# Circuit Analysis with an ideal diode

I'am trying to solve this problem using superposition but can't seem to get the right answer.

The problem is to look for the voltage in R3 in the following circuit:

• The V1 graph is green
• The R3 voltage graph is red.
• (The red graph, R3's voltage, goes up to about 11V and stays there until the .5ms and then goes down to at about -71V)

I used LTSpice to find the voltage in R3 and it came out to be this:

Could someone tell me what technique to use and how to start please.

Edit 1: It was hard to see the R3 voltage graph colored blue, so changed it to red.

• Superposition does not hold true since that's not a linear circuit. Could be used to Theveninize everything but the diode, but in this case is not convenient since you would loose output quantity Vout. – carloc Sep 5 '16 at 6:28

Depending whether the diode is in the conducting or non-conducting state, your circuit could have two different equivalent circuits:

simulate this circuit – Schematic created using CircuitLab

A is the equivalent circuit when the diode is conductiong. B is the equivalent circuit when the diode is not conducting.

You should be able to see that solving Vout is trivial in case A.

In case B you can solve using mesh analysis, or using modified node analysis. Superposition could be applied, but is not needed.

Then you must examine the solution for case B and find any situations where the diode would not be biased correctly to be non-conducting, and replace the solution at those points in time with the solution from case A.

• Could you explain me again what you meant in the last paragraph? In circuit A the voltage in R3 is 10V, right? – laco13 Sep 5 '16 at 16:14
• Yes. Now solve circuit B. Anywhere where R30 has more than 10 V across it, that's a case where the preconditions for circuit B to model the nonlinear circuit are not valid, so you need to use circuit A as the model instead. – The Photon Sep 5 '16 at 16:21
• What do you mean to use circuit A as a model for the circuit B? Sorry if I'm not understanding it, but I'm just starting to use diodes. – laco13 Sep 5 '16 at 16:55
• Circuit A and circuit B are two models for your original nonlinear circuit. – The Photon Sep 5 '16 at 16:56

Let's call the node the anode of the diode is connected to "node A" and the node the cathode is connected to "node C" and the voltages of those nodes $v_A$ and $v_C$ respectively.

Now look at the circuit without the diode and find out what V1 needs to be for

• case1: $v_A > v_C$
• case2: $v_A < v_C$

Now do circuit analysis for each of the two cases where in

• case1: diode is replaced by a short between node A and node C
• case2: diode is removed (no connection between node A and node C)

Case1 is the part where the red graph is flat.
Case2 is the part where the red graph is curved.

Solve for threshold Vt of V1, when I(D1)=V(D1)=0 then loops are isolated.

When V1 > Vt is when diode conducts, then Vout=V2

and for V1 < Vt diode is not conducting and solve for Vout.

• What do you mean to solve for the threshold? When I tried to solve for Vout when the diode is not conducting, I tried using mesh analysis but couldn't get the answer. – laco13 Sep 5 '16 at 5:00
• ... cut D1 out of the loop and can you solve the Vout for Vin=-200? Then compute V1 when Vout =V2 using same ratio for currents in each loop – Tony Stewart Sunnyskyguy EE75 Sep 5 '16 at 5:42