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Question
When is the load line of bipolar junction transistor nearly-1)horizontal,2)vertical?

Answer
Hello Premashis,

As it says on the website
http://en.wikipedia.org/wiki/Load_line_(electronics)
the load line represents "the constraint other parts of the circuit place" on the transistor. So we need to choose the circuit we'll talk about. Let's assume a basic switch with a 2N2222 with the emitter tied to ground, the base tied to a resistor whose other end is connected to the input signal, and the collector is tied to a resistor, Rc, whose other end is tied to the supply voltage, Vs.

When the signal at the base provides enough base current to turn the transistor fully on (or to saturate it), the voltage between the collector and emitter, Vce, is small compared to Vs. (If Ic=150 ma, Vce(sat)=0.3V. With smaller Ic, Vce(sat) is even smaller.) So the collector current, Ic, determines the value of the load line where it intercepts the vertical axis. For the purpose of drawing the load line, we assume that the fully on condition has Vce=0 and Ic=Vs/Rc. This gives us the data point where the line intercepts the y axis, or the Ic axis.

When the signal into the base resistor is zero, there is no base current so the transistor is turned off and the collector current is very small. And therefore there is little voltage drop across Rc and the voltage from collector to emitter is approximately the full supply voltage. For the purpose of drawing the load line, we assume that the fully off condition has Vce=Vs and Ic=0. This gives us the data point where the line intercepts the x axis, or Vce.

1) To get a horizontal load line:
When the signal at the base provides enough base current to turn the transistor fully on, the collector current, Ic, must be very small. A large value of Rc would accomplish that. Let's assume Vs=10V and Rc=10 Mohms. So the maximum collector current we can get is 1 uamp. That's where the load line intercepts the Ic axis, nearly at zero. And when the signal at the base is zero, that condition turns the transistor fully off, and the collector current, Ic, is therefore zero (approximately zero in the real world). With no collector current there is no voltage across Rc which gives is the condition Vce=Vs (approximately). That's where the load line intercepts the Vce axis, at 12 volts. So a line connecting the fully on condition and the fully off condition is nearly a horizontal line.

2) To get a vertical load line:
We need the intercept on the Vce axis to be close to the Ic axis. And the intercept on the Ic axis needs to be a reasonable current. Let's see if Vs=0.5V and Rc=10 ohms will do that. When the signal at the base is zero, that condition turns the transistor fully off, and the collector current, Ic, is therefore zero. In that condition, Vce = 0.5V. That's where the load line intercepts the Vce axis, at 0.5 volts. (If the chart is scaled in the usual way, that is very close to the origin.) My discussion for a horizontal load line ignored the transistor's Vce during saturation condition. But with this circuit which has only 0.5 V for the supply voltage, Vce(sat) is more significant. Vce(sat) may be 0.2 V meaning the voltage across Rc will be 0.3 V and therefore Ic = 30 ma. The load line's intercept on the Ic axis is at 30 ma. So when you connect those 2 data points, the line is close to vertical.

Another website I referred to in writing this reply,
http://aries.ucsd.edu/NAJMABADI/CLASS/ECE65/06-W/NOTES/BJT1.pdf

I hope this helps,
Steve

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I would be delighted to help with questions up through the first year of college Physics. Particularly Electricity, Electronics and Newtonian Mechanics (motion, acceleration etc.). I decline questions on relativity and Atomic Physics. I also could discuss the Space Shuttle and space flight in general.

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