Why is base current not what it should be?

[arjob]

My multimeter shows:
Ib=218\[\mu\]A why?

But my calculation shows, it should be: 8.5\[\mu\]A

I used the formula:
Ib=(3.1-0.7)/(10k+271k) [I have \[\beta\] = 270 ]

What is the problem? Please help

 

[Vineeth m p]

The equation which you used is accurate when the transistor is operating in active region. I mean, the current gain or beta value of a transistor at saturation region is different from that at active region, which is less than the former. It means, you cannot put the value as 270, but a lower value. which will cause a higher value of base current. In short, you should know where your BJT is actually operating in; active region or saturation region, and put appropriate value of beta.

 

[chuckey]

with R5 = 100 k, the transistor is saturated, with a current of 3.1/101K through its collector circuit, so there will be a lower voltage dropped across R4 (30 microA X 1K ~ 30mV), so the current is about 3.1 -.7V /10K.
Frank

 

[arjob]

Yes, thanks. I realized the mistake I was doing in calculating the Ib.