# Early Effect! Pls help

Discussion in 'General Electronics Chat' started by sahil_time, Nov 1, 2011.

1. ### sahil_time Thread Starter New Member

Nov 1, 2011
1
0
Early effect states the collector current increases as we increase CE voltage in a common emmiter circuit due to base-width modulation. But does the Early effect change the base current? Does base current remain constant or does it change?

2. ### nigelwright7557 Senior Member

May 10, 2008
488
71
If CE current increases then so does base current.
Base current will be CE current / HFE.

3. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
β
It depends on the forcing function.
If Ib is held constant, Ic will increase as Vce increases.
If Ic is held constant, Ib will decrease as Vce increases.

The implication here is that β increases as Vce increases.

4. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
But HFE is modulated by Vce.
See post #3.

Dec 26, 2010
2,147
300
Not necessarily: the base current may be determined by external circuit constraints, such as a high-impedance current source.

In particular, the Early effect can be evaluated from measurements of the Ic vs Vce characteristics obtained as a family of curves with base current as a parameter, where the base current is held constant for each individual curve.

In this case the base current is determined independently, not as a function of other things.

Edit: Others have put this better: Early effect modifies β.

6. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
This is one of those times where it would have been useful to quote the post you were replying to. You said "not necessarily". Not necessarily what? I assume you were replying to post #2.

Dec 26, 2010
2,147
300
I did not put that very well. I had wanted pretty much to contradict post #2, in the sense that although it usually true that
in this context this comment seemed likely to mislead the OP, given that here HFE is not fixed.

That started me off woffling about characteristic curves, and how the effect could be measured. When I saw your comment about HFE being modulated by Vce, this was clearly a better approach. Unfortunately by then I had submitted my post. (I have never been very fast typing, and now some deterioration of the faculties has slowed me down even more.)

8. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
Ahh, a fellow geezer!

Oct 29, 2011
82
30
10. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
cabraham likes this.