BJT- thevenins or Voltage Divider to use

[sankalani2003]

in BJT problems some use Voltage Divider to find Base voltage and then going to find other parameters.

in some cases they start from thevanins voltage,resistance and going to find other parameters.

question is when to use above methods in correct way.

 

[neils_arm_strong]

You can use any method you like.But a thevenin equivalent gives a systematic approach in most cases.

 

[sankalani2003]

what if transistor's gain is High

 

[Pheetuz]

The gain of the transistor shouldnt have an impact on which of the above you should use, personally i use the voltage divider theorem to find the base voltage and this is a good starting point to find out the rest of the values associated with the transistor.

 

[jimmy neutron]

I think the methods (voltage divide rule) and Thevenin's will give different results. Look at the attached figure:



The voltage divide rule makes the approximation that the current flow into the base is so small that it can be neglected. This is a reasonable approximation since expect the current through the 62k and 9.1k resistors to be of the order millamp while the current into the base is expected to be oforder microamp, i.e. a thousand times smaller.

In this case the current through the 9.1k is the same as the current through the 62k and then you can calculate the pd across the 9.1k as 9.1/(9.1 + 62)*16 V.

[As aside: In this particular example the pd between the base and ground is 2.05 V. Allowing for the 0.7 V drop across the BE junction this gives the pd across the .68k as 2.05 - 0.7 = 1.35 V. Now can calculate IE and if β is known then can determine IB to check the assumption]