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# Characteristics of a NPN-Junction Transistor in Common Emitter Configuration

To study the characteristics and to determine the current gain of a NPN junction transistor in common emitter configuration.

CHARACTERISTICS OF A NPN-JUNCTION TRANSISTOR IN COMMON EMITTER CONFIGURATION

### AIM

To study the characteristics and to determine the current gain of a NPN junction transistor in common emitter configuration.

### APPARATUS REQUIRED

Transistor - BC 548/BC107, bread board, micro ammeter, milli ammeter, voltmeters, variable DC power supply and connecting wires.

### FORMULA Where,

ri → Input impedance (Ω)

∆VBE → The change in base-emitter voltage (volt)

∆IB → The change in base current (µA)

ro → Output impedance (Ω)

∆VCE → The change in collector-emitter voltage (volt)

∆IC → The change in collector current (mA)

β → Current gain of the transistor (No unit)

### CIRCUIT DIAGRAM Figure (a) NPN - Junction transistor and its symbol (Transistor is held with the flat surface facing us) Figure (b) NPN junction transistor in CE configuration

### Note

A resistor is connected in series with the base to prevent excess current flowing into the base.

### Precautions

·        Care should be taken to connect the terminals of ammeters, voltmeters, and dc power supplies with right polarity.

·        The collector and emitter terminals of the transistor must not be interchanged.

## PROCEDURE

·        The connections are given as shown in the diagram.

·        The current and voltage at the input and output regions can be varied by adjusting the DC power supply.

## i) Input characteristic curve: VBE vs IB (VCE constant)

·        The collector-emitter voltage VCE is kept constant.

·        The base-emitter voltage VBE is varied in steps of 0.1V and the corresponding base current (IB) is noted. The readings are taken till VCE reaches a constant value.

·        The same procedure is repeated for different values of VCE. The readings are tabulated.

·        A graph is plotted by taking VBE along x-axis and IB along y-axis for both the values of VCE.

·        The curves thus obtained are called the input characteristics of a transistor.

·        The reciprocal of the slope of these curves gives the input impedance of the transistor.  ## ii )Output characteristic curve: VCE vs IC (IB constant)

·        The base current IB is kept constant.

·        VCE is varied in steps of 1V and the corresponding collector current IC is noted. The readings are taken till the collector current becomes almost constant.

·        Initially IB is kept at 0 mA and the corresponding collector current is noted. This current is the reverse saturation current ICEO.

·        The experiment is repeated for various values of IB. The readings are tabulated.

·        A graph is drawn by taking VCE along x-axis and IC along y-axis for various values of IB.

·        The set of curves thus obtained is called the output characteristics of a transistor.

·        The reciprocal of the slope of the curve gives output impedance of the transistor. ## iii) Transfer characteristic curve: IB vs IC (VCE constant)

·        The collector-emitter voltage VCE is kept constant.

·        The base current IB is varied in steps of 10 µA and the corresponding collector current IC is noted.

·        This is repeated by changing the value of VCE. The readings are tabulated.

·        The transfer characteristics is a plot between the input current IB along x-axis and the output current IC along y-axis keeping VCE constant.

·        The slope of the transfer characteristics plot gives the current gain β can be calculated. ## RESULT

i) The input, output and transfer characteristics of the NPN junction in common emitter mode are drawn.

ii)

(a) Input impedance = ________Ω

(b) Output impedance = ________Ω

(c) Current gain β = ____(no unit)

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