Hi....My name is sunil and im doing my electronics engineering in India. I have a minor query in the basics of transistors.
In a npn common base circuit, it says that the collector current Ic is normally much greater than the base current Ib. How is that possible??
i mean, the base and the collector both form the output circuit rite....then how can there exist two different currents at two diff parts of the circuits??
As long as the collector is more positive than the base, the transistor exhibits current gain or beta, with the collector current beta times the base current. The emitter carries both the base and collector currents.
Common base means that the base is part of both input and output circuits. The input circuit is the voltage between emitter and base, while the output voltage is between collector and base. Often, in a common base circuit, the base is held at a steady DC voltage, with the emitter voltage swinging through a very small range, about one diode drop more negative than the base voltage, and the collector swinging a much larger voltage, centered at some operating point quite a bit more positive than the base voltage.
The output current does not pass through the base, but the output voltage swings with respect to the base voltage. The collector current passes through the emitter through whatever is impressing the input voltage swing on the emitter. In the common base configuration, the input and output currents are not separate, as they are with the common emitter configuration. But the word "common" refers to voltage swings. Both the input and output voltage swings are with respect to the base voltage.
Your mistake is in thinking the base forms an important part of the output circuit.
Graham
Sunil,
Think of it as a water pipe connected to a tap (the emitter) with the open end being the collector. Now punch a tiny hole in the pipe and the leak you have is the base current.
This analogy has several holes in it (three actually) but never mind :-)
Cheers - Joe
thanx a lot
thanx a lot
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