A typical CE circuit is shown in the figure below, where , , and .
The DC operating point (also known as bias point, quiescent point, or Q-point) is the DC steady-state operating condition of a transistor, (with no AC input signal applied), determined by and of the input port, and and of the output port.
From the current-voltage plot of the output characteristics, we see that the operation of a transistor can be in one of the three possible regions:
When , or even negative, , the output current is , , i.e., the transistor (between collector and emitter) is cut off (immediate above the horizontal axis of the output plot).
When the input voltage is about , , the transistor is in the linear range where the collector current is proportional to base current , and . The CE transistor circuit in the linear region is widely used for amplification.
When the input voltage is further increased will be significantly increased (due to the exponential relationship between and ), the linear relationship no longer holds as approaches its maximum . The transistor is is saturated and , independent of (to the immediate right of the vertical axis of the output plot).
Example: In the CE circuit shown below, , , , . The load line can be determined by two points: and . Find output voltage when takes the following values:
The transistor is in linear region.
We get this unreasonable negative voltage because the base current is so high that the transistor is working in its saturation region where the linear relationship is no longer applicable (It is only valid in linear region). The actual output voltage can be estimated to be about , and the actual can be found to be .