The below figure shows the output characteristic curves for the transistor in CE mode. The DC load line is drawn on the output characteristic curves.
- Load line
To draw load line, we have to find saturation current and the cutoff voltage. After plotting these values on the vertical and the horizontal axes, a line is drawn joining these two points, which represents DC load line. It represents all possible combinations of the collector current Ic and the collector voltage Vc (or Vce) for the given load resistor Rc.
- Saturation point
The point at which the load line intersects the characteristic curve near the collector current axis is referred to as the saturation point. At this point of time, the current through the transistor is maximum and the voltage across collector is minimum for a given value of load.
Therefore saturation current for the fixed bias circuit,
Ic (sat) =Vcc/Rc
- Cutoff point
The point where the load line intersects the cutoff region of the collector curves is referred as the cutoff point (i.e. end of load line). At this point, collector current is approximately zero and emitter is grounded for fixed bias circuit.Therefore,
Vce (cut) = Vc = Vcc
- Operating point
The "Q point" for a transistor amplifier circuit is the point along its operating region in a "quiescent ", where no input signal gets amplified.