Basic Electronics (3110016)

BE | Semester-1   Winter-2019 | 06-01-2020

Q3) (b)

Explain the selection of a Q point for a transistor bias circuit and discuss the limitations on the output voltage swing.

  • The process of increasing the signal strength is called as Amplification. This amplification when done without any loss in the components of the signal, it is called Faithful amplification.
  • Faithful amplification is the process of obtaining complete portions of input signal by increasing the signal strength. This is done when AC signal is applied at its input.
  • As shown in Fig. - 1, the input signal applied is completely amplified and reproduced without any losses. This can be considered as Faithful Amplification.
  • The operating point is so chosen that it lies in the active region/middle of the DC load line and it helps in the reproduction of complete signal without any loss.
  • If the operating point is considered near saturation point, then the amplification will be as Fig. - 2.
  • When Q point is set near saturation, then negative half cycle of output voltage gets distorted/clipped as shown in Fig. - 2.
  • If the operation point is considered near cut off point, then the amplification will be as Fig. - 3.
  • When Q point is set near cut-off, then positive half cycle of output voltage gets distorted/clipped as shown in Fig. - 3.
  • Hence the placement of operating point is an important factor to achieve faithful amplification. But for the transistor to function properly as an amplifier, its input circuit (i.e., the base-emitter junction) remains forward biased and its output circuit (i.e., collector-base junction) remains reverse biased.