Th

1
Thévenin's Theorem (4.3)

• Prof. Phillips
• February 24, 2003

2
Thevenins Theorem

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

3
Implications

• We use 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.

4
Independent Sources (Thevenin)
RTh

Voc
Circuit with independent sources
Thevenin equivalent circuit
5
No Independent Sources
RTh
Circuit without independent sources
Thevenin equivalent circuit
6
Example CE Amplifier
10V
1kW

Vin

2kW
Vo

7
Small Signal Equivalent
Ib

2kW
1kW
50W

Vin
Vo
100Ib

8
Thevenin Equivalent _at_ Output
Ib

2kW
1kW
50W

Vin
Vo
100Ib
-
RTh


Voc
Vo

9
Computing Thevenin Equivalent

• Basic steps to determining Thevenin equivalent
  are
• Find voc
• Find RTh ( voc / isc)

10
Thevenin/Norton Analysis

• 1. Pick a good breaking point in the circuit
  (cannot split a dependent source and its control
  variable).
• 2. Thevenin Compute the open circuit voltage,
  VOC.
• Norton Compute the short circuit current,
  ISC.
• For case 3(b) both VOC0 and ISC0 so skip step
  2

11
Thevenin/Norton Analysis

• 3. Compute the Thevenin equivalent resistance,
  RTh.
• (a) If there are only independent
  sources, then short circuit all the voltage
  sources and open circuit the current sources
  (just like superposition).
• (b) If there are only dependent sources,
  then must use a test voltage or current source in
  order to calculate
• RTh VTest/Itest
• (c) If there are both independent and
  dependent sources, then compute RTh from VOC/ISC.

12
Thevenin/Norton Analysis

• 4. Thevenin Replace circuit with VOC in series
  with RTh.
• Norton Replace circuit with ISC in parallel
  with RTh.
• Note for 3(b) the equivalent network is merely
  RTh, that is, no voltage (or current) source.
• Only steps 2 4 differ from Thevenin Norton!

13
Class Examples