Diodes Current–voltage characteristic

[du.art]

Homework Statement

http://img39.imageshack.us/img39/7496/59187398.th.jpg
V_DON=0,7
First it is asked to calculate V₀ for V_in=-1V and V_in=-5V.
Secondly it is asked to draw V₀(V_in) for -10<V_in<10

Homework Equations

The Attempt at a Solution

Using Kirchhoff Volt Law I've established that for V_in=-1V D1 is ON and D2 is OFF and for V_in=-5V D1 is ON and D2 is ON an I believe I got it right.
I have also determined that D1 conducts for a V_in< -0.7V and that D2 needs V_in<-1.4V. Right now I'm stuck at linking their current–voltage characteristic and determining the slope value, which depends somehow of the resistance values, so that I can build the V0(V_in) graph

 

[Redbelly98]

Homework Statement

http://img39.imageshack.us/img39/7496/59187398.th.jpg
V_DON=0,7
First it is asked to calculate V₀ for V_in=-1V and V_in=-5V.
Secondly it is asked to draw V₀(V_in) for -10<V_in<10

Homework Equations

The Attempt at a Solution

Using Kirchhoff Volt Law I've established that for V_in=-1V D1 is ON and D2 is OFF and for V_in=-5V D1 is ON and D2 is ON an I believe I got it right.

I agree.

I have also determined that D1 conducts for a V_in< -0.7V...

Yes.

... and that D2 needs V_in<-1.4V.

Not quite. Think of it this way:
With D2 right at the "on/off" threshold, it has 0.7V and 0 current. That means the current through the 1kΩ is ____?
And therefore the current through the 4kΩ is _____?
So the voltage across the 4 kΩ is ______?
And Vin would equal _____?

Right now I'm stuck at linking their current–voltage characteristic and determining the slope value, which depends somehow of the resistance values, so that I can build the V0(V_in) graph

You can replace each diode with either an open circuit, or a 0.7V source, depending on whether it is off or on, respectively.

 

[du.art]

Not quite. Think of it this way:
With D2 right at the "on/off" threshold, it has 0.7V and 0 current. That means the current through the 1kΩ is ____?
And therefore the current through the 4kΩ is _____?
So the voltage across the 4 kΩ is ______?
And Vin would equal _____?

With D2 right at the "on/off" threshold, it has 0.7V and 0 current. That means the current through the 1kΩ is 0.7mA
And therefore the current through the 4kΩ is 0.7mA
So the voltage across the 4 kΩ is 2.8V
And Vin would equal 2.8V

Am I right?

You can replace each diode with either an open circuit, or a 0.7V source, depending on whether it is off or on, respectively.

I can do 1 graph for each diode. My problem is when I try to put them together in order to represent the whole circuit nothing that I do makes enough meaning for me to call it an answer to the problem
If the diodes were in parallel I knew how to solve it or even if they were Zener's. But with them connected in series I'm not making any sense of this.

From what I can tell I'm getting a positive diode clipper similar to this one http://www.circuitstoday.com/wp-content/uploads/2009/10/Output-Waveform-Positive-Clipper-and-Negative-Clipper-300x123.jpg" (graph on the left) but with its max V_out at -0,7V? Is that it? But wouldn't I get the same with D1 only?

 

[Redbelly98]

With D2 right at the "on/off" threshold, it has 0.7V and 0 current. That means the current through the 1kΩ is 0.7mA
And therefore the current through the 4kΩ is 0.7mA
So the voltage across the 4 kΩ is 2.8V

So far so good.

And Vin would equal 2.8V

Am I right?

There are two problems with this last step.
1. Vin is not equal to the voltage across the 4kΩ resistor. Rather, it is the sum of the voltages across the 4kΩ, D2, and D1.
2. Keep in mind the polarity: which end of D2 has a higher potential at the on/off threshhold, and therefore in what direction is current flowing through the resistors?