COMMON EMITTER CHARECTERISTICS OF NPN TRANSISTOR

AIM   

To plot the characteristics of BC 107 transistor and to find

1. Dynamic input resistance

2. Dynamic output resistance

3. Common emitter current gain

 

COMPONENTS AND EQUIPMENTS NEEDED
 

1. Transistor

2. Variable power supplies

3. Resistors

4. Voltmeters

5. Ammeters

 

CIRCUIT DIAGRAM

THEORY
 

         A transistor is a 3 terminal device. It can be considered as the combination of two diodes. In a transistor there are 3 regions: 1.emitter 2.base 3.Collector. In an npn transistor the emitter and collector are n types, and base is p type. In any transistor emitter is heavily doped, base is lightly doped and collector is moderately doped. For the proper working of transistor the emitter base junction should be forward biased and collector base junction should be reverse biased. In a common emitter configuration, emitter is common to both input and output.

          Transistor (bipolar transistor-BJT) is a current controlled device. The input characteristics are a plot between the base current and base emitter voltage. The dynamic input resistance can be calculated by taking the slope of the input characteristics by keeping the output voltage constant. The output characteristics is a plot between collector current and collector emitter voltage by keeping the input current constant..Now the common emitter current gain ß can be calculated as a ratio between collector current and base current at a particular value of output voltage (collector emitter voltage)

 

PROCEDURE
 

1. Check the components and identify the leads of transistor.

2. Keeping the rheostats at minimum position, switch on the power supplies

3. Make the voltage at collector emitter as zero and vary the rheostat at input side in small steps

4. Do the above step for other collector –emitter voltages (eg: Vce=3 V, Vce=5 V) and tabulate the readings.

5. Switch off the power supplies and switch on the supplies and make the input current at 40 uA.

6. By varying the rheostat at output note down the readings of ammeter and voltmeter readings.

7. Repeat the above step, for a few number of times for various base current values (eg:IB =60 uA)

8. Plot the graph

9. Calculate the dynamic input resistance, dynamic output resistance and common emitter current gain.

OBSERVATIONS


1.     
To plot input characteristics

 

For VCE=0V                               For VCE=3 V                              

IB

VBE

IB

VBE

 

 

 

 

 

 

 

 

2.      To plot the output characteristics

 

           For IB=40 uA                             For IB=60 uA                          For IB=80 uA

Ic (mA)

VCE(volts)

     Ic

   VCE

    Ic

  VCE

 

 

 

 

 

 

 

 

 

 

 

 

GRAPH

 

1.  INPUT CHARACTERISTICS

 

2.OUTPUT CHARACTERISTICS

 

 

CALCULATIONS

 
Dynamic input resistance= slope of input characteristics=………..

Dynamic output resistance=slope of out put characteristics=……..

Common emitter current gain=………………

 

RESULT

 

CE characteristics of an NPN transistor were plotted    .

Dynamic input resistance=……………..

Dynamic output resistance=…………….

Common emitter current gain=…………..

 

SAMPLE VIVA QUESTIONS

 

1.      In the name of BC 107 what B, C and 107 stands for?

          B –For silicon (A –For germanium)

         C-Audio frequency low power (Application of device)

         107-is the serial number

 

2.      Define α, β, γ

          First one is the common base current gain. Second one is the common emitter current gain. Third one is the common collector current gain.

 

3.      What is the advantage of silicon over germanium?

          Easily available.

 

4.      Draw the circuit of a Darlington pair

5.      Write the name of a Darlington package

2N999

6.      What are the advantages of Darlington pair?

High input resistance, more gain

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