1. V-I characteristics of a transistor
The behavior of a transistor is observed with the help of its V-I characteristic.
There are two types of characteristic.
1. Input Characteristic
2. Output Characteristic
Input Characteristic: Input characteristic of a transistor gives the
relationship between input current and input voltage for a constant
output voltage.
Output Characteristic: Output characteristics of a transistor gives
the relationship between output current and output voltage for
constant input current.

2. V-I characteristics in cb configuration

3. Input characteristics
The input characteristic is a curve plotted between IE and VEB. At constant base voltage VCB.
Keeping VCB constant(e.g.2V) note down IEB for different VEB.
This will give input characteristic.

4. V-I characteristics in cb configuration

5. Input characteristics
Observations:--
(i)Emitter current is very small ,when emitter
Base voltage VEB is below to the knee voltage.
(ii)After knee voltage the emitter current increasers rapidly.
Input Resistance(ri):--It is the ratio of change in emitter voltage ∆VEB to the resulting change in emitter current∆IE at constant collector base voltage VCB.

6. Output characteristics
Output Charecteristic:--
It is the curve between IC and VCB at constant emitter current.
Keeping emitter current IE fixed note the collector current IC for various values of collector base voltage VCB.

7. Output characteristics
The output characteristics has 3 basic regions:
Active region – defined by the biasing arrangements
Cutoff region – region where the collector current is 0A
Saturation region- region of characteristics to the left
Of VCB = 0V

8. The curves (output characteristics) clearly indicate that a first approximation to the relationship between IE and IC in the active region is given by
IC ≈IE
Once a transistor is in the ‘on’ state, the base-emitter voltage will be assumed to be
VBE = 0.7V

10. OBSERVATION:--
1. The collector current Ic varies withVCB only at very low voltage (VCB<0).The transistor is never operated in this region.
2. Collector current IC is almost constant when the voltage is increased beyond the cutin voltage.It means that now Icis independent of VCB and depends upon IE.
OUTPUT RESISTANCE:--It is the ratio of change in collectorbase voltage (∆VCE) to change in collector current(∆IC)at constant emitter current.

11. V-I characteristics of CE configuration

12. V-I characteristics of CE configuration

13. The input characteristic is a curve plotted between IBand VBE
The input characteristic is a curve plotted between IBand VBE. At constant Collecror Emitter voltage VCE.
Keeping VCE constant note down IB for different VBE.
This will give input characteristic.

14. Input characteristics
IB is microamperes compared to miliamperes of IC.
IB will flow when VBE > 0.7V
for silicon and 0.3V for germanium
Before this value IB is very small and no IB.
Base-emitter junction is forward bias
Increasing VCE will reduce IB
for different values.
Input characteristics for a
common-emitter NPN transistor

15. Observation:--As compared to CB configuration,IB increases less rapidly with increase of VBE.Therefore input resistance of a CE configuration is higher than that of CB configuration.
Input Resistance:- It is the ratio of change in base emitter voltage(∆VBE) to the change in base current(∆IB) at constant VCE

16. Output characteristics
For small VCE (VCE < VCESAT), IC increase linearly with increasing of VCE
VCE > VCESAT IC not totally depends on VCE  constant IC
IB(uA) is very small compare to IC (mA). Small increase in IB cause big increase in IC
IB=0 A  ICEO occur.
Noticing the value when IC=0A. There is still some value of current flows.

17. Observation:-
1.The collector current IC varies with VCE between 0 and 1V only.After this collector current is almost constant,independent of VCE.
2. For any value of VCE above knee voltage, the collector current IC is approximately equal to βIB..
3. The transistor are operated in the region above knee voltage.
Output Resistance:- It is the ratio of change in collector emitter voltage(∆VCE) to change in collector current(IC) at constant IB.

18. Comparison of transistor configuration
S.NO
CHARECTERISTIC
COMMON BASE
COMMON EMITTER
COMMON COLLECTOR 1
Input resistance
Low
(about 100 Ω)
high
(about 750 Ω)
Very high (about 750 Ω) 2
Output resistance
Very high (about 450 Ω)
High
(about 45k Ω)
(about 50 Ω) 3
Voltage gain
(about 150)
(about 500)
Very Low
(Less than 1) 4
Leakage current
Very small
(5 μA for Ge)
Very large
(500μA for Ge)
(500μA forGe) 5
Application
For high frequency application
For audio frequency application
For impedence matching

19. Conclusion:--
Common emitter circuit is most efficient
reason:--
1.High current gain
2.High voltage and Power gain.