basic transistor action

Discussion in 'Electronic Basics' started by Sunil, Jul 11, 2007.

1. SunilGuest

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??

2. John PopelishGuest

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.

3. EeyoreGuest

Your mistake is in thinking the base forms an important part of the output
circuit.

Graham

4. Joe McElvenneyGuest

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

5. neon

1,325
0
Oct 21, 2006
there are two diodes in a transistor controlled by inpurity in the base once a transistor saturates current can flow both way.

thanx a lot

thanx a lot