8/18/2005
Example_Analysis of a Complex Diode Circuit.doc
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Example: Analysis of a
Complex Diode Circuit
Consider this circuit with two ideal diodes:
10.0 V
+ vDi 2
R1 =10K
+
iDi 1
vDi 1
−
iDi 2
R2 =5K
−
-10.0 V
Let’s analyze this circuit and find vDi 1 , iDi 1 , vDi 2 , and iDi 2 !
Remember, we must accomplish each of the five steps:
Step 1: ASSUME that both D1 and D2 are “on” (might as well!).
Step 2: ENFORCE the equalities vDi 1 = 0 = vDi 2 , by replacing
each ideal diode with a short circuit.
Jim Stiles
The Univ. of Kansas
Dept. of EECS
8/18/2005
Example_Analysis of a Complex Diode Circuit.doc
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Step 3: ANALYZE the resulting circuit, and find iDi 1 and iDi 2 .
+ vDi 2 =0 −
R1 =10K
10.0 V
i
i
i
D1
+
vDi 1 =0
iDi 2
R2 =5K
−
-10.0 V
Begin with KCL:
i = iDi 1 + iDi 2
where
i =
and
iDi 2 =
Therefore,
iDi 1 =
Jim Stiles
The Univ. of Kansas
Dept. of EECS
8/18/2005
Example_Analysis of a Complex Diode Circuit.doc
3/5
Step 4: Now we must CHECK inequalities to see if our
assumptions are correct!
iDi 1 =
iDi 2 =
One assumption is therefore INCORRECT. We must proceed to
step 5—change our assumptions and completely start again!
Q: Wait a second! We don’t
have to completely start from
the beginning, do we? After all,
our assumption about diode D2
turned out to be true—so we
and
already know that iDi 2 =
vDi 2 = 0 , right?
A: NO! The solution for diode D2 is dependent on the state of
both diodes D1 and D2. If the assumption of just one diode
turns out to be incorrect, then the solutions for all diodes are
wrong!
So, let’s change our assumption and start all over again!
Jim Stiles
The Univ. of Kansas
Dept. of EECS
8/18/2005
Example_Analysis of a Complex Diode Circuit.doc
4/5
Step 1: Now ASSUME that D1 is “off” and D2 is “on”.
Step 2: ENFORCE iDi 1 = 0 (D1 open) and vDi 2 = 0 (D2 short).
Step 3: ANALYZE resulting circuit, and find vDi 1 and iDi 2 .
10.0 V
+ vDi 2 =0 −
R1 =10K
i
+
iDi 1 =0
iDi 2
vDi 1
R2 =5K
−
-10.0 V
Note i = iDi 2 =
and from KVL:
∴ vDi =
Jim Stiles
The Univ. of Kansas
Dept. of EECS
8/18/2005
Example_Analysis of a Complex Diode Circuit.doc
5/5
4) CHECK our assumptions.
iDi 2 =
vDi 1 =
∴ Assumptions are correct! We are finished!
Jim Stiles
vDi 1 =
iDi 1 = 0
vDi 2 = 0
iDi 2 =
The Univ. of Kansas
Dept. of EECS