To Study The Characteristics Of A Common Emitter NPN (or PNP) Transistor And To Find Out The Values Of Current And Voltage Gains

Transistor is a semiconductor device which is used to amplify or change the direction of electronic signals and electrical power. NPN and PNP are bipolar junction transistors which are mainly used in amplifiers, oscillators, switches, and many other electrical circuits. Let us now learn about the characteristics of a common emitter NPN or PNP transistor through the experiment.

Keywords Takeaways: Semiconductor, Transistor, Output resistance, Input resistance, Diodes, Amplifiers, Oscillators, Switches, Electrical circuit

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Aim Of The Experiment 

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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 gains.

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Apparatus Required 

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The materials required for this experiment are as follows:

• n-p-n transistor, 
• Three volt battery, 
• 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.

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Theory

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In the common-emitter circuit of a transistor, the emitter-base makes the input section and emitter-collector makes the output section. As usual, the base junction (input junction) is forward biased and the collector junction (output junction) is reverse biased. Input resistance (RI) is defined as the resistance offered by the collector junction and has a very small resistance due to forward biasing and it is known as input resistance. 

Common Emitter Circuit

Output resistance (RO) is defined as the resistance offered by the collector junction and has a high value due to reverse biasing and is called output resistance. Current gain (β) is defined as the ratio of change in collector current to the change in the base current is known as current gain.

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Formula Used

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The formula used for these experiments are as follows.

Input Resistance, RI = ΔVb/ ΔIb

Output Resistance, RO = ΔVc/ Ic

Current Gain, β = ΔIc/ Ib

Voltage Gain = Current gain x Resistance gain

Av = β x RO/ RI

Diagram

Common Emitter Transistor Circuit

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Procedure

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1. Make a circuit as shown in the figure above.
2. Make all the connections neat, clean and tight. Double-check the connections
3. Now, note the least count and zero errors of voltmeters and ammeters carefully.
4. Make the voltmeter readings zero in V1 and V2 and insert the keys.
5. Now, apply forward bias voltage on the base junction. Read the base voltage ( Vb) from and the base current (Ib) from μA.
6. Go on increasing Vb till Ib rises. Corresponding note the values of Ib for each value of Vb.
7. Make the collector voltage Vc = 10 V and repeat steps 5 and 6.
8. Repeat step 7 with the following readings Vc = 20 V and 30 V.
9. Make all the readings zero.
10.  Now Keep the value of collector voltage (Vc) as zero. Adjust the base voltage Vb to make the base current Ib = 10 μA. Though the value of collector voltage Vc is zero, there is also collector current Ic. Note the readings.
11. Make the collector voltage as follows 10 V, 20 V and 30 V and note the corresponding collector currents carefully.
12. Repeat steps 10 and 11 with the following readings Ib = 20μA, 30 μA, and 40 μA.
13. Write down your observations.

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Observations

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Least count of voltmeter, V1 = ……….V

Zero error of voltmeter, V1 = ……

Least count of voltmeter, V2 = ……….V

Zero error of voltmeter, V2 = ……….V

Least count of milli-ammeter = ………… mA

Zero error of milli-ammeter = ………….mA

Least count of microammeter = ……….uA

Zero error of microammeter = ………uA

Table for base voltage and base current

Note down the observations in the following table.

Table for collector voltage and collector current

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Calculations

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The following calculations are to be done.

Calculation of input resistance (RI ) from the observations

To calculate the input resistance, Plot a graph between the base voltage Vb values and base current Ib values for the zero collector voltage Vc, take Vb along X-axis and Ib along Y-axis. Plot the graphs for different values of Vc. 

The slope of the graph becomes large towards the end. These graphs are known as ‘input characteristics’ of the transistor.

Calculation for output resistance (R0) from the observation.

Now Plot a graph between the values of collector voltage Vc as observed and the values of collector current Ic as observed above for 10 μA base current Ib, now take Vc along X-axis and Ic along Y-axis. Plot the graphs for different values of Ib.

The slope of the graph becomes zero.These graphs are known as ‘output characteristics’ of the transistor.

Calculation for current gain (β) as observed 

To start with the calculation of current gain, plot a graph between the values of base current Ib as observed during the experiment and the values of corresponding collector current Ic as observed for 30 volts collector voltage Vc, now take Ib along X-axis and Ic along Y-axis. The graph that comes is a straight line. The graph is known as the current gain characteristic of the common emitter transistor.

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Result 

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The result for the given common emitter transistor is,

Current gain, β = …….

Voltage gain, Av = …….

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Precautions

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1. Check whether the connections are neat, clean and tight.
2. The key should be used only when the circuit is being used.
3. Be careful, Beyond the breakdown point, forward bias voltage should not be applied.
4. Be careful, Beyond the breakdown point, reverse bias voltage should not be applied.

Sources Of Error

Faulty junction diodes can be the source of error when applied.

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Things To Remember

• A transistor is a semiconductor device which is used to amplify or to change the direction of electronic signals and electrical power.
• Input resistance (RI) is defined as the resistance offered by the collector junction and has a very small resistance due to forward biasing and it is known as input resistance.
• Output resistance (RO) is defined as the resistance offered by the collector junction and has a high value due to reverse biasing is called output resistance.
• Current gain (β) is defined as the ratio of change in collector current to the change in the base current is known as current gain.
• In the common-emitter circuit of a transistor, the emitter-base makes the input section and emitter-collector makes the output section. As usual, the base junction (input junction) is forward biased and the collector junction (output junction) is reverse biased.
• Faulty junction diodes can be the source of error when applied is the source of error.

Also Read:

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