Input and Output characteristics of transistor in Common Emitter Configuration

Input Characteristics:-

Input characteristics in Common Emitter configuration means input voltage Vs input current by keeping output voltage  as constant.

i.e,  Vs  by keeping  constant.

Therefore the curve between Emitter current  and Emitter to Base voltage  for a given value of Collector to Base voltage  represents input characteristic.

for a given output voltage  , the input circuit acts as a PN-junction diode under Forward Bias.

from the curves there exists a cut-in (or) offset (or) threshold voltage  below which the emitter current is very small  and a  substantial amount of Emitter-current flows after cut-in voltage ( 0.7 V for Si and 0.3 V for Ge).

the emitter current  increases rapidly with the small increase in . with the low dynamic input resistance of a transistor.

i.e, 

This is calculated by measuring the slope of the input characteristic.

i.e, input characteristic determines the input resistance .

The value of  varies from point to point on the Non-linear portion of the characteristic and is about  in the linear region.

Output Characteristics:-

Output Characteristics are in between output current Vs output voltage with input current as kept constant.

i.e, 

i.e, O/p characteristics are in between  Vs  by keeping  as constant.

basically it has 4 regions of operation Active region, saturation region,cut-off region and reach-through region.

active region:-

from the active region of operation  is almost independent of  

i.e, 

when  increases, there is very small increase in  .

This is because the increase in  expands the collector-base depletion region and shortens the distance between the two depletion regions.

with  kept constant the increase in  is so small. transistor operates in it’s normal operation mode in this region.

saturation region:-

here both junctions are Forward Biased.

Collector current  flows even when (left of origin)  and this current reaches to zero when  is increased negatively.

cut-off region:-

the region below the curve  ,transistor operates in this region  when  the two junctions are Reverse Biased. 

 even though  mA.  this is because of collector leakage current (or) reverse-saturation current  (or) .

punch through/reach through region:-

 is practically independent of  over certain transistor operating region of the transistor.

• If  is increased beyond a certain value,  eventually increases rapidly because of avalanche (or) zener effects (or) both this condition is known as punch through (or) reach through region.
• If transistor is operated beyond the specified output voltage () transistor breakdown occurs.
• If  is increased beyond certain limit, the depletion region() of o/p junction penetrates into the base until it makes contact with emitter-base depletion region. we call this condition as punch-through (or) reach-through effect.
• In this region , the large collector current destroys the transistor.
• To avoid this  should be kept in safe limits specified by the manufacturer

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