BC547



BC547 is an npn  bi-polar junction transistor. a transistor stands for transfer of resistance, mainly used for amplification and switching purposes. it has a maximum current gain of 800. The  transistor  terminals  require  a  fixed  dc  voltage  to  operate  in  the  desired region  of its characteristic curves. bc547 is used in common emitter configuration for amplifiers.  the  voltage  divider  is  the  commonly  used  biasing  mode.  for  switching  applications, transistor is biased so that it remains fully on if there is a signal at its base. in the absence of base signal, it gets completely off. its equivalent transistors are bc548 and bc549. together with other electronic components, such as resistors, coils, and capacitors, it can be used as the active component for switches and amplifiers. 


Figure 1:  pin diagram of bc547B



  •   VCE=.7 t0 30v
  • Imax=100mA
  •  gain 200 to 450
  • VBE on =0.7
  • Vinput max =5V
  •   maximum frequency nearly 1MHz (750ns)
Sample application


Figure 2: TRANSISTOR AS RELAY DRIVER  

BASE RESISTOR DESIGN

From data sheet,
Maximum Collector current Ic = 100 mA
Current gain β = 200 – 450
take  β=200 and Ic = 100 mA
β = Ic/Ib
Ib = Ic/ β
   = 100/200 = 0.5mA
There will be a drop of  0.7V across base emitter junction of transistor
Ie,.. VBE = 0.7V;
Voltage across  the resistor Rb ,Vb =input base voltage control signal -  VBE
 = 5-0.7 = 4.3V
R = Vb/Ib
=4.3/(0.5*10-3) = 8.6 KΩ
Select the value less than above value
Rb=7.2K