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.
