HOW TO USE TRANSISTOR

Use a transistor as amplifier

AMPLIFIER: An amplifier has two input and two output terminals, so a
three terminal transistor used as an amplifier must have
one of its terminals common to both input and output as
shown (left). Which terminal is used as the common
connection has a marked effect on the performance of the
amplifier.
These differences can be exploited by the circuit designer
to give and amplifier that most suits a particular purpose.
In common emitter small changes in base emitter current cause large changes in collector emitter
current. The circuit is that of a CURRENT amplifier. For VOLTAGE amplification, we connect a load
resistor (or an impedance such as a tuned circuit) in the collector circuit so that the change in collector
current causes a change in the voltage across the load resistor. The value of the load resistor will
affect the VOLTAGE GAIN of the amplifier.

In COMMON COLLECTOR (emitter follower) mode the transistor is normally used as a BUFFER
AMPLIFIER for matching impedances between two other circuits. This mode gives the amplifier high
input impedance and low output impedance. The voltage gain is unity (1). It is a useful CURRENT
AMPLIFIER and is often used for driving high current devices such as motors etc.