# Common emitter configuration of BJT

## Main Question or Discussion Point

in common emitter configuration of BJT , why do we have linear relationship between output current (collector current ) & output voltage ( collector emitter voltage) ? Has it anything to do with early effect ? If yes then why don't we have linear relationship in case of common base configuration ?

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in common emitter configuration of BJT , why do we have linear relationship between output current (collector current ) & output voltage ( collector emitter voltage) ? Has it anything to do with early effect ? If yes then why don't we have linear relationship in case of common base configuration ?
There is no relation between output current and output voltage. It only has linear relation IF you use a linear load at the collector. Typical example is a resistor and if you have one with NO voltage coef. Then the voltage is V=IR. You put a non linear load, the voltage is absolutely non linear. Case in point, I can make the output voltage logarithmic by using a forward biased diode as the collector load and make it a logarithmic amplifier. People in Guitar amp design use carbon composite resistors that has voltage coef to introduce some even harmonics.

erece,

in common emitter configuration of BJT , why do we have linear relationship between output current (collector current ) & output voltage ( collector emitter voltage) ? Has it anything to do with early effect ? If yes then why don't we have linear relationship in case of common base configuration ?
Yes, the variation of Ic vs. Vce in a common emitter configuration is caused by the Early effect. Notice that Early is capitalized, because it was named after James Early. In a common base configuration, one sees flat lines on the Ic vs. Vcb curve. Notice that the x-axis of the CE is Vce and on the CB is Vcb. The Early effect acts like a shunt resistor across the current generator of the collector. For a CB, the Early resistance is Re, and for the CE configuration, the Early resistance is Re/(β+1). This can be verified by a study of transistor models. Since the Early resistance looks smaller in a CE configuration, the Ic lines start to show a slope.

Ratch

erece,

Yes, the variation of Ic vs. Vce in a common emitter configuration is caused by the Early effect. Notice that Early is capitalized, because it was named after James Early. In a common base configuration, one sees flat lines on the Ic vs. Vcb curve. Notice that the x-axis of the CE is Vce and on the CB is Vcb. The Early effect acts like a shunt resistor across the current generator of the collector. For a CB, the Early resistance is Re, and for the CE configuration, the Early resistance is Re/(β+1). This can be verified by a study of transistor models. Since the Early resistance looks smaller in a CE configuration, the Ic lines start to show a slope.

Ratch
I don't know whether OP meant linear relation of Ic vs Vce when using input signal or output resistance as you describe.

Do you have any article of early resistance?

yungman,

I don't know whether OP meant linear relation of Ic vs Vce when using input signal or output resistance as you describe.
Ideally, the Ic should not change with Vce in the active region of a BJT. In the active region, the BJT is a voltage controlled current source (VCCC).

The OP asked why the Early effect does not show up as much in the CB configuration. As stated in my previous post, it is because the shunt resistance is "betatized" downward in a CE configuration.

The Early effect causes more Ic to exist than would be present without the Early effect. That is equivalent to a shunt resistor across the almost infinite resistance of the collector current generator.

Do you have any article of Early resistance?

See the "Small-signal model" paragraph in this link below
http://www.enotes.com/topic/Early_effect

Ratch

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yungman,

Ideally, the Ic should not change with Vce in the active region of a BJT. In the active region, the BJT is a voltage controlled current source (VCCC).

The OP asked why the Early effect does not show up as much in the CB configuration. As stated in my previous post, it is because the shunt resistance is "betatized" downward in a CE configuration.

The Early effect causes more Ic to exist than would be present without the Early effect. That is equivalent to a shunt resistor across the almost infinite resistance of the collector current generator.

See the "Small-signal model" paragraph in this link below
http://www.enotes.com/topic/Early_effect

Ratch
I know about early effect, ro and Va, I just have not heard of Early Resistance. Do you have formulas on this Early Resistance?

yungman,

Do you have formulas on this Early Resistance?
You can get the resistance from the slope of the Ic vs. Vce curve.

Ratch

erece,

Yes, the variation of Ic vs. Vce in a common emitter configuration is caused by the Early effect. Notice that Early is capitalized, because it was named after James Early. In a common base configuration, one sees flat lines on the Ic vs. Vcb curve. Notice that the x-axis of the CE is Vce and on the CB is Vcb. The Early effect acts like a shunt resistor across the current generator of the collector. For a CB, the Early resistance is Re, and for the CE configuration, the Early resistance is Re/(β+1). This can be verified by a study of transistor models. Since the Early resistance looks smaller in a CE configuration, the Ic lines start to show a slope.

Ratch
re is conventionally is emitter resistance and is 1/gm!! Unless you call output resistance re. What you are talking is output resistance using early voltage to calculate ro≈Va/Ic.

I am not sure what the OP was asking, I take that he was asking about change of Ic with change of Vce with input voltage like any ordinary amplifier. What you are talking is mainly change of Ic with change of Vce with Ib or Vbe keeping constant. These are two totally different thing depend on what the OP meant.

yungman,

re is conventionally is emitter resistance and is 1/gm!!
I said Re, not re. I was using the "e" subscript to represent "Early".

Unless you call output resistance re.
Nothing of the sort. I am assuming the output resistance "ro" is very high in a CB configuration.

What you are talking is output resistance using Early voltage to calculate ro≈Va/Ic
Yes, that is what the second link I posted says it is.

I am not sure what the OP was asking, I take that he was asking about change of Ic with change of Vce with input voltage like any ordinary amplifier.
He was asking why the Ic current lines have a slope in the CE configuration, and are almost horizontal in the CB configuration.

What you are talking is mainly change of Ic with change of Vce with Ib or Vbe keeping constant. These are two totally different thing depend on what the OP meant.
Not really. If Ic changes with Vce, the Early effect is in play. Otherwise not.

Ratch

Good evening Ratch,

Not really. If Ic changes with Vce, the Early effect is in play. Otherwise not.
The slight difficulty with this approach is that Vce is not something you can force onto a transistor, unlike Ic (without lots of blue smoke that is).

The mode of action of a CE circuit is that the transistor adopts a Vce compatible with the set Ic, not the other way around.

This is why transistor manufacturers publish curves of the variation of beta with Ic, not curves of the variation of Ic with Vce.

Studiot,

The slight difficulty with this approach is that Vce is not something you can force onto a transistor, unlike Ic (without lots of blue smoke that is).
Of course you can. See this link. http://www.ecelab.com/vce-ic-curves.htm The Vce voltage varies from 5 to 15 volts while Ic hardly changes at all.

The mode of action of a CE circuit is that the transistor adopts a Vce compatible with the set Ic, not the other way around.
In the curve shown in the above link, Vce changes much and Ic changes little. Even less if a CB configuration and Ic is plotted agains Vcb. They are mostly independent of each other unless the Early effect comes into play.

This is why transistor manufacturers publish curves of the variation of beta with Ic, not curves of the variation of Ic with Vce.
Someone did not tell the manufacturer in the link of that stipulation.

Ratch

yungman,

I said Re, not re. I was using the "e" subscript to represent "Early".

I have not seen this notation yet.

Nothing of the sort. I am assuming the output resistance "ro" is very high in a CB configuration.

Output resistance is defined as change of Ic vs change of Vce with everything else remain constant. Be that as CE or CB. Of cause, it is know CB has the highest output resistance and the curve is almost horizontal.

Yes, that is what the second link I posted says it is.

He was asking why the Ic current lines have a slope in the CE configuration, and are almost horizontal in the CB configuration.

I read back the OP over and over and I don't see that's what he asked. He just asked is there a linear change of Vce vs Ic. Unless he specified with everything else remain constant, I read it as face value of whether Vce vary linearly with Ic which is not as I explained. In day to day design question, you almost never deal with early voltage, mostly about input to output. My only confusion is the "Re" term that I never seen before.

Not really. If Ic changes with Vce, the Early effect is in play. Otherwise not.

Ratch
OP has to specify the condition. If you specified everything else remain constant and Ic change with Vce, that would be very obvious you are talking about early voltage. We all know that.

Read back the OP, the Va statement is a separate statement. Particular if he was asking about output resistance like you suggested, CB should absolutely be a linear relation also, it's just has much higher output resistance, It is still linear. OP need to clarify this.

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erece,

Ratch

Yep, because it is day and night difference!!!

i was studying from microelectronics by donald neaman
According to the book, in CE configuration for a constant Vbe , if Vce increases then Vcb also increases thus making more reverse bias. NOW the effective base width will decrease and gradient changes will come into the picture ultimately increasing the Ic. That was the EARLY EFFECT. So i think this whole thing is applicable to CB config also because in this case increase in Vcb for constant Vbe will increase the Ic for the same reasons. So, i want to know that why in books they always show horizontal lines for CB config. ?

uart
i was studying from microelectronics by donald neaman
According to the book, in CE configuration for a constant Vbe , if Vce increases then Vcb also increases thus making more reverse bias. NOW the effective base width will decrease and gradient changes will come into the picture ultimately increasing the Ic. That was the EARLY EFFECT. So i think this whole thing is applicable to CB config also because in this case increase in Vcb for constant Vbe will increase the Ic for the same reasons. So, i want to know that why in books they always show horizontal lines for CB config. ?
The Early effect is the increase in base transport factor (and hence increase in both $\alpha = I_C/I_E$ and $\beta = I_C/I_B$) due to the base narrowing as $V_{CE}$ increases.

This effect is exactly the same for both the CE and the CB configuration. However, since only tiny changes are required in $\alpha$ to give a significant change in $\beta$, the effect is simply not very noticeable in the CB case (since you are plotting Ic for a given Ie in this case, rather than Ic for a given Ib as in the CE case).

Take a numerical example. Say that the base transport factor is 0.995 and the emitter injection efficiency is also 0.995 then the overall $\alpha$ will be approximately their product, giving $\alpha \simeq 0.990$ and $\beta = \frac{\alpha}{1-\alpha} \simeq 99$.

Now imagine that we increase Vce so that the base narrows and the base transport factor increases by just 0.1% to 0.996. The overall $\alpha$ will now increase to about $\alpha \simeq 0.991$, however $\beta = \frac{\alpha}{1-\alpha}$ increases to approximately 110, an 11% increase!

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thanks

erece & uart,

I believe I offered an explanation of why the Ic current lines are horizontal in a CB configuration and have a slope in the CE configuration. See post #3 of this thread. The Early effect effectively puts a shunt resistor across the collector current generator and reduces the transistor's performance. The slope of Ic shows the resistance of the collector circuit. I said that the more vertical slope of the CE configuration was because the transistor see its output resistance as rc/(β+1), which is smaller than the CB impedance of just rc. The lower resistance is why the lines show a more vertical slope. Any good book on transistor circuits will show that CE and CB have a different output impedance.

I don't think that the smaller value of α compared to β makes any difference. They both have a one to one correspondance to each other. The output resistance of CE can be expressed as also as rc*(1-α), which is identical to the value above.

Ratch