In this tutorial, the following concepts are covered.
Outline:
- What is a load line of a PN junction diode? How to draw a load line?
- What is the characteristic curve? How to find the characteristic curve?
- What is Q point? How do find out its value?
Load line is a straight line. From this line, we can find exact values of a semiconductor diode current and voltages. By changing the value of resistance and applied voltage value (these are the circuit parameters) we can move the Q point up and down along the y-axis, This straight line is called as called it the load line.
If the load changes, the slope of the load line changes.
If the load changes, the slope of the load line changes.
The characteristic curve or V-I curve or voltage current graph shows the diode current and voltage relationship. The voltage applied is independent variable and is along x-axis and the current is a dependent variable and is on y-axis.
Below is the V-I graph of a semiconductor diode. The curve is the diode current which increases rapidly as the voltage approaches to 0.7V. the straight line is the load line. The operating point is also marked.
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| Q point/Operating point |
Look at point A:
At this point the current is maximum and the voltage is zero. This point is called 'Saturation'.
Look at point B:
At this point the voltage is maximum and the current is zero. This point is called 'Cut off'.
At this point the current is maximum and the voltage is zero. This point is called 'Saturation'.
Look at point B:
At this point the voltage is maximum and the current is zero. This point is called 'Cut off'.
The intersection of the load line and characteristic curve will define the Operating / bias / Q point of a network.
There are two methods of finding Q point.
Here is a video that cover all the step; draw a load line, draw the characteristic curve and then the Q point. Have a look at this.
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