•DC biasing circuits
•DC and AC equivalent circuit
•Q-point (Static operation point)
•DC and AC load line
•Saturation Cutoff Condition
DC Biasing Circuits
•The ac operation of an amplifier depends on the initial dc values of IB, IC, and VCE.
•By varying IB around an initial dc value, IC and VCE are made to vary around their initial dc values.
•DC biasing is a static operation since it deals with setting a fixed (steady) level of current (through the device) with a desired fixed voltage drop across the device.
Purpose of the DC biasing circuit
•To turn the device “ON”
•To place it in operation in the region of its characteristic where the device operates most linearly, i.e. to set up the initial dc values of IB, IC, and VCE
•The voltage – divider (or potentiometer) bias circuit is by far the most commonly used.
Þ voltage-divider to set the value of VB , IB
Þ to short circuit ac signals to ground, while not effect the DC operating (or biasing) of a circuit
(RE Þ stabilizes the ac signals)