May 08, 2013



In  this  configuration  the  input  is  applied  between the  base  and  the  collector and  the  output  is  taken  from  the  collector  and  the  emitter.  Here  the  collector  is common to both the input and the output circuits as shown in Fig.

                                                       Common Collector Transistor Circuit

In  common  collector  configuration  the  input  current  is  the  base current  IB  and  the output current is the emitter current IE. The ratio of change in emitter current to the  change in the base current is called current amplification factor.

It is represented by


A test  circuit  for determining the  static characteristic  of an NPN transistor is shown in Fig. In this circuit the collector is common to both the input and the output circuits.   To   measure   the   base   and   the   emitter   currents,   milli   ammeters   are connected in series with the base and the emitter circuits. Voltmeters are connected   across the input and the output circuits to measure VCE and VCB


                                                Common Collector Input Characteristic Curve

  • It  is  a  curve  which  shows the  relationship  between the  base  current,  IB and the collector base voltage VCB at constant VCE This method of determining the characteristic is as follows.

  • First, a suitable voltage is applied between the emitter and the collector. Nextthe  input  voltage  VCB  is  increased  in  a  number  of  steps  and  corresponding values of IE are noted.

  • The base current is taken on the y-axis, and the input voltage is taken on the x-axis. Fig. shows the family of the input characteristic at different collector- emitter voltages.

  • The following points may be noted from the family of characteristic curves.  1.Its  characteristic  is  quite  different  from  those  of  common  base  andcommon emitter circuits.
2.When VCB increases, IB is decreased.

Output Characteristics

  • It is a curve which shows the relationship between the emitter current l and collector-emitter voltage, the method of determining the output characteristic is as follows.

  • First,  by  adjusting  the  input  a  suitable  current  IB  is  maintained.  Next  VCB increased in a number of steps from zero and corresponding values of IE are  noted.

  • The above whole procedure is repeated for different values of IB. The emitter current  is  taken  on  the  Y-axis  and  the  collector-emitter  voltage is  taken  on the X-axis.

  • Fig shows the family of output characteristics at different base current values. The following points are noted from the family of characteristic curves.
                                         Common Collector Output Characteristic Curves

1.This  characteristic  is  practically  identical  to  that   of  the  common  emitter circuit.

2.Its current gain characteristic for different values of VCE is also similar to that of a common emitter circuit.