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I need to calculate the circuit below. (The used diode in the schematics is not the same from the task we have to solve. In our task we work with a ideal diode.)

schematic

simulate this circuit – Schematic created using CircuitLab

In school we learned, that a ideal diode could be substituted with a normal voltage source with the forward voltage of the diode (in our case 0.7V). Now I am not sure how to solve this task and how would a general approach on solving such problems look like?

According to the solution, the voltage meter should show V = 3.975V and I = 198.75uA

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  • \$\begingroup\$ Are you positive you're supposed to use an "ideal diode"? Why do you talk about a forward voltage? \$\endgroup\$ – Dan Laks Apr 2 '15 at 7:27
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Since it is clear that the diode is forward biased, it acts as a short circuit but with a constant voltage drop. So you can replace the diode with a 0.7V and the circuit can be redrawn as shown.

schematic

simulate this circuit – Schematic created using CircuitLab

Now it is just a matter of writing and solving two linear equations.

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Let the voltage at the junction of R1 and R2 be V, then apply KCL to this junction. Hint: the current through R3 is (V-0.7)/20k

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  • \$\begingroup\$ Chu, just a suggestion. In your equation, the denominator "20 [ma]", looks like you're saying to divide by 20mA. I know that you're really just indicating that the answer will be in mA, but it's a little confusing at first glance. \$\endgroup\$ – Dan Laks Apr 2 '15 at 7:43

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