CB TRANSISTOR-Transistor characteristics in common base configuration.

Transistor characteristics in common base configuration.

When the transistor is connected with its base terminal common to both the input junction
and the output junction, it is said to be in common base mode configuration.

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

APPARATUS: CB mode kit, patch chords.

FORMULAE: 1. i/p resistance: ri = (ΔVEB) / ΔIE at constant VCB
2. o/p resistance: ro = ΔVCB / ΔIC at constant IE

THEORY
When transistor is connected with its base terminal common to both the input junction
and the output junction, it is said to be in common base mode configuration.

Input characteristics:
The input characteristics for CB mode are
plotted by keeping collector to base voltage
(VCB) constant and varying emitter to base
voltage (VBE) and noting down the corresponding
value of emitter current IE. The equivalent circuit
for transistor can be considered as shown in the above fig.
For VCB = 0 V: The E-B junction just behaves as
forward biased p-n junction diode for VCB = 0V. Hence
the characteristics curves are identical to it. Current
increases rapidly beyond the cut in voltage.
For VCB > 0 V: The curve shifts toward the left for VCB > 0V. This is because; as the
collector to base voltage is increased; positive potential on collector accelerates the
electrons from the base into the collector thereby increasing collector current. This results in
increase in emitter current (since IE = IB+IC). Thus for same value of VEB for VCB> 0 V
larger emitter current flows and hence curve shifts towards left compared to for VCB = 0V.

Output characteristics:
The output characteristics for CB mode are plotted by keeping emitter current IE constant
and varying collector to base voltage (VCB) and noting down corresponding value of
collector current IC.
For IE = 0 mA: The C-B junction behaves
as reverse biased junction and hence a small collector
current of the order of few μA flows and remains almost
constant even if VCB is varied. Thus for IE = 0 mA; both
E-B and C-B junction are reverse biased and hence this
region is called cut off region.

For IE > 0 mA: The transistor is properly biased (i.e. E-B junction forward biased and CB
junction reverse biased). The majority charge carriers start diffusing into the collector
and hence initially the collector current increases with increase in VCB. The collector
current then attains some saturation value and is nearly independent of VCB. This value of
collector current increases if IE is increased following same trend of variation. Thus a set
of collector i.e. output curves are obtained for different constant values of IE. The region in
which the collector current is independent of VCB, varies with (and nearly equal to) IE; is
called active region. The transistor is operated in this region to work as an amplifier; i.e.
to amplify weak ac signals. The dynamic output resistance is found using the formula
o/p resistance:
ro = ΔVCB / ΔIC at constant IE

Transfer characteristics:
When the transistor is operated in its active region; the collector current is independent of
collector voltage and varies with emitter current. The ratio of change in collector current
with change in emitter current defines the current gain and is denoted as α.

PROCEDURE
Input and transfer characteristics:
1. Make the connections as shown in figure.
2. Keep VCB = 0 V by shorting collector to emitter terminal .
3. For IE = 0 mA of emitter current (IE) note the value of VEB.
4. Vary IE in steps of 2 mA up to IE = 15 mA. Each time record the value of VEB.
5. Remove the short and reconnect the output circuit. fix the output voltage VCE to say 2 V.
Repeat step 4. Then repeat above step 4 for VCE = 5 v.

Output characteristics:
1.Fix the value of IE = 2 mA
2.Vary VCB from 0v to 1v in steps of 0.2V. Note down value of IC. Beyond 1 V;
vary voltage in steps of 1 V noting down IC each time.
3.Repeat step 2 for two more values of IE i.e. for IE = 4 and 6 mA.

RESULT
1. The i/p resistance of transistor in CB mode is found to be --------
2. The o/p resistance of transistor in CB mode is found to be --------

Answer the following
Q1. What do you mean by ‘α’ of the transistor?
Q2. Write relation between α and β of transistors.
Q3. Can emitter and collector be interchanged?
Q4. Compare three modes; CE, CB and CC w.r.t. their o/p and i/p resistances.
Q5. What are the principal carriers in p n p transistor.
Q6. Draw the ckt. diagram for p n p transistor in CB mode.
Q7. Why the collector current varies initially with VCB and then remains constant?
Q8. Can you find cut in voltage for transistor?
Q9. Why the value of α is less than 1?
Q10. What do you mean by term ‘BJT’?
Q11. What do you mean by an amplifier ckt?
Q12. Why base is lightly doped?
Q13. What do you mean by ‘transfer characteristics’ of a transistor?
Q14. What is the basic difference between CE and CB modes?
Q15. Do you know any other semiconductor device that has similar amplifying action?
Q.16 - Why is the base current small in a transistor ?