# Ideal Diode Problems

#### tquiva

Joined Oct 19, 2010
176
Could someone please assist me w/ two of the following circuit analysis problems on ideal diodes.

I know the following steps are as follows:

(1) Make a plausible assumption (Reverse bias or forward bias)
(2) Proceed w/ analysis
(3) Check if assumption from (1) is consistent

The first problem is: For this problem, I'm not sure whether or not my approach to determine V_A is correct. Could someone please confirm this with me?

The second problem is: For this problem, I assumed both diodes D1 & D2 are in forward biased. In determining I2, I had a nodal equation at B and got 0.67mA. Is this correct? Or since there is a voltage after D2, is this a reverse bias?

Many thanks in advance to anyone who is able to help me out on these problem.

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#### t_n_k

Joined Mar 6, 2009
5,455
One should first make the point that points A & B, being directly connected, are at the same potential. I am also assuming the reference potential is ground.

For case (c) D1 is indeed reverse biased and non-conducting. The same current therefore flows in D2 and both resistors. As you have correctly shown the current is 0.33mA. However VA=VB=5k*0.33mA=1.67V rather than 3.3V.

For case (e) I'm reading the schematic as showing the 10kΩ tail is tied to -5V. Hence, if D1 was initially assumed to be reverse biased then one would have ....

VA=VB=10V*10k/15k-5V=1.67V which would make the assumption that D1 is reversed biased incorrect. Hence we must assume D1 is forward biased making VA=AB=0V and therefore ...

Voltage drop across the 5k = 5V.
Current in 5k=1mA

Voltage drop across the 10k = 5V
Current in D2 and 10k = 0.5mA

Current in D1=1mA-0.5mA=0.5mA

#### tquiva

Joined Oct 19, 2010
176
VA=VB=10V*10k/15k-5V=1.67V which would make the assumption that D1 is reversed biased incorrect.
Thank you so much!

However, I get kind of lost on instances as these. What characteristic of 1.67V would deem the assumption of reverse bias incorrect? For this case, or any case... how would I use such a voltage value to determine any initially made assumptions?

#### t_n_k

Joined Mar 6, 2009
5,455
This is precisely an example of what you stated in post #1 .....

(1) Make a plausible assumption (Reverse bias or forward bias)
(2) Proceed w/ analysis
(3) Check if assumption from (1) is consistent

(1) Plausible assumption - D1 is reverse biased
(2) Proceed with analysis - If D1 is not conducting then the only possible current path is from +5V via 5K, D2 and 10k to -5V. If this were the case one can show VA=VB=1.67V relative to ground [0V].
(3) Check if assumption in (1) is consistent - No it's not. If D1 cathode is tied to ground (0V) then it's implausible that D1 anode could be at 1.67V. The initial assumption is incorrect. If we found that D1 anode was at -1.67V then that would be consistent with the initial assumption. For instance, if we swapped the 5k and the 10k positions then that would be exactly the case and the initial assumption would be plausible/consistent.

#### tquiva

Joined Oct 19, 2010
176
Thank you, that makes sense now.

I am given a problem similar to that of (e).
I applied the same approach as in (e), but received a negative current for I1. I'm not so sure if this is correct. Did I do it right?

#### t_n_k

Joined Mar 6, 2009
5,455
If you (correctly) assume D1 is conducting then VA=VB=0. This means the voltage drop across D2 & 5k in series (in parallel with D1) would also be 0V. Hence no current would flow in D2 & the 5k.

#### tquiva

Joined Oct 19, 2010
176
Ok, I see that now.

I have:
V_10k=5V
I_10k=5V/10k=0.5mA

I2=0

With the given information, I have the following nodal equation at node A:
I1+0=0.5mA
Therefore, I1=0.5mA

Is this correct now?