Diodes & Rectifiers: Homework Solutions

[Cocoleia]

Homework Statement

I am practicing problems like this:

where Vs=10V, R=2kohm, they say it is like a battery + ideal diode with VD=0.7V
I have to sketch the transfer characteristic Vo=f(Vs) and draw the waveform of Vo(t)

Homework Equations

The Attempt at a Solution

I am really unsure how to go about these problems. I have the answer, I just don't know they got to it. Can someone explain please?

I am also unsure of how to draw the waveform, but I think it could be something like:

Is this correct?

[/B]

 

[mfb]

V_I = V_s?

Where did you take the voltage drop at the diode into account?

The problem statement asks for V₀(t), not V_D(t).

 

[Cocoleia]

V_I = V_s?

Where did you take the voltage drop at the diode into account?

The problem statement asks for V₀(t), not V_D(t).

I do not understand, I just found this graph in my textbook. Can you explain even the first part, since it too is just a drawing from the book.

 

[mfb]

Your first plot has an axis label V_I. What is that?
Where did you find that graph? The symbols should be introduced somewhere.

The diode won't conduct until there is a 0.7 V potential difference between its sides.

 

[Cocoleia]

Your first plot has an axis label V_I. What is that?
Where did you find that graph? The symbols should be introduced somewhere.

The diode won't conduct until there is a 0.7 V potential difference between its sides.

V_I is supposed to be Vs, sorry the example in the book had different variables than my question. I don't understand what a transfer characteristics is. I think if it is forward then you put a short circuit and if it is reverse you put an open circuit? But I don't know how that translates on the graph

 

[cnh1995]

V_I is supposed to be Vs, sorry the example in the book had different variables than my question. I don't understand what a transfer characteristics is. I think if it is forward then you put a short circuit and if it is reverse you put an open circuit? But I don't know how that translates on the graph

Is your voltage source ac or dc? You have shown it as dc but you have drawn the waveforms assuming it to be ac.

 

[Cocoleia]

Is your voltage source ac or dc? You have shown it as dc but you have drawn the waveforms assuming it to be ac.

I don't know, I guess it is DC?

 

[cnh1995]

I don't know, I guess it is DC?

Well, go with dc since you have shown it that way.
So, your dc source is variable, say from 0V to 10V. The diode drop is assumed to be 0.7V. What can you say about the i-v characteristic of the diode from this? Can you plot the diode voltage first, as a function of the input dc voltage?

 

[Cocoleia]

Well, go with dc since you have shown it that way.
So, your dc source is variable, say from 0V to 10V. The diode drop is assumed to be 0.7V. What can you say about the i-v characteristic of the diode from this? Can you plot the diode voltage first, as a function of the input dc voltage?

Would it be forward biased and therefore a short circuit ? I'm not sure about the plot

 

[cnh1995]

they say it is like a battery + ideal diode with VD=0.7V

Now for input voltage V_s≤0.7V, what is the diode voltage? For V_s>0.7V, what is the diode voltage?

 

[Cocoleia]

View attachment 111968
Now for input voltage V_s≤0.7V, what is the diode voltage? For V_s>0.7V, what is the diode voltage?

When it is less, it is 0. When it is more it is 0.7?

 

[cnh1995]

When it is less, it is 0. When it is more it is 0.7?

We are talking about "voltage" across the diode. If Vs=0.5V, what is the voltage across the diode? Use the above graph. You are right about Vs>0.7V.

I don't know, I guess it is DC?

draw the waveform of Vo(t)

It seems that the input is supposed to be ac, since the output voltage is a function of time. With dc input, you can draw the transfer characteristic but I don't think they are expecting you to draw V_o(t) for dc input.

 

[Cocoleia]

We are talking about "voltage" across the diode. If Vs=0.5V, what is the voltage across the diode? Use the above graph. You are right about Vs>0.7V.It seems that the input is supposed to be ac, since the output voltage is a function of time. With dc input, you can draw the transfer characteristic but I don't think they are expecting you to draw V_o(t) for dc input.

I am unsure on how to draw transfer characteristics and the V_o(t), it was never explained to me in class.

 

[cnh1995]

I am unsure on how to draw transfer characteristics and the V_o(t), it was never explained to me in class.

Ok. We can work that out here. We will take the voltage source as AC, with magnitude 10V.

You are right about Vs>0.7V.
What is the volage across the diode when Vs<=0.7V? Refer the graph in #10. Once you know that, you can draw the transfer characteristic easliy.

Or answer this: If Vs<=0.7V, what is the voltage across the 2k resistor?

 

[mfb]

V_I is supposed to be Vs, sorry the example in the book had different variables than my question. I don't understand what a transfer characteristics is. I think if it is forward then you put a short circuit and if it is reverse you put an open circuit? But I don't know how that translates on the graph

There is still the resistor.

At V₀=-10 V, analyze the circuit. What is the voltage at the resistor?
At V₀=-5 V, analyze the circuit. What is the voltage at the resistor?
At V₀=0 V, analyze the circuit. What is the voltage at the resistor?
At V₀=+5 V, analyze the circuit. What is the voltage at the resistor?
At V₀=+10 V, analyze the circuit. What is the voltage at the resistor?
With those values (and suitable intermediate values), you can plot the voltage at the resistor as function of the source voltage. That is the curve you need in the first part.

I would expect your voltage source to be AC, otherwise plotting things as function of time is useless.

 

[Cocoleia]

Ok. We can work that out here. We will take the voltage source as AC, with magnitude 10V.

You are right about Vs>0.7V.
What is the volage across the diode when Vs<=0.7V? Refer the graph in #10. Once you know that, you can draw the transfer characteristic easliy.

Or answer this: If Vs<=0.7V, what is the voltage across the 2k resistor?

There is still the resistor.

At V₀=-10 V, analyze the circuit. What is the voltage at the resistor?
At V₀=-5 V, analyze the circuit. What is the voltage at the resistor?
At V₀=0 V, analyze the circuit. What is the voltage at the resistor?
At V₀=+5 V, analyze the circuit. What is the voltage at the resistor?
At V₀=+10 V, analyze the circuit. What is the voltage at the resistor?
With those values (and suitable intermediate values), you can plot the voltage at the resistor as function of the source voltage. That is the curve you need in the first part.

I would expect your voltage source to be AC, otherwise plotting things as function of time is useless.

Ok thank you, I understand now.