Bipolar Transistors
A bipolar transistor consists of a three-layer "sandwich" of doped (extrinsic)
semiconductor materials, either P-N-P or N-P-N. Each layer forming the
transistor has a specific name, and each layer is provided with a wire contact for
connection to a circuit. Shown here are schematic symbols and physical
diagrams of these two transistor types:
For any given state of operation, the current directions and voltage polarities for
each type of transistor are exactly opposite each other
Bipolar transistors work as current-controlled current regulators. In NPN mode
the main current (electron flow) being controlled moves from emitter to
collector. The much smaller current controlling moves from emitter to base.
Bipolar transistors are called
bipolar because the main flow of
electrons through them takes
place in two types of
semiconductor material
The transistor can function as a switch because a transistor's collector current is
proportionally limited by its base current, the device can be used as a currentcontrolled switch. A small flow of electrons through the base of the transistor can
exert control over a much larger flow of electrons through the collector. Examples of
circuits are
Common Emitter, Common Base and Common Collector connections.
Common Emitter
In the simple circuits above the transistors are connected in the common-emitter
configuration because both the input and output load share the emitter lead as a
common connection point. The common emitter configuration provide high
current and voltage amplification ( Gain)
Common Collector
It is called the common-collector configuration because (ignoring the power
supply battery) both the input signal source and the output load share the
collector lead as a common connection point:
This circuit provides high current gain
and unity ( ie one) voltage gain.It is
commonly called the emitter follower
because the voltage Vout at the
emitter changes in line with Vin.
The emitter follower is a very useful
buffer or impedance matching circuit.
Common Base
It is called the common-base configuration because (DC power source aside), the
input signal source and the output load share the base of the transistor as a
common connection point:
The current gain is slightly less than 1 and the voltage gain is Rout / Re. The
circuit is not as popular as the other 2 but has uses as a non inverting
amplifier or a trans-impedance amplifier where changes in input current are
translated to an output voltage