State the need of biasing and describe the concept of DC load line

1 Answer

Answer :

Need of Biasing for Transistor: The transistor should be biased in the active region if it is to be used for amplification and in saturation and cut off if it is used as a switch. 2. The Q point should be adjusted approximately at the center of the load line for voltage amplifier application. 3. The value of stability factor (S) should be as small as possible. 4. Q point should be stabilized by introducing a negative feedback in the biasing circuit. 5. The Q-point should not be affected due to temperature changes or device to device variation. 6. Bypass capacitor should be included to avoid reduction in voltage gain due to negative feedback. 7. Transistor should be biased in the linear region of the transfer characteristics.

Concept of DC load line: The DC word indicates that this line is drawn under the dc operating conditions without any ac signal at the input. And the word load line is used because the slope of this line is -1/Rc is the load resistance. Operating point is the point on the load line which represents the dc current through a transistor (ICQ) and the voltage across it (VCEQ) When no ac signal is applied. The dc load line is a set of infinite number of such operating points and the user or designer can choose any point on the dc load as the operating point. The position of operating point on the load line is dependent on the application of the transistor.

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