Homework Help: Is it possible to find Vout in this case ?

1. Dec 3, 2011

Femme_physics

Last edited by a moderator: May 5, 2017
2. Dec 3, 2011

technician

FP
I think that diode symbol you have drawn is a zener diode. If so then the Vout is 2V.... that is what zener diodes do.... they set the voltage to what they are designed
It means the voltage across the 1Ω is 4 volts so the current down the series circuit is 4A

3. Dec 3, 2011

Staff: Mentor

That's a zener diode, so it wants to clamp (limit) the voltage across itself at 2V. So right away you can say that Vout will be in the neighborhood of 2V. This is true so long as the supply voltage is greater than the zener voltage, where the diode starts conducting.

For a more precise figure for Vout you would need to have more information about the particular zener diode's characteristics. They tend to exhibit some resistance that increases with current flow. This one will be drawing about 4 amps!

4. Dec 3, 2011

Femme_physics

Yes, it's a zener diode.

REally? I didn't know that.

That makes sense! Thanks.

5. Dec 3, 2011

Femme_physics

Excellent. Yes, we're in basic zener diodes exercises, yet, so I don't think it has any special characteristics.

Thanks a lot gneill, techhie :)

6. Dec 3, 2011

technician

It's a pleasure
You keep us on our toes
Have probably given too much help.... just realised this is a homework forum

7. Dec 3, 2011

Staff: Mentor

You're welcome, as always!

If it's early days in zener school , then assume an ideal diode: zero on resistance and a sharp turn-on at its specified voltage.

8. Dec 4, 2011

Femme_physics

No, I love your help, keep it up please :)

Gotcha.

Actually, this question I asked comes from this question:

http://img35.imageshack.us/img35/8592/myworkp.png [Broken]

I mostly now have a problem figuring out if I should treat the section just above the diode zener (the place I marked "e") as a crossroad, or it should really be a little loop like this:

http://img27.imageshack.us/img27/8527/webcam1322984748.png [Broken]

Last edited by a moderator: May 5, 2017
9. Dec 4, 2011

technician

This is another weird circuit I would say the diagram should have the loop so that the Zener diode (3V?) is in series with the R above it....but... What is that resistor connected to?
Also I do not understand the 3resistors on the left!
Also.....this is another circuit with no feedback to the - input so the output is certain to be +Vs or -Vs.
It will be interesting to hear the other responses that come your way.
Thanks for your kind comment

10. Dec 4, 2011

Femme_physics

I would be very interested, too. And I'd like to know if I can solve it using KCL and KVL only?

11. Dec 4, 2011

Staff: Mentor

I don't think you'll need even Ohm's law for this one! If that odd loop of resistors between the +12V supply and the + input is correct (no missing ground or power connection) then what determines the voltage on the + input of the op-amp? Hint: What current flows into op-amp inputs?

12. Dec 5, 2011

Femme_physics

Last edited by a moderator: May 5, 2017
13. Dec 5, 2011

Staff: Mentor

Actually, not much changes in terms of the result at the output. Can you determine the voltages present on the op-amp inputs?

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14. Dec 5, 2011

technician

Looks "easy" now !!!!!!!!!!

15. Dec 5, 2011

Femme_physics

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16. Dec 5, 2011

technician

That is what that symbol means. If they were in contact it would (should !!!!!) be shown clearly with a dot on the connection

17. Dec 8, 2011

Femme_physics

Ah, thanks! So in that case, answering the questions...

A) Find Vout

B) Find Vout when C is shortened to the ground

http://img585.imageshack.us/img585/4126/zelat.jpg [Broken]

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18. Dec 8, 2011

I like Serena

For (A) the plus supply voltage is not -7.5 V.

After that we'll get to (B).

Last edited: Dec 8, 2011
19. Dec 8, 2011

Femme_physics

What did I do wrong?

20. Dec 8, 2011

I like Serena

You picked the wrong (the negative) power supply.

21. Dec 8, 2011

Femme_physics

Last edited by a moderator: May 5, 2017
22. Dec 8, 2011

I like Serena

Much better!
And nice "m" in your [mA]!

One thing though.
You write Vout=(V+ - V-)=1
But this is not correct.

You wrote before that V+ was greater than V-, so Vout was the positive voltage supply.
That was correct.

You can also write it like this, but then you should write something like:
Vout=(V+ - V-) 1000000 = 1000000 [V] which is greater than Vss, so Vout=Vss=8.4 [V].

23. Dec 10, 2011

Femme_physics

Got it fixed. I'll scan my solution when I get home

Much obliged!

24. Dec 10, 2011

Femme_physics

Last edited by a moderator: May 5, 2017
25. Dec 10, 2011

I like Serena

There!

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