Thevenins Theorem

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Thevenins Theorem
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Thevenins Theorem

• Any circuit with sources (dependent and/or
  independent) and resistors can be replaced by an
  equivalent circuit containing a single source and
  a single resistor.
• Thevenins theorem implies that we can replace
  arbitrarily complicated networks with simple
  networks for purposes of analysis.

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Implications

• We Thevenins theorem to justify the concept of
  input and output resistance for amplifier
  circuits.
• We model transducers as equivalent sources and
  resistances.
• We model stereo speakers as an equivalent
  resistance.

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TT Independent Sources
Circuit with independent sources
Thevenin equivalent circuit
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TT No Independent Sources
RTh
Circuit without independent sources
Thevenin equivalent circuit
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Finding the Thevenin Equivalent

• I-V characteristics are straight lines (a
  consequence of the linearity of resistor models).
• Circuits with independent sources
• Intercepts are voc(t) and isc(t)
• Circuits w/o independent sources
• Slope is RTh

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Independent Sources
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Example CE Amplifier
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Small Signal Equivalent
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Thevenin Equivalent _at_ Output

Vo
-
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Computing Thevenin Equivalent

• Find voc
• Find isc
• Compute RTh

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Find voc
Ib
V1

2kW
1kW
50W

Vin
Voc
100Ib
-
-
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Nodal Analysis
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Solve for Voc

• Voc Vin

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Find Isc
Ib
V1
2kW
1kW
50W

Vin
Isc
100Ib
-
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Computing Isc
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Thevenin Equivalent