DC biasing is setting up a circuit to operate a transistor at a desired operating point on its characteristic curve. Three bias networks for the common emitter amplifier are shown in figure 5.7.
Figure 5.7: Bias circuits for the common emitter amplifier.
In figure 5.7a the only path for DC bias current into the base is through . is a power supply voltage which is generally greater than 10 V such that can be ignored. The DC voltage at the collector should be large enough to provide at least a 2 V drop between collector and emitter and clearly must be less than . In the absence of other circuit requirements, a convenient algebraic choice for is . DC circuit analysis results in the following relative sizes of the two resistors:
Although the circuit works reasonably well, the fact that is quite variable among samples leads to a bad design. A well-designed circuit should have an operating point that is less dependent on this parameter.
Figure 5.7b shows a network with the base-biasing resistor connected to the collector instead of . acts as a negative feedback resistor since it feeds the collector current back into the base. Analysis gives
Therefore a change in has only half the effect of the previous design.
A more common bias stabilization technique employs a series resistor between the emitter and ground. This circuit has about the same sensitivity to changes in as the previous circuit.
A further improvement can be made by introducing a second base-bias resistor as shown in figure 5.7c. The bias voltage is determined almost entirely by the two bias resistors. These biasing methods can also be used for the common collector and common base configurations.