early effect

[bakkis]

Hi,
Somebody please explain how early effect changes Vbe. I know that: with changes in collector-base voltage the width at the collector-base junction varies causing current due to minority carriers add up to the collector current. But how does Vbe get affected here ?

Thanks

 

[bakkis]

SOmebody, please answer this is important for me.

 

[Ratchit]

bakkis,

Somebody please explain how early effect changes Vbe. I know that: with changes in collector-base voltage the width at the collector-base junction varies causing current due to minority carriers add up to the collector current. But how does Vbe get affected here ?

Since the Ic is exponentially related to Vbe, it is to be expected that Vbe will rise and fall as Ic rises and falls due to variations in Vbc. The Early name is capitalized because it was named after James Early.

Ratch

 

[BrownOut]

Not quite. The Early effect produces an output current that is independent of vbe, and is linear with VCE. vbe should not be affected in any unilateral confuguration. In non-unilateral configurations, small signal and large signal analysis must be performed to determine the effect of vbe, but generally, vbe will be smaller when considering the Early effect due to degenerative feedback current produced from VCE. See Sedra & Smith, bipolar transistors ( ch 4 in the 3rd edition ) for more details.

 

[Ratchit]

bakkis,

Look at the second to the last paragraph of this link. **broken link removed**
Looks like Vbe will be slightly less, especially at higher reverse voltages.

Ratch

 

[BrownOut]

The small change in vbe is due to the degenerative feedback current, as I described in post #4. Note however, that the current caused by the Early effect is indpenedent of vbe. To see this, note that current goes up as vbe goes down.

 

[bakkis]

Thanks you all for the replies. I get the point.

 

[MrAl]

The small change in vbe is due to the degenerative feedback current, as I described in post #4. Note however, that the current caused by the Early effect is indpenedent of vbe. To see this, note that current goes up as vbe goes down.

Hi,

Partly joking here, how can the current be independent of vbe if when vbe goes down the current goes up? We arent allowed inverse dependence all of a sudden? (har har)

I know what you mean though, that it's MOSTLY independent of vbe, and i agree, because vbe doesnt usually change much so the major effect is not from that. If vbe changes by 0.01 volt that's only a change of 0.01v in vbc, which is negligible compared to most changes we see in the collector base voltage.

I think a better way to describe it is to say that the early effect causes a change in collector current with vbc due to base width modulation from the collector base reverse voltage. The graphs all show vce though so we can extrapolate back to get the actual value of the Early voltage.


BTW, nice article linked to written by Bob Pease Ratch.

 

[BrownOut]

I didn't say that current is independent of vbe. I said current due to the Early effect is independent of vbe. In other words, the current is composed of two components, the normal 'forward' transconductance current which is dependent on vbe, and the current that is dependent on VCE. That current, independent of vbe, is degenerative and decreases vbe ( notice I don't say here is vbe in independent of output curret ) If you read up on it, you would see I"m correct.

 

[bakkis]

Mr Al,
Show us the link to the article by Bob Pease Ratch.

Srinivas

 

[bakkis]

Hi BrouwnOut,
I thought i had got the point. But your reply seems to confuse me, here is what i understand, kindly let me know if am right or wrong :
collector current changes with collector-base junction width modulation(reverse biasing). This current change has nothing to do with Vbe.

Q: Whats the current that's dependent on Vce ?

Srinivas

 

[BrownOut]

Hi bakkis. Every transistor has a forward transconductance, given by iC=gmvBE. That's the normal foward current where the output is a function of the input, at vBE. The current that's linearly proportional to vCE is the current from the Early effect. Look at the equation:

Ic=Ise^(VBE/VT)*(1+VCE/VA) source

the last quantity in parenthesis: (1+VCE/VA) VA is the so-called Early voltage. As VCE rises, this component of collector current rises independently of vBE. Now, the reason is base-width modulation, but the effect is a linear current rise in collector current proportional to VCE. In other words, hold vBE constand and vary VCE, and you measure a collector current that varies lineraly with VCE. If you change vBE to a different value, then also it constant, you'll see again a linear current proportional to VCE, but the slope of the resultant current wil be different. Look at the graph in the right margin of the link provided, it shows the linear current rise with VCE rise, and also shows -VA, the early voltage.

 

[BrownOut]

Actually, maybe I should not say it's independent of vBE, because vBE does effect the slope of the current rise. However, what I mean is the current due to the Early effect is linearly proportional to VCE only, but the constant of proportionality is a function of vBE. Again, I refer you to the graph in the link above.

 

[bakkis]

Thanks a lot Mr Brownout !!! That was very helpful.

 

[MrAl]

I didn't say that current is independent of vbe. I said current due to the Early effect is independent of vbe. In other words, the current is composed of two components, the normal 'forward' transconductance current which is dependent on vbe, and the current that is dependent on VCE. That current, independent of vbe, is degenerative and decreases vbe ( notice I don't say here is vbe in independent of output curret ) If you read up on it, you would see I"m correct.

BrownOut:
Yes, but saying that a current I goes up as a voltage V goes down indicates a dependence, so the wording is a bit confusing that's all.
Also, that link isnt considered the best way to represent this effect and i would think they would have acknowledged that. It think it is an accepted model though so no harm done i guess.

bakkis:
See post #5 for Ratch's link to RAP.

 

[BrownOut]

Mr Al, I've already explained this. There is absolutely no dependence of the Early effect on vbe (other than what I've already pointed out), and none implied. The link was a fine way of representing the effect, and prefect understanding has resulted. ( see post #14 ) There is really no reason to try to continue to confuse the issue.

 

[MrAl]

Hello again,


Sorry if i confused you. I didnt think the discussion would be complete without talking about the true cause of the effect rather than a mostly behavioral aspect. If you're still confused, read more in the link you provided. They talk about more of this there.

 

[BrownOut]

I'm not confused. Never was. We already talked about what causes the effect, but not in much detail because that wasn't the question. I believe the OP has his answer though. I'm available anytime to talk about causes if need be.