# Diodes Circuit analysis

Discussion in 'Electronics Homework Help' started by ABCH, Nov 9, 2015.

1. ### ABCH

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0
Nov 9, 2015
Hi guys...
I got this assignment and I have no idea how to solve it.

the question is in the image in the attached file

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2. ### dorke

2,342
665
Jun 20, 2015
Look at the attached pic: looking at point X (=Vo) :
you need to ask, at what voltage at point X does D1 actually conducts?
1v +1ma*1kohm+0.7=2.7 v
if X is at 2.7v what is the state of D2 and D3 ?
The answer is they are cut-off(not conducting).
Now you ask what is the voltage at Vi
If Vi is greater than 5.7v you have :
Vo=(Vi-1-0.7)*1/4 +1+0.7=Vi/4 +1.275
at Vi=10v we get Vo=3.775v
look at the graph to complete the task you need to look at D2 and D3 and ask at what voltage do they conduct both and only one ( point Y).

Tha fios agaibh likes this.
3. ### Colin Mitchell

1,417
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Aug 31, 2014
Yes. You are correct. The diodes are reverse-biased and they are not conducting.

Last edited: Nov 17, 2015
4. ### Colin Mitchell

1,417
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Aug 31, 2014
The diodes are reverse-biased and are not conducting.

Last edited: Nov 17, 2015
5. ### dorke

2,342
665
Jun 20, 2015
Colin your analysis is totally wrong!

"Suppose we have no input voltage on Vi.
What is the state of the circuit?
The voltage-difference between top and bottom is 3v.
we will assume the diodes are conducting and dropping 0.7v"

Wrong!
the diodes are reversed bias and thus not conducting!

6. ### Ratch

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Mar 10, 2013
Sorry I did not get to this problem sooner, but I was on vacation.

It is assumed that the diodes will turn off if their voltage is less than 0.7 volts, or their current is less than 1 ma. Starting at -10 volts, it is easily observed that diode D1 will not conduct at this voltage, and D3 and D4 are conducting. That makes the output voltage -3.4 volts. At input voltage -8.5 diode D3 is current starved and turns off. Diode D2 is on until the input voltage is -6.7 volts, and it also turns off due to current starvation. All diodes are now off so the output voltage is the same as the input voltage. At input voltage +5.7 volts diode D1 turns on at 1 ma and conducts until the +10 volt limit is reached. See the plot below. Ratch

7. ### dorke

2,342
665
Jun 20, 2015
Ratch,
This a "homework Help".
You are not supposed to put the full solution, just help and give direction...

8. ### Ratch

1,075
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Mar 10, 2013
This problem has been hanging around for ten days now. If the OP hasn't been given the solution by the teacher by now, he should change schools. Sometimes, the full solution is a way of learning something. Others who read this thread might want to see the solution, too. Since the OP has not responded to this thread, it can be assumed that he has moved on to something else.

Ratch  