Aim
To study the characteristics of a common emitter npn or pnp transistor and to find out the values of current and voltage gains.
Materials Required
- An n-p-n transistor
- A 3-volt battery
- A 30-volt battery
- Two high resistance rheostats
- One 0-3 volt voltmeter
- One 0-30 volt voltmeter
- One 0-50 Î¼A micro-ammeter
- One 0-50 mA milli-ammeter
- Two one way keys
- Connecting wires
Theory
What is input resistance?
Input resistance (R_{I}) is defined as the resistance offered by the collector junction and has a very small resistance due to forward biasing. It is given as:
\(R_{I}=\frac{\Delta V_{b}}{\Delta I_{b}}\) |
What is output resistance?
Output resistance (R_{O}) is defined as the resistance offered by the collector junction and has a high value due to reverse biasing. It is given as:
\(R_{O}=\frac{\Delta V_{c}}{\Delta I_{c}}\) |
What is current gain?
Current gain (Î²) is defined as the ratio of change in collector current to the change in base current. It is given as:
\(\beta =\frac{\Delta I_{c}}{\Delta I_{b}}\) |
Diagram
Procedure
- The experimental setup should be according to the diagram.
- All the connections should be neat, clean and tight.
- The least count and zero error of voltmeters and ammeters should be noted.
- Before inserting the keys, the voltmeter readings in V_{1} and V_{2} should be zero.
For input characteristics
- To read the base voltage (V_{b}) and base current (I_{b}) from Î¼A, a forward bias voltage needs to be applied on the base junction.
- V_{b} should be increased till I_{b} increases suddenly and for every value of I_{b}, V_{b} should be noted.
- Repeat steps 5 and 6 by making the collector voltage V_{c} = 10 V.
- With V_{c} = 20 V and 30 V, repeat step 7.
- Make all the readings zero.
For output characteristics
- Note the collector current I_{c} by adjusting collector voltage V_{c} zero, and base current I_{b} = 10 Î¼A by adjusting base voltage V_{b}.
- To note the collector current, make the collector voltage 10V, 20V and 30V.
- With base current I_{b}=20Î¼A, 30Î¼A and 40Î¼A, repeat steps 10 and 11.
- Record the observations.
Observations
Least count of the voltmeter, V_{1} = â€¦..V
Zero error of voltmeter, V_{1} = â€¦â€¦.V
Least count of the voltmeter, V_{2} = â€¦..V
Zero error of voltmeter, V_{2} = â€¦â€¦.V
Least count of milli-ammeter = â€¦â€¦..mA
Zero error of milli-ammeter = â€¦â€¦.mA
Least count of micro-ammeter = â€¦â€¦..Î¼A
Zero error of micro-ammeter = â€¦â€¦.Î¼A
Table for base voltage and base current
Sl. no |
Base voltage (V_{b}) in V |
Base current (I_{b}) for collector voltage |
|||
V_{c}= 0V in Î¼A |
V_{c}= 10V in Î¼A |
V_{c}= 20V in Î¼A |
V_{c}= 30V in Î¼A |
||
Table for collector voltage and collector current
Sl.no |
Collector voltage (V_{c}) in V |
Collector current (I_{c}) for base current |
|||
I_{b} = 10Î¼A in mA |
I_{b} = 2 0Î¼A in mA |
I_{b} = 30Î¼A in mA |
I_{b} = 40Î¼A in mA |
||
Calculations
1. Calculation for input resistance (R_{I})
Plot graph between base voltage V_{b} from table-1 and base current I_{b} from table-2 for zero collector voltage V_{c}. V_{b} is on the x-axis and I_{b} is the y-axis. The graph obtained is called input characteristics of the transistor. The slope gives the value of \(\frac{\Delta I_{b}}{\Delta V_{b}}\) which is the reciprocal of \(\frac{\Delta V_{b}}{\Delta I_{b}}\)
2. Calculation for output resistance (R_{O})
Plot graph between collector voltage V_{c} and collector current I_{c} from table-2 for 10 Î¼A. V_{c} is taken on x-axis and I_{c} on the y-axis. The graph obtained is called output characteristics of the transistor. The slope gives the value of \(\frac{\Delta I_{c}}{\Delta V_{c}}\) which is the reciprocal of \(\frac{\Delta V_{c}}{\Delta I_{c}}\)
3. Calculation for current gain (Î²)
Plot graph between base current I_{b} and its corresponding collector current I_{c} from table-2 for 30V collector voltage V_{c}. I_{b} is taken on x-axis and I_{c} on the y-axis. The graph obtained will be a straight line and is called the current gain characteristic of the common emitter transistor. The slope of the straight line gives the value of \(\frac{\Delta I_{c}}{\Delta I_{b}}\) which is equal to Î².
4. Calculation for voltage gain A_{v}
\(A_{v}=\beta *\frac{R_{0}}{R_{1}}\)Result
For the given common emitter transistor,
Current gain, Î² = â€¦â€¦.
Voltage gain, A_{v} = â€¦â€¦.
Precautions
- The connections should be neat, clean and tight.
- Key should be used when the circuit is being used.
- Beyond breakdown, forward bias voltage should not be applied.
- Beyond breakdown, reverse bias voltage should not be applied.
Sources Of Error
Faulty junction diode might be supplied.
Viva Questions
Q1. What is semiconductor junction transistor?
Ans: A semiconductor junction transistor has three sections and they are:
- Emitter (E)
- Base (B)
- Collector (C)
Q2. How are the three sections arranged in a transistor?
Ans: A thin layer of extrinsic semiconductor is used in the base and on the other two sections the second type of extrinsic semiconductor is used.
Q3. What are the different ways to use a transistor?
Ans: Following are the three different ways in which a transistor can be used:
- Common base circuit
- Common collector circuit
- Common emitter circuit
Q4. What is the importance of the input characteristics?
Ans: The importance of the input characteristics is that it helps in calculating the input resistance of the transistor.
\(R_{I}=\frac{\Delta V_{b}}{\Delta I_{b}}\) |
Q5. What is the importance of the output characteristics?
Ans: The importance of the output characteristics is that it helps in calculating the output of the transistor.
\(R_{O}=\frac{\Delta V_{c}}{\Delta I_{c}}\) |
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