Transistor in CE mode

AIM: 1. To plot the input characteristics and transfer characteristics.
2. To plot output characteristics of Transistor in common emitter mode.
3. To calculate input resistance (ri), output resistance (ro), and current gain (β).

ce transistor

The transistor is a three-terminal device namely emitter, base, and collector. Thus
it has two junctions viz; base-emitter (B-E) and collector base (C- B) junctions. The
transistor when properly biased; provides power gain that is needed for most
electronic applications. The transistor is said to be properly biased; when E-B junction
is forward biased while C-B junction is reverse biased.

APPARATUS: Transistor in CE mode kit, patch chords, dc power supply.

FORMULAE
1. input resistance ri = (ΔVBE / ΔI B ) at VCE = constant
2. output resistance ro = (ΔVCE/ ΔIC) at IB = constant
3. current gain β = (ΔIC/ΔIB) V CE = constant

THEORY
Transistor is a three-terminal device namely emitter, base and collector. Thus
it has two junctions viz; base- emitter (B-E) and collector base (C- B) junctions. The
transistor when properly biased; provides power gain that is needed for most
electronic applications. The transistor is said to be properly biased; when E-B junction
is forward biased while C-B junction is reverse biased.

Depending upon the type of application a transistor is connected in three
configurations: (i) common base (CB)
(ii) common emitter (CE)
(iii) common collector (CC).

The common emitter connection is widely used because of its high current
amplification, where emitter is the common point with the input and output terminals.
Any electronic device is analyzed by studying I-V characteristics i.e. variation of
current with applied voltage. If the voltages are dc then I-V characteristics are said to
be static characteristics while if voltages are ac then I-V characteristics are said to be
dynamic characteristics. For transistor since it is mainly used as amplifier; dynamic
characteristics are of more interest than static. Common Emitter transistor analysis is
based on the following characteristics.

Input characteristics:

Variation of IB with respect toVBE , by keeping VCE constant gives i/p characteristics
which is shown in fig. From the graph input dynamic resistance ri can be calculated. In
forward bias, the input resistance ri is expected to be small. It is given by,
ri = (ΔVBE / ΔIB ) at vCE = constant
Static input resistance is instantaneous resistance of the transistor and is given as
ri = (VBE / IB) at VCE = constant

Output characteristics:

 typical output characteristics curves give the relation
between output variables Ic and vCE for constant IB. Since the collector-base junction
is reverse biased, the output resistance ro is larger. It is defined as, ro = (ΔVCE/ ΔIC)
at IB = constant
Transfer characteristics :
By keeping VCE constant, variation of output collector current Ic, with respect to base
current IB shows Transfer characteristics. This graph is helpful for calculating current
gain β. It is the ratio of a small change in IC to the corresponding change in IB for
constant VCE. Current gain β varies from 20 to 500 according to the transistor type. β
can also be calculated from the transfer characteristics- the plot of IC and IB for fixed
VCE values.

PROCEDURE
1. Input and transfer characteristics:
1. Make the connections as shown in figure.
2. Keep VCE = 0 v by shorting collector to emitter. For zero μA of base current (IB)
note the value of voltage vBE.
3. Vary value of IB in steps of 10 μA up to IB = 100μA. Each time note down the
corresponding value of VBE.
4. Remove the short. Fix output voltage vCE to say 1V. Repeat step 3.
This time; note down the value of ic also along with VBE.
6. Repeat above step for VCE = 3 v also.
2. Output characteristics:
1.Fix the value of IB = 0 μA
2.Vary VCE from 0V to 10 V in steps of 1V. Note down value of IC.
3.Repeat step 2 for three more values of IB i.e. for IB = 10, 30 and 50μA.
After completion of experiment knobs of different power supplies
should be kept at minimum positions.

RESULT:
1. The i/p resistance of the transistor in CE mode is found to be -------------
2. The o/p resistance of the transistor in CE mode is found to be --------------
3. The current gain of the transistor in CE mode is found to be ----------------

Answer the following
Q1.why the circuit configuration called common emitter configuration?
Q.2. if the collector circuit is pen and the emitter circuit is closed, does base current
flow? Will it be high or low?
Q3. Define static and dynamic resistances.
Q4. State doping levels of three regions.
Q5. What are ‘input characteristics’?
Q6. Draw symbols of npn and pnp transistors.
Q7. A transistor is a current controlled device than voltage controlled
one. Explain.
Q8. Show cut off, saturation & active regions on collector characteristics.
Q9. Why semiconductor devices are preferred to vacuum tubes?
Q10. What are principle carriers in npn and pnp transistors?
Q11. Define ‘β’ of transistor.
Q12. How transistor is biased normally?
Q13. What are the applications of transistors?
Q14. In digital circuits, transistor is used as switch. What is mode of
biasing in this case?
Q15. What does arrow on emitter lead indicate?
Q16. Write equation involving transistor currents.
Q17. What are the modes other than CE, in which transistor may be operated?
Q18. Why is the collector made wider than emitter and base?