I test
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16 i B = I test iB iB RBRB RERE RCRC I test I test + I test + I test R B – + I test R E – ( +1) + V test = I test R B + ( +1)R E – = V test I test RBRB + ( + 1)R E R Th = Done! pause
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17 End of This Module Don’t forget to do the homework!
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