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david90
Joined: 05 May 2004
Posts: 368
Helped: 1
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 30 Sep 2004 8:21 Diode problem |
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go to http://www.csupomona.edu/~bolson/courses/ECE220/Homework/HW1_ece220.doc
and read # 1.a.i
Isn't the voltage across the diode, VD, just .7v for all foward biased cases? It seems too easy and obvious.
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barrybear
Joined: 22 Nov 2001
Posts: 254
Helped: 5
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| 30 Sep 2004 11:30 Re: Diode problem |
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I beleive that the questions are designed to test your ability to make simplecalculations around diode and resistor circuits . Diodes forward biased are genraly exceped to have a voltage drop of .7 volts this is not always the case as the voltage across a diodewill depend on current and diode type.
Barrybear
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Element_115
Joined: 23 Mar 2002
Posts: 312
Helped: 21
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| 30 Sep 2004 23:53 Re: Diode problem |
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No it might not be 0.7V. In some cases it might be 0V or
some negative value. "IF" the diode is on! it should have 0.7V.
Hope this helps
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zmath
Joined: 14 Jun 2003
Posts: 62
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| 01 Oct 2004 15:18 Diode problem |
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V forward bias for siligon diode =0.7 v.
V forward bias for germanium diode =0.3 v.
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TomSpade
Joined: 02 Oct 2004
Posts: 5
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| 02 Oct 2004 5:49 Re: Diode problem |
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| It looks like they're just running through all the different scenarios a beginner might have issues with: Negative supplies, diodes without a terminal attached to ground, voltages across a reversed bias diode. These things aren't obvious if it's the first time you've encountered them.
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thinkraghu
Joined: 28 Sep 2004
Posts: 1
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| 03 Oct 2004 10:05 Re: Diode problem |
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Isn't the voltage across the diode, VD, just .7v for all foward biased cases? It seems too easy and obvious.
The voltage across diode in Forward Bias is 0.7v when it is "ON" otherwise it will violate the laws if has 0.7v in "OFF" state.
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Kevin Weddle
Joined: 10 Jul 2004
Posts: 101
Location: San Antonio, TX
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| 03 Oct 2004 22:26 Re: Diode problem |
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| The resistance of a diode can be fairly constant. This is what makes it good for signals. You can count on the resistance of the diode to remain constant over a small range of currents. Some good advice. Always use the peak to peak when dividing signals. Don't try and divide the DC. You can only do this resistors. If you try it with diodes it won't work because the voltage is not dependent on the resistance. The voltage is first a difference of .7 and that does not create a voltage divider.
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golfbumb
Joined: 29 Sep 2004
Posts: 40
Helped: 5
Location: California
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| 04 Oct 2004 1:47 Re: Diode problem |
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No, the voltage across a forward biased diode is not necessarily 0.7 volts. It is only approximately 0.7 volts when the diode is nearly fully forward biased with sufficient current to do so.
As an easy demonstration, just place a 100 kilohm resistor in place of R1 in figure 1a of your homework. The approximately 100 microamp current will barely even begin to turn on the most sensitive of diodes. In that case Vd would be slightly higher than 0 VDC (the diode is technically forward biased, but only barely.)
Your homework paper says to use the "offset" method and UNDERLINES it. I do not recall a diode offset method and would use an iterative method to solve these. Go back and review the offset method. If Cal Poly uses course numbers like San Jose State did when I was there (they are both CSU schools) a 200 series course is a Master's course. If so and it looks easy, you are likely assuming incorrectly.
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