# Homework Help: Diode numerical (calculation of current and voltage across diode)

1. Sep 21, 2014

The Diode in the circuit shown below has the non linear terminal characteristics as shown in the figure.Let the voltage be "coswt" V . Question 3.1.33 and my attempt is shown in the figure.

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2. Sep 23, 2014

### Staff: Mentor

The formula ηVT / Id is the slope of a Si diode's exponential V-I characteristic, and gives the Silicon diode's small signal resistance. You don't have a Si diode for this question, you have some clunky diode whose V-I graph has a fixed slope regardless of Id.

I would much prefer that you typed out the text of a question, rather than forcing me to jockey back and forth between multiple graphics (and wasting my 3G data quota unnecessarily in the process).

Last edited: Sep 23, 2014
3. Sep 23, 2014

Sorry for inconvenience.

4. Sep 23, 2014

Can i calculate the voltage at output using superposition theorem . Considering dc voltage and ac voltage sources separately.

5. Sep 23, 2014

### Staff: Mentor

Yes, that should work out.

Did you get the right answer for 3.1.33?

6. Sep 23, 2014

No i didn't , i get very confused with this concept. When using superposition theorem and considering DC signal first i took cut off voltage and forward resistance of diode into consideration to get output voltage 1,when i considered AC signal then also should i take cut off voltage of diode into consideration ?

7. Sep 23, 2014

But i did understood that i shouldn't use dynamic resistance of diode in this case .

8. Sep 24, 2014

### Staff: Mentor

AC is synonymous with "small-signal" and also "incremental", it imposes what amounts to just a tiny wobble in the DC. So this small wobble does not see the 0.6 V cut-off, it sees just how the current increases for a tiny increase in voltage: the dynamic resistance or incremental resistance, and this is the slope of the diode's graph at the DC bias point.

Of course, you should from the outset confirm that the AC component really is small compared with the DC. There may be occasions when an examiner deliberately sets a trick question to catch anyone not fully awake.

9. Sep 26, 2014

### rude man

Since you're given multiple choice, you can proceed as follows:
Let Vin = 2 + 1 = 3V (highest input voltage)
Sum currents to zero at the anode
write expression for diode current assuming v > 0.5V. This should be obvious from the diagram.
Solve for diode current, then diode voltage.

10. Sep 27, 2014