Welcome Cyril,

In the active mode of operation where the transistor can act as an amplifier the relation between IC and IB is 

IC= beta IB + ICEO,

Alternatively,  IC depends exponentially on VBE as,

IC = alphaF IES exp VBE/VT +IC0,

The symbols have their usual meaning.

So, either IB or VBE can be used as an independent parameter  for IC.

The prefer of IB rather than VBE stems from simplicity of the function. The linear relation is more straight forward to calculate than the exponential relation. 

With the advent of the computer aided analysis method of electronic circuits, the tranistor is represented such that the independent parameters are the junction voltages VBE and VCB. The model built in the electronic circuit simulator for the transistor is the Gummel-Poon mpdel.

So, the use of IB makes the manual circuit analysis easier. Conceptually both can be used.

My best wishes.

The Ic curves are linearly distributed when using Ib as a paremeter, and 'exponencially' distrubuted when using Vbe. Then, the transistor is almost linear if governed by Ib.

Leonardo, but the shape of the characteristics will be the same? Even when the temperature changes and under the influence of the Early effect?

why not use the base-emitter voltage instead of the base current

The Ib is in micro amperes and it is easy to use by designers and instruments. Whereas VBE is in volts and change in VBE is in millivolts. This is not comfortable to use. Hence Ib is more popular than VBE.

Welcome Cyril,

In the active mode of operation where the transistor can act as an amplifier the relation between IC and IB is 

IC= beta IB + ICEO,

Alternatively,  IC depends exponentially on VBE as,

IC = alphaF IES exp VBE/VT +IC0,

The symbols have their usual meaning.

So, either IB or VBE can be used as an independent parameter  for IC.

The prefer of IB rather than VBE stems from simplicity of the function. The linear relation is more straight forward to calculate than the exponential relation. 

With the advent of the computer aided analysis method of electronic circuits, the tranistor is represented such that the independent parameters are the junction voltages VBE and VCB. The model built in the electronic circuit simulator for the transistor is the Gummel-Poon mpdel.

So, the use of IB makes the manual circuit analysis easier. Conceptually both can be used.

My best wishes.

A constellation of wonderful answers! And what is most impressive, these answers are real, practical and human friendly... what is not so typical for "pure scientists":)

It seems this question is closely related to the question about how to investigate the diode IV curve:

https://www.researchgate.net/post/How_do_we_investigate_the_IV_curve_of_a_forward_biased_diode-by_a_current_voltage_or_real_source

So it turns out that we choose Ib not because of any special "mystic" advantages which are incomprehensible to us "mere mortals" ... but simply because of some most ordinary practical considerations and facilities. A very strong argument to explain this choice to students...

I think the shape is closely similar, but I don't really know if it is exactly the same. One should analyze the complete Ebers-Moll equations in order to be sure.

The Early voltage is clearly present in both types of curves as shown by the slopes of the curves in the aciive region for both types of curves.

The fact that Ic is directly related to Ib by BetaF is what makes me think that there should be a difference between both types of characteristics. This relationship does not appear when the transistor Ic current is governed by Vbe, which is an independent variable.

I have the feeling that an output characteristic which is measured at fixed VBE will be more inclined (more sensitive to the Early effect) than a characteristic measured at fixed IB. In addition, it seems it should be more sensitive to the temperature variations. I will explore it experimentally in the lab.

AR Redd,

Really, can this be a reason "not to use the base-emitter voltage instead of the base current"?

 Dear Cyril ,

Dear colleagues,

This is my answer on a similar question indicated at the end of the answer.

I am sorry to ay it did not find any attention in spite of the effort spent in it. I hope this time to get what it deserve.

I will try to derive a relationship between the Early voltage VA for constant IB and constant VBE based on the basic relations in the bipolar transistor.It may be tedious to follow, but i think it brings the evidence.

Assume that the transistor operates in the active mode such that theEJ i forward biased and he CJ is reverse biased.

According to the charge control model of the npn transistor the collector current is given by equation 1

IC= A Dn q delta n / Wbn with the symbol have their usual meaning and the reverse saturation current of the collector current is neglected.

The quasineutral base region width Wbn is that parameter what decreases by increasing VCE causing the collector current to increase. This effect is the Early effect..

Let us express Wbn by its value at VCB=0, Wbn0 and its incremental decrease dW because of increasing VCB and reverse biasing the collector junction, that is 

Wbn= Wbn0- dW, equation 2 , where dW is because of the collector pace charge widening in the base region.

Now let us expres IB in terms of the excess charges in the base region and emitter region

IB = A q delta n Wbn/ 2 Taun + A Dp q deltap / We ,wih Taun the minority carrier lifetime in the base region and We is the neutral region of the p-emitter.

The base current is composed of two components the recombination in the base region which is the first term, and the back infected current in the emitter which is the second term.

This IB relation is the most important one to explain the different observations concerning the Early voltage.

It is so, that The base current may be dominated by the recombination in the base region or may be dominated by back injected current. Integrated circuits very miniature transistors the base current is dominated by back injected base current.

In power transistors with the base width relatively larger, the base current may be dominated by the recombination in the base. With the improvement of technology, the base current is observed to be dominated by the back injection in the emitter.

There is remaining one fundamental equation to complete the required equations for the proof.

IT is the junction law of the EJ

delta n= nb0 exp VBE/ Vt with VBE>Vt,

delta p= pe0 exp VBE/ Vt with VBE>Vt,

The case of VBE is constant is straight forward,

Then delta n and delta p will be constants and the collector  current can be put in the form by combining equations 1 and 2

IC= ICSAT( I+ dW/ Wb0), With ICSAT, the collector current at VCB=0,

Now we come to the case of IB= constant and dominated by the back in injection in the emitter, IB= A q Dp delta p /we, From this equation if IB is costant, delta pwill be constant and from the junction law, VBE will be constant and consequently the collector current dependence on the neutral base width.

The last case, where the base current is dominated  by the recombination in the base region IB =A q delta n Wbn/ 2 Taun,

Combining with collector current and rearranging we get

IC= ICSAT ( 1- dw/ Wb0)^-2 = approximately ICSAT( 1+2 dw/ Wb0)

which means that the Early effect will increase in case of constant base current which is dominated by the recombination in the base.

In this way an explanation is introduced to Lutz observations concerning the Early effect in biploar transistors.

0 / 0 · Jun 5, 2015

How is the Early voltage defined? - ResearchGate. Available from: https://www.researchgate.net/post/How_is_the_Early_voltage_defined/1 [accessed Sep 22, 2015].

Here is my answer in pictures. The two characteristics are almost equal although the first (at IB = const) seems a little better than the second (at VBE = const).

Very intriguing observations... obviously based on a good practice... There is no risk to sleep while I read them:)

A. And how can we explain this odd behavior of the high-power transistor? It can cause big non-linear distortions in power amplifiers.

B. Here the answer is quite easy. The negative half-wave "sucks" the charge stored in the base during the positive half-wave.

There is a simple answer to this question, the ratio of collector current to base current does not depend strongly on temperature while the VBE does. Most designs try to control the current to remove any dependence of the performance on temperature.

So, in this case, the shape of the output characteristic will not change with the temperature (because of the self heating)?

Cyril, no, unfortunately not. The VBE generally falls as the temperature rises so the base current increases. The result is thermal runaway and can result in destruction of the device if the circuit is not well designed.

I think the choice makes sense when measuring power transistors, as the curve shifts far less rapidly with changing junction temperature than if you Voltage drive the base (typically 1%/OC versus 10%/OC).   Current-drive is also more informative at low collector-base Voltages, as this characteristic is the more sensitive to the effect of short transit times, as well as to the effect of large collector-base junctions (compared with the emitter-base junction). 

As an example, the collector current curves for a BC107A-BC109C series are almost indistinguishable in shape for current drive, as well as for Voltage drive, but a (presumably gold-doped) 2N2222 has significantly lower current gain at VCE between 0 and 0.4 Volts - even though the characteristic looks very similar to the BC107 series for Voltages down to about 100-mV when the base is Voltage driven.

At the end of the day, the reason has been mentioned indirectly above. Of the electrons  flowing through the emitter, most go to the collector but a small proportion go to the base creating the base current. The fraction is well defined by the doping and geometry and base-emitter voltage is a secondary effect.

Very interesting!!!!

Luciano, thanks for the original interpretation. Nowadays, this is not so common; that is why it is so valuable.

I would add only one practical consideration. When measuring the output characteristic, it is much more convenient to set the current instead of voltage as a parameter.

Similarly, when measuring the diode IV curve, it is much more convenient to change the current instead of voltage as an input variable.

Regarding "there is no problem in representing the output stage as a voltage source in series with a high value resistance", the (constant) voltage source is the power supply VCC, and the "high value resistance" is the high differential output (collector-emitter) resistance of the transistor. The combination of both forms the traditionally mentioned constant current source.

If you want to explain what this "differential resistance" is (what does the transistor do), you can present it as a variable ohmic resistor:

https://en.wikibooks.org/wiki/Circuit_Idea/Negative_Differential_Resistance#Virtual_infinite_resistance

Dear Luciano,

Thanks for the warm response to my comment. As you will see in many discussions here, goodwill is the most important thing for they to be interesting and fruitful. Where it is missing, the discussion dies... But as I see, you ask interesting questions, and you can keep discussions alive; so I predict you a great success in this endeavor:)

With your last comment, you have aroused my interest in something that I have been thinking on for a long time - the so-called "Early voltage". What interests me is whether there can be some physical explanation of this quite artificial definition. I even think it is appropriate to ask a question, "Can the Early voltage be physically explained?":

https://www.researchgate.net/post/Can_the_Early_voltage_be_physically_explained

Here is my view on this issue:

IMO this concept is an (not so successful) attempt to present the high differential resistance of the electronic transistor in the active region of its output characteristic by an equivalent electrical circuit consisting of a constant voltage source (producing the Early voltage) and constant ohmic resistor with resistance equivalent to the transistor differential resistance.

As a rule, the transistor is n-p-n type, and its emitter is grounded; so its characteristic starts at the origin of the coordinate system and goes to right. The Early voltage is negative; so, the combination of the voltage source and the resistor (the "electrical transistor") acts as a current sink.

If the transistor is p-n-p type, its emitter is connected to VCC; so its characteristic starts from a point on the X coordinate shifted from the origin to right with VCC, and goes to left. Now the Early voltage is positive; so, the combination of this positive voltage source and the resistor acts as a current source.

In short, the transistor is presented as a simple resistor-type current source/sink.

But I think this substitution is quite misleading... and it does not give a true idea of the transistor action in this part of the characteristic. Actually, there is no any voltage source inside the transistor, and it acts just as a varying (dynamic) resistor.

I use this equivalent circuit in the lab by telling, in a joking form:), to my students how in electrical engineering they make a current source in a "silly" way - by using a constant voltage source with infinite voltage and resistor with infinite resistance... while in electronics we make it in a "clever" way - by a constant voltage source with relatively low voltage and resistor with dynamic resistance...

May be I am a bit late in my answer, nevertheless :

1) As mentiond already, both sets of curves do exist, of course and can be found in the literature. With Vbe= onst. we have exponentially shaped distances of the various curves and with Ib=-const. they are practically equidistant.

HOWEVER: It is important to know that in both cases we get DIFFERENT values for the slopes - hence, also for the artificial quantity called Early voltage. This can also be proved using the four-pole equations expressed

using the hybrid parameters or the y-parameters.

2) Here is another reason for the popularity of the curves with Ib as a parameter: Remember the so called “4-quadrant “graph that connects all 4 characteristic quantities: Vce, Ic, Vbe and Ib.

In the upper left part of the graph we have Ic=f(Ib). This automatically requires to have also Ib as a fixed parameter in the upper right part of the picture.

3) There is another practical point regarding the selection and fixation of the Dc bias point. We are using the “working line“ determined by the collector resistor to select a suitable bias point. However, due to known reasons we make a good guess for a suitable Vbe value, but we need a value for the correponding Ib current for calculatong the base voltage divider.