02 February 2018 16 3K Report

I have been thinking for a long time on the so-called "Early voltage". What interests me is whether there can be some physical explanation of this quite artificial definition.

IMO this concept is a (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 that is 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...

See also:

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

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

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