Diode numerical (calculation of current and voltage across diode)

[lazyaditya]

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.

 

[NascentOxygen]

The formula ηV_T / I_d 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 I_d.

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).

 

[lazyaditya]

Sorry for inconvenience.

 

[lazyaditya]

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

 

[NascentOxygen]

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

Yes, that should work out.

Did you get the right answer for 3.1.33?

 

[lazyaditya]

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 ?

 

[lazyaditya]

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

 

[NascentOxygen]

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 ?

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.

 

[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.

 

[lazyaditya]

Thank you for help. Problem solved :)