Diode Circuits: Find Q-Point for D1

[Gotejjeken]

Homework Statement

For the figure below, use the ideal diode model and the constant voltage drop diode model (assume a turn on voltage of 0.6V) to find Q-point for diode D1:

[PLAIN]http://gotejjeken.webs.com/ckt.jpg

Homework Equations

KCL/KVL

The Attempt at a Solution

For the ideal model, I assumed the diode was on, and thus acted as a wire. Doing that, I get a Q-point of (0V, 1mA), as there is 1mA flowing into the node from the 10V source and 2mA flowing out of the node from the 20V source, so there must be 1mA coming in through the diode branch.

For the CVD model, I get a Q-point of (0V, 0.88mA), as there is 1.06mA coming in from the 10V source, and 1.94mA flowing out through the 20V source, leaving 0.88mA to flow in through the 0.6V source.

I am a bit concerned about my definition of the I3 current (I1 and I2 are defined by the problem). If I define it flowing out of the node instead of in, the current is negative, which doesn't make sense for either model of the diode. If I assume the diode is off, I get a positive voltage across it with no current, which also doesn't make sense.

I always get confused with circuits that have multiple sources like this, can anyone check my reasoning over and see if it is sound?

 

[KataKoniK]

Your reasoning seems sound and your calculations are correct. However, one thing to keep in mind is that the direction of current flow through the diode will depend on the polarity of the voltage across it. In this case, since the voltage across the diode is positive, the current will flow in the direction you have defined (out of the node for I3). If the voltage across the diode were negative, the current would flow in the opposite direction. So, it is important to consider the polarity of the voltage when defining the direction of current flow through a diode. Other than that, your approach and calculations seem correct.