Download Some hints regarding Thevenin Equivalents and Circuits with

Some hints regarding Thevenin Equivalents and Circuits with Dependent Sources
In the online HW problems for Thevenin Equivalent Circuits, you have two circuits that
are shown below.
Circuit A
Circuit B
In order to find RTH, one approach is to find VTH = VOC and ISC. However, if you do the
analysis (either nodal or mesh), you find VOC = 0V and ISC = 0A (because there are no
independent sources of power in the circuit, only a dependent voltage source).
Normally, one can find RTH by using RTH = (VOC / ISC ). However, what is the value of
0V/0A ??? The answer: we do not know.
(RTH is NOT equal to 2R || R. You can only zero out independent sources.)
A different approach is needed. RTH is the resistance seen looking back into the circuit
when all independent sources are zeroed out (exactly the case for the circuits above).
So, one technique is to apply a known voltage (or current) source at terminals A-B and
calculate the corresponding current (or voltage) demanded. The, RTH is the
voltage/current ratio of these two values.
For example, in circuit A, apply a 1mA source across points A-B, which forces Ib to
equal 1mA. Now that Ib is known the value of the dependent voltage source is known
(AIb). Use basic circuit techniques (Ohm’s Law, KVL and KCL) to find VS, the voltage
across the 1mA source. Then RTH = VS/1mA.
A similar approach can be applied to circuit B, but it is more convenient to apply a
known voltage source (i.e., 1V) and determine the current demanded from this source.