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EXPERIMENT NO. 8

AIM:- A study of input, output and transfer characteristics of a transistor in common base configuration

APPARATUS :- Two d.c power supplies ( 0 to 10 V), two voltmeters (0 to 10 V), two milliammeters (0 to 10 mA), two rheostats, a pnp transistor, connecting wires.

FORMULAE:-
i) Dynamic input resistance, ri = [delta(VEB) / delta(IE)] at VCB = constant
ii) Output resistance, ro = [delta(VCB) / delta(IC)] at IE = constant
iii) DC current gain, alphaDC = (IC) / (IE)
iv) AC current gain, alphaAC = [delta(IC) / delta(IE)] at VCB = constant.

CIRCUIT DIAGRAM:-

PROCEDURE:-

a) Input Characteristics
Make the connections as shown in the circuit diagram. The collector - base voltage VCB is kept constant at 2 V. The voltage VEB is varied in steps of 0.1 V and for each setting of the voltage, the current IE is noted. The value of VCB is then set at 8 V and the above procedure is repeated. The reading are recorded in table 1.

b) Output characteristics
The input current, i.e. emitter current, IE is kept constant at 5 mA. Voltage VCB is varied in steps of 1 V and for each setting of the voltage, the output current, i.e. collector current IC is noted. The value of IE is then set at 7.5 mA and the above procedure is repeated. The value of IE is then set at 10 mA and the above procedure is again repeated.The reading are recorded in table 2.

c) Transfer characteristics
By keeping output voltage VCB constant at some value, for different values of input current IE, corresponding values of output current IC are noted down and recorded in table 3.

OBSERVATIONS:-

  1. For input characteristics
  2. VCB = 0 V

    VCB = 4 V

    VCB = 8 V

    VEB V

    IE mA

    VEB V

    IE mA

    VEB V

    IE mA

    0
    0.1
    0.2
    0.3
    0.4
    0.5
    0.55
    0.6
    0.65

     

    0
    0.1
    0.2
    0.3
    0.4
    0.5
    0.55
    0.6
    0.65

     

    0
    0.1
    0.2
    0.3
    0.4
    0.5
    0.55
    0.6
    0.65

     

  3. For output characteristics
  4. IE = 5 mA

    IE = 7.5 mA

    IE = 10 mA

    VCB V

    IC mA

    VCB V

    IC mA

    VCB V

    IC mA

    0
    0.2
    0.4
    0.6
    1.0
    2.0
    3.0
    4.0
    5.0
    6.0
    7.0
    8.0

     

    0
    0.2
    0.4
    0.6
    1.0
    2.0
    3.0
    4.0
    5.0
    6.0
    7.0
    8.0

     

    0
    0.2
    0.4
    0.6
    1.0
    2.0
    3.0
    4.0
    5.0
    6.0
    7.0
    8.0

     

  5. For transfer characteristics
  6. VCB = constant at 5 V.

    IE mA

    IC mA

    0
    2.5
    5
    7.5
    10
    12.5
    15
    17.5

     

GRAPH:-

i) To draw input characteristics, VEB values are taken along x - axis and IE values are taken along y - axis and the graphs are plotted for all three constant VCB values.
ii) To draw outut characteristics, VCB values are taken along x - axis and IC values are taken along y - axis and the graphs are plotted for all three constant IE values.
iii) To draw transfer characteristics, IE values are taken along x - axis and IC values are taken along y - axis and the graph is plotted.

CALCULATIONS:-

i) ri = [delta(VEB) / delta(IE)] at VCB = constant =
ii) ro = [delta(VCB) / delta(IC)] at IE = constant =
iii) alphaDC = (IC) / (IE) =
iv) alphaAC = [delta(IC) / delta(IE)] at VCB = constant =

RESULT:-

i) Dynamic input resistance, ri =
ii) Output resistance, ro =
iii) DC current gain, alphaDC =
iv) AC current gain, alphaAC =

PRECAUTIONS:-

  1. Make sure that the connections are tight.
  2. Take care to apply i)suitable forward voltage across emitter - base junction and ii) suitable reverse voltage across the base - collector junction. Otherwise the transistor may be damaged.