Introduction
Transistor is commonly used
as an amplifier or a miniature electronic switches . Transistor is a static device It is a three terminal
device consisting of Collector Emitter and Base. It is also called BJT (bipolar junction transistor). There are two PN junctions in one transistor: Emitter base junction
and Collector base junction. 
In transistor symbol the arrow head is always at emitter terminal and arrow head points from p-type base
to n-type base.
 Transistor Parameter In the datasheet you will find the transistor package and identify which lead is C,B and E. Vceo Collector to Emitter breakdown voltage This is the maximum voltage that can be placed from the collector to the emitter. It is normally measured with the base open circuit - hence the letter "o" in the abbreviation. The value should not be exceeded in the operation of the circuit otherwise damage may occur Ic Collector current Normally defined in milliamps, but high power transistors may be quoted in amps. The important parameter is the maximum level of collector current. This figure should not be exceeded otherwise the transistor may be subject to damage Hfe
This is the current gain of the transistor (
Ic/Ib)
Frequency Transition The frequency where current gain falls to unity. The transistor should normally be operated well below this frequency.
Working of Transistor
In electronic circuits emitter-base junction is always forward-biased while the collector base junction is always reversed biased.
When small amount of voltage is applied to base terminal the large current flow from collector terminal to emitter can be controlled.
In common emitter(for NPN),the input signal is applied between base and emitter and the output is taken from collector and emitter. Transistor as an Amplifier
UM66 is a melody generator IC. It is activated when positive supply is given at its pin 2 (here I/P)
. Zener Diode reduces this power supply to 3.3V required for UM66.1u 25V is filtering capacitor. Output of UM66 is fed to the loud speaker (8 ohm, 0.5 W) via driver NPN transistor BC547
. Preset resistor 1Kohm is used to control the volume of loud speaker.Transistor Vcc is 9 Volt. Here the small signals from UM66 output are amplified by the transistor and you can here loud melody music. Transistor as a switch
The transistor can turn ON ot OFF current in load (here LED) depending upon whether the input base voltage is positive or negative.Here the transistor acts as a switch. 
Selection of Transistor
Vcc and Ic should be less than the Vceo and Icmax data from datasheets respectively. Input Frequency should be less than FT . Choose the Base resistor Rb using this formula
Rb=0.2 x Load Resistance x Hfe Load connection for PNP and NPN Transistor 
TESTING TRANSISTORS Select the diode testing mode in Digital multimeter(DMM). Use datasheets to find which lead is base. 
NPNNow connect the positive terminal (red probe) to the base of the transistor.Then place the other DMM terminal (black probe) to remaining transistor lead
For working transistor DMM will show some values for B-C junction and B-E junction. PNPNow Place the black probe to base terminal.Then connect the red terminal to remaining leads of transistor respectively.
For working transistor DMM will show some values for B-C junction and B-E junction There should be high resistance between Collector and emitter terminals for working transistor.
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