Bipolar transistor biasing


Posted on Feb 6, 2014

Bipolar transistor amplifiers must be properly biased to operate correctly. In circuits made with individual devices (discrete circuits), biasing networks consisting of resistors are commonly employed. Much more elaborate biasing arrangements are used in integrated circuits, for example, bandgap voltage references and current mirrors. The operat


Bipolar transistor biasing
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ing point of a device, also known as bias point, quiescent point, or Q-point, is the point on the output characteristics that shows the DC collector emitter voltage (Vce) and the collector current (Ic) with no input signal applied. The term is normally used in connection with devices such as transistors. For analog operation of a Class A amplifier, the Q-point is placed so the transistor stays in active mode (does not shift to operation in the saturation region or cut-off region) when input is applied. For digital operation, the Q-point is placed so the transistor does the contrary - switches from the "on" (saturation) to the "off" (cutoff) state. Often, the Q-point is established near the center of the active region of a transistor characteristic to allow similar signal swings in positive and negative directions. The Q-point should be stable; in particular, it should be insensitive to variations in transistor parameters (for example, should not shift if transistor is replaced by another of the same type), variations in temperature, variations in power supply voltage and so forth. The circuit must also be practical; both easily implemented and cost-effective. At constant current, the voltage across the emitter base junction VBE of a bipolar transistor decreases 2 mV (silicon) and 1. 8mV (germanium) for each 1 °C rise in temperature (reference being 25 °C). By the Ebers Moll model, if the base emitter voltage VBE is held...




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