Early Effect

Summary about the Lecture on 18-08-2014

1.      The generalised transistor equation is  

2.      Large signal current gain (CB) is

3.      Typical value of α is between 0.90-0.995.

4.      The consequences of Early effect

a)      α increases with |V_CB| as there is less chance of recombination within the base region.

b)      I_E increases with |V_CB| because the concentration gradient of minority charges within the base region is increased. This is due to decrease in the effective base width.

c)      For extremely large reverse voltage across the collector junction makes the effective base width reduced to zero causing a breakdown called punch-through.

5.      There exhibits a voltage in the input characteristics of the BJT called cutin, offset or threshold voltage below which the input current is very small.

6.      There are three regions in the output characteristics of the BJT namely

a)      Active region:

                                            i.            In this region the J_E is FB and the J_C is RB.

                                          ii.            The I_C is independent of V_C and dependent only upon I_E

                                        iii.            A small increase in |I_C| is observed with increase in |V_C|, this is due to Early effect.

b)      Saturation region:

                                            i.            In this region both the J_E and J_C are FB.

                                          ii.            BJT behaves as a switch in ON state.

                                        iii.            This region is characterised by the region closure to the ordinate where the output current increases rapidly with the output voltage.

c)      Cut-off region:

                                            i.            In this region both the J_E and J_C are RB.

                                          ii.            This region is characterised by the region below the curve corresponding to output current equal to zero.

References: 

Millman and Halkias, Integrated Electronics, McGrawHill.

Robert  L.  Boylestad and Louis  Nashelsky, Electronic Devices and Circuit Theory, Pearson.