Bias circuit requirements:

Signal requirements for Class A amplifiers

The Q-point is placed thus the transistor stays in active mode (does not shift to operation in the saturation region or cut-off region) while input is applied for analog circuit operation. Whereas For digital operation, the Q-point is placed thus the transistor does the contrary - switches from "on" to "off" state. Frequently, Q-point is established near the center of active region of transistor characteristic to permit identical signal swings in positive and negative directions. Q-point should be stable. Particularly, it should be insensitive to differences in transistor parameters (for instance, should not shift if transistor is replaced by another of similar type), variations in temperature, variations in power supply voltage and so forth. The circuit has to be practical: easily implemented and cost-effective.