How does a transistor work also describe its three main regions? Why it has two separate voltage sources. What are the advantages of the transistor also discuss its uses?
July 16, 2020
Consider the three regions, in series, the Emitter, the Base and the Collector. We use the letters E, B, and C to refer to them. For a NPN transistor, the two ends, E and C, are of N type material while the base is of P type. The 2N2222 is an NPN transistor. Generally, Vc, the voltage at the collector, is larger than Ve, the voltage at the emitter. The electrons would then eventually flow from E to C.
NOTATION: Vxy == Vx - Vy, ie, the difference of voltage at x and the voltage at y.
Case where Vc > Ve.
If Vbe = Vb – Ve, the voltage drop between the Base ans the Emitter, is less than 0.7 volt for a discrete transistor (0.6 volt for a transistor in an integrated circuit, an IC), the transistor is blocking the currents. Indeed, the diode made of the Base and Emetter is then reversed biased. If Vbe > 0.7 but Vb < Vc (Vc = voltage at the collector), the transistor is in the active forward mode. The diode Emitter-Base is forward biased such that a very light variation of the voltage at its end, Vbe, can matches a humungous change in the current through it, Ie. (Remember the curve of a diode, for the current, on the vertical axis, and the voltage on the horizontal, pass the diode threshold, the current varies rapidly for a small change of the voltage). But for the transistor, the Base region is really thin, so most of the electrons, when under these voltages, pass through it unaffected such that Ie = Ic (the current emitted by the Emitter is almost the same in the Collector. A small number of electrons would escape through the Base terminal, though. Note that the current Ic is dependent on the voltage of the diode Base-Emitter, Vbe, and not (in first approximation) on Vc, the voltage at the Collector terminal. The proportion of Ic over Ib, the current which escape, is called beta, of hFE. The transistor is used under this “polarization” for amplification purpose (amplify the voltage, the current, or the power).
If Vb = Vc, if the voltage at the Base equals the voltage at the collector, the transistor enters in the saturation mode: the electrons of the B-E diode are now as attracted to exit through the Base than through the Emitter. Beta, or hFE, starts to drop. If Vb >> Vc (Vb much greater than Vc ), the transistor is in deep saturation mode. The electrons are more attracted to Vb than to Vc, since Vc < Vb. Vce = Vc – Ve, that is, the voltage drop across the Collector and the Emitter, drops up to as small as 0.2 volt. The transistor is thus as close as an ideal closed switch (at ON) (for low frequency and for case where the time it may take to get back at the OFF state is irrelevant).
Case where Ve > Vc..
With this polarization, the role played by the Emitter and the Collector are switched. The transistor is less performing in that Backward Active mode, such as having a much lower value for beta, or hFE, and the manufacturer is not testing much the characteristics of his production under that polarization. It is sure, though, in this polarization, if Vbc = Vb – Vc, is less than 0.7 (or 0.6) volt, the transistor is blocking all current.
July 16, 2020
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