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# DC operating point

A typical CE circuit is shown in the figure below, where , , and .

The DC operating point (also known as bias 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.

• The input current and voltage can be determined by
• The base characteristics of the PN-junction between base B and emitter E, and
• The external circuit including the voltage source and resistor , represented by the load line , which is the straight line that passes through the two special points corresponding to the open-circuit voltage and short-circuit current:

The actual current and can be found at the intersection of the two curves, so that both the internal current-voltage characteristics of the transistor and the external circuit parameters are both satisfied. The voltage can also be approximated to be about if is not too low and is not too large, so that is within its typical range in practice.

• The output current and voltage can also be determined by
• The output characteristics of the transistor, and
• The external circuit including the voltage source and resistor , represented by the load line , which is the straight line that passes through the two special points corresponding to the open-circuit voltage and short-circuit current:

The actual current and voltage , the DC operating or Q point, can be obtained as the intersection of the load line and the curve of the current-voltage characteristics, corresponding to a given base current .

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:

• , and , , the transistor is cut off.

• , , and . . .

The transistor is in linear region.

• . , , and .

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the base current is so high that the transistor is working in its saturation region where the linear relationship is no longer applicable (It is only valid in linear region).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 .

From the current-voltage plot of the output characteristics, and also the example above, we see that the operation of a transistor can be in one of the three possible regions:

• Cut-off region:

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).

• Linear region:

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.

• Saturation region:

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).

Next: AC Signal Amplification Up: ch4 Previous: Bipolar Junction Transistor (BJT)
Ruye Wang 2018-04-18