Lecture - 17: Biasing Techniques

Example-1

Determine the Q-point for the CE amplifier given in fig. 1, if R1 = 1.5K W and Rs = 7K W . A 2N3904 transistor is used with β = 180, RE = 100W and RC = Rload = 1K W . Also determine the Pout(ac) and the dc power delivered to the circuit by the source.

Fig. 1

Solution:

We first obtain the Thevenin equivalent.

and

Note that this is not a desirable Q-point location since VBB is very close to VBE. Variation in VBE therefore significantly change IC.We find Rac = RC || Rload= 500 W and Rdc = RC + RE =1.1KW. The value of VCE representing the quiescent value associated with ICQ is found as follows,

Then

Since the Q-point is on the lower half of the ac load line, the maximum possible symmetrical output voltage swing is

The ac power output can be calculated as

The power drawn from the dc source is given by

The power loss in the transistor is given by

The Q-point in this example is not in the middle of the load line so that output swing is not as great as possible. However, if the input signal is small and maximum output is not required, a small IC can be used to reduce the power dissipated in the circuit.

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