ALL ABOUT DARLINGTON TRASISTOR.
The figure is typical of the Darlington circuit, two transistors were set extremely its characteristics, but also because there is a common-collector will not be completely saturated with.
Overall value of β seems, is βQ1 * βQ2, so the equivalent current magnification is very large, but should be noted that drive-level Q1 collector is connected with the output level Q2 together, if fully saturated zone into Q2 (Vce of about 0.3 V), then there would be no IC1 Q1 production, produce must meet VceQ2 IC1 must be greater than or equal to VceQ1 (saturated) + VbeQ2, this value is close to about 1V. Darlington type normal action, Q2 is not completely saturated, completely saturated because Q1 will not be able to play a "flow to help" function (reason mentioned above), when VceQ2 too low, IC1 is reduced, which also led to IB2 reduced, so VceQ2 will rise, and vice versa, when VceQ2 too high, IC1 will increase, the associated IB2 will increase, so VceQ2 will drop, and finally VceQ2 will reach an equilibrium point, this point how much voltage will ? It depends on IB1, IC current value may be, it is certain that Q2 will not work in a fully saturated zone. Darlington circuit with one exception makes Q2 into saturation, it is large enough to IB1, IB1 * βQ2 large greater than the IC, then IB1 = IB2, and Q1 is not some feature of the play, really into two back-docking of the diode ....... but with a Darlington transistor is aimed at a small current to control high current, when there is so much IB can drive, that's a single one with a general power like crystals. 34063 is a good example of the control, 34063 is similar to the output Darlington transistor structure, the difference is that no two transistor collector connected together, are, respectively, from two feet out the PIN, in order to meet the needs of different circuit types, as step-down circuit, the transistor operates in the high-end, then do not have to choose only access into the standard Darlington, as a boost when the transistor operates in the low-end, drive level to another collector resistor to the power supply voltage, which differences between the two very different but also to the output loss, but a relatively high efficiency step-up type, because the output transistors can operate at full saturation.
