Aim
To study the characteristics of a common emitter npn (or pnp) transistor and to find out the values of current and voltage gains.
Apparatus
An n-p-n transistor, a three volt battery, a 30 volt battery, two high resistance rheostats, one 0-3 volt voltmeter, one 0-30 volt voltmeter, one 0-50 μA micro-ammeter, one 0-50 mA milli-ammeter, two one way keys, connecting wires.
Theory
In common-emitter circuit of a transistor, emitter-base make input section and emitter- collector make output section. As usual, base junction (input junction) is forward biased and collector junction (output junction) is reverse biased. ,
Resistance offered by base junction, is called input resistance (RI) It has a very small resistance due to forward biasing.
Resistance offered by collector junction, is called output resistance (R0). It has a high value due to reverse biasing.
Due to high output resistance (resistance in output section), a high resistance can be used as load resistance (RL). Generally RL = R0.
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Also emitter current (Ie) divides itself into base current (Ib) and collector current (Ic). In n-p-n transistor, Ic is about 98% of Ie, base current Ib remains only 2% of Ie. A little change in Ib causes a large change (about 49 times) in Ic. The ratio of change in collector current to the corresponding change in base current, measures current gain in common emitter transistor. It is represented by symbol β.
to-study-the-characteristics-of-a-common-emitter-npn-or-pnp-transistor-and-to-find-out-the-values-of-current-and-voltage-gains-2
For the example given above, P becomes 49.
The product of current gain and the resistance gain measures voltage gain of the common emitter transistor. It is about fifty times the resistance gain.
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Diagram
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Procedure
  1. Make circuit diagram as shown in figure.
  2. Make all connections neat, clean and tight.
  3. Note least count and zero errors of voltmeters and ammeters.
  4. Make voltmeter readings zero in V1 and V2 and insert the keys.
    For input characteristics
  5. Apply forward bias voltage on base junction. Read base voltage ( Vb) from and base
    current (Ib) from μA.
  6. Go on increasing Vb till Ib rises suddenly. Note corresponding values of Ib for each value of Vb.
  7. Make collector voltage Vc = 10 V and repeat steps 5 and 6.
  8. Repeat step 7 with Vc = 20 V and 30 V.
  9. Make all readings zero.
    For output characteristics
  10. Keep collector voltage (Vc) zero. Adjust base voltage Vb to make base current Ib = 10 μA. Though collector voltage Vc is zero ; but there is collector current Ic. Note it.
  11. Make collector voltage 10 V, 20 V and 30 V and note the corresponding collector currents.
  12. Repeat steps 10 and 11 with Ib = 20μA, 30 μA, and 40 μA.
  13. Record your observations as given below :
Observations
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to-study-the-characteristics-of-a-common-emitter-npn-or-pnp-transistor-and-to-find-out-the-values-of-current-and-voltage-gains6
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Calculations
  1. Calculation for input resistance (RI )
    Plot a graph between base voltage Vb (column 2—table 1) and base current Ib (column 3a—table 1) for zero collector voltage Vc, taking Vb along X-axis and Ib along Y-axis. Plot graphs for different values of Vc. The graphs come as shown.
    These graphs are called ‘input characteristics’ of the transistor.
    to-study-the-characteristics-of-a-common-emitter-npn-or-pnp-transistor-and-to-find-out-the-values-of-current-and-voltage-gains-9
    to-study-the-characteristics-of-a-common-emitter-npn-or-pnp-transistor-and-to-find-out-the-values-of-current-and-voltage-gains-10
  2. Calculation for output resistance (R0)
    Plot a graph between collector voltage Vc (column 2—table 2) and collector current Ic (column 3a – table 2) for 10 μA base current Ib, taking Vc along X-axis and Ic along Y-axis. Plot graphs for different values of Ib.
    to-study-the-characteristics-of-a-common-emitter-npn-or-pnp-transistor-and-to-find-out-the-values-of-current-and-voltage-gains-11
    These graphs are called ‘output characteristics’ of the transistor.
    to-study-the-characteristics-of-a-common-emitter-npn-or-pnp-transistor-and-to-find-out-the-values-of-current-and-voltage-gains-12
  3. Calculation for current gain (β)
    Plot a graph between base current Ib (columns 3a, 36, 3c and 3d—table 2) and corre-sponding collector current Ic (from same columns) for 30 volts collector voltage Vc, taking Ib along X-axis and Icalong Y-axis. The graph comes to be a straight line. The graph is called current gain characteristic of the common emitter transistor.
    to-study-the-characteristics-of-a-common-emitter-npn-or-pnp-transistor-and-to-find-out-the-values-of-current-and-voltage-gains-13
    to-study-the-characteristics-of-a-common-emitter-npn-or-pnp-transistor-and-to-find-out-the-values-of-current-and-voltage-gains-14
  4. Calculation for voltage gain Av
    to-study-the-characteristics-of-a-common-emitter-npn-or-pnp-transistor-and-to-find-out-the-values-of-current-and-voltage-gains-15
Result
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Precautions
Same as given in Experiment 8.
Sources of error
Same as given in Experiment 8.