LOAD LINE ANALYSIS confusion pls help

[als_757]

Hi All,

Currently we had a quiz for load line analysis and the question was:

Determine the value of R for the circuit below that will result in a diode current of 10mA if E=7V. Use approximate analysis method and graph.


I answered Idq=10mA, Id=11.25mA, and R=622ohms. Because of the statement "result in a diode current" I thought that Idq is the given which is 10mA. But my instructor says it is wrong Idq should be 9.1mA, .

I was confused of which is correct.
Prior the examination I found the same question at "Basic Electronics and Circuit theory by boylestad" but the statement " use approximation method" was not included in the book.


Pls help which is correct.
How should it be solved? and pls explain the difference with approximation method.

i attached the question in the book.

your answers would be a great help. I don't know how will i defend my answer to my instructor.

[pebe]

I don't know what 'Idq' is because you have not defined it. But if the Fig 2.147b (that you have not shown) is the Vf/Id curve of the diode, then there is no need for approximation, because the result is definite.

From Fig 2.147b, read off the forward voltage for a current of 10mA. call that Vread. The voltage applied across the resistor will be 7V-Vread = Vapp (it will be around 6.3V). The resistor value will be Vapp/10mA.

[als_757]

hi,

my apology for not attaching the other fig. i already attached it.

'Idq' is were the lines intersect. and is referred to as the operating point or (quiescent) Q-point.

and here is a link for load-line analysis.

http://www.ece.jcu.edu.au/subjects/ee23 ... e%2010.pdf

so that we could have the same understanding..

thank you!

[pebe]

I think your instructor is making 'a mountain out of a molehill' by applying load line analysis to the question that was asked. If he had come up with the question of what current flows through the 2.2K resistor in the circuit shown, then I could have understood his reasoning. But the question he asked was much more straightforward, ie. What value of resistor will result in a current of 10mA?

The 'Forward Voltage vs Current' curve given in Fig 3b clearly shows that a current of 10mA will produce a voltage drop of 0.8V across the diode. From there on it's simply an application of Ohms Law to find the resistor value, like this:

7V is applied to the diode and the (unknown) resistor in series. The voltage drop across the diode @10mA = 0.8V.
So 7V - 0.8V = 6.2V is applied across the resistor ,R, and 10mA current flows through it.
So R = 6.2V/.01A = 620ohms