Early Effect! Pls help

sahil_time

Joined Nov 1, 2011
1
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?

nigelwright7557

Joined May 10, 2008
532
If CE current increases then so does base current.
Base current will be CE current / HFE.

Ron H

Joined Apr 14, 2005
7,014
β
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?
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.

Ron H

Joined Apr 14, 2005
7,014
If CE current increases then so does base current.
Base current will be CE current / HFE.
But HFE is modulated by Vce.
See post #3.

Joined Dec 26, 2010
2,148
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.

Ron H

Joined Apr 14, 2005
7,014
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.
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.

Joined Dec 26, 2010
2,148
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
If CE current increases then so does base current. Base current will be CE current / HFE.
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.)

Ron H

Joined Apr 14, 2005
7,014
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.)
Ahh, a fellow geezer!