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Does transistor need pull down?
Since an NPN transistor’s base is not a high impedance point like a MOSFET or a JFET, and the HFE of the transistor is less than 500, and at least 0.6V is needed to turn the transistor ON, a pull-down resistor is not critical, and in most cases is not even needed.
What is the function of base in BJT?
Both injection efficiency and recombination in the base reduce the BJT gain. Another useful characteristic is the common-base current gain, αF. The common-base current gain is approximately the gain of current from emitter to collector in the forward-active region.
How is the BJT used as a switch?
The BJT as Switch REVIEW: • Transistors may be used as switching elements to control DC power to a load. The switched (controlled) current goes between emitter and collector; the controlling current goes between emitter and base.
Why is the base of a BJT not connected to the base?
In the above figures, the base of either BJT is not connected to a suitable voltage, and no current is flowing through the base. Consequently, the transistor cannot turn on. Perhaps, the simplest thing to do would be to connect a switch between the base and collector wires of the transistor as in figure (a) below.
What happens when a switch is closed on a bipolar junction transistor?
If the switch is closed as in figure (b), current will be able to flow from the base to the emitter of the transistor through the switch. This base current will enable a much larger current flow from the collector to the emitter, thus lighting up the lamp.
How does R2 help to turn off the BJT?
The very short answer: R2 helps to turn the BJT off fast. A bit longer: When the current ceases to be provided through R1, all there is left to stop the base from being forward biased is the current that flows through the base itself. This works for slow circuits.