![]() Integrated devices can take less space than two individual transistors because they can use a shared collector. A typical integrated power device is the 2N6282, which includes a switch-off resistor and has a current gain of 2400 at I C=10 A. The maximum collector current I C(max) of the pair is that of Q 2. The Darlington pair has more phase shift at high frequencies than a single transistor and hence can more easily become unstable with negative feedback (i.e., systems that use this configuration can have poor performance due to the extra transistor delay).ĭarlington pairs are available as integrated packages or can be made from two discrete transistors Q 1, the left-hand transistor in the diagram, can be a low power type, but normally Q 2 (on the right) will need to be high power. This resistor provides a low-impedance discharge path for the charge accumulated on the base-emitter junction, allowing a faster transistor turn-off. To alleviate this, the second transistor often has a resistor of a few hundred ohms connected between its base and emitter terminals. The increased low output level can cause troubles when TTL logic circuits are driven.Īnother problem is a reduction in switching speed or response, because the first transistor cannot actively inhibit the base current of the second one, making the device slow to switch off. For equal collector currents, this drawback translates to an increase in the dissipated power for the Darlington transistor over a single transistor. Β D a r l i n g t o n = β 1 ⋅ β 2 + β 1 + β 2 always.) Thus the "saturation" voltage of a Darlington transistor is one V BE (about 0.65 V in silicon) higher than a single transistor saturation voltage, which is typically 0.1 - 0.2 V in silicon. It typically creates a high current gain (approximately the product of the gains of the two transistors, due to the fact that their β values multiply together).Ī general relation between the compound current gain and the individual gains is given by: That's the sort of power we are able to get from a Darlington transistor configuration, and an ordinary looking transistor could be turned into a hugely rated device just with a simple modification.A Darlington pair behaves like a single transistor, meaning it has one base, collector, and emitter. When the two are combined as a Darlington pair, the gain shoots up substantially to 40 x 400 = 16000, awesome isn't it. Th average current gain (hFE) of 2N3055 is around 40, while for BC547 it's 400. However as soon as another BJT which is a BC547 here is connected with 2N3055 in a Darlington pair, the unified current gain jumps up into a very high value, and allows the lamp to glow at full brightness. This is because the current gain of 2N3055 is very low to process the low base current into high collector current. Normally with such low current at the base, the 2N3055 alone can never illuminate a high current load such as a 12V 2 amp bulb. Here's a practical Darlington design by connecting a 2N3055 power transistor with a small signal BC547 transistor.Ī 100K resistor is used at the signal input side to reduce the current to a few millamps. I B = V CC - V BE / R B + β DR E - (12.9) ![]() Here the base current can be calculated using the formula: The following figure shows a common Darlington circuit using transistors with a very high current gain β D. How to DC Bias a Darlington Transistor Circuit ![]() This kind of packaged Darlington transistors have external features similar to a normal transistor but have very high and enhanced current gain output, compared to the normal single transistors. The unit comes with 3 standard terminals externally, namely base, emitter, collector. ![]() The indicated current gain, is the net gain from the two BJTs. The following table provides the datasheet of an example Darlington pair within a single package. Β D = β 2 - (12.8) Packaged Darlington Transistorĭue to its immense popularity, Darlington transistors are also manufactured and available ready made in a single package which have two BJTs internally wired up as one unit. When matched transistors are used in a Darlington connection such that β 1 = β 2 = β, the above formula for the current gain gets simplified as: If the Darlington connection comprises of two individual BJTs with current gains β 1 and β 2 the combined current gain can be calculated using the formula: The main advantage of this configuration is that the composite transistor behaves like a single device having an enhanced current gain equivalent to the product of the current gains of the each transistor. A Darlington transistor can be defined as a connection between two BJTs that allows them to form a single composite BJT acquiring a substantial amount of current gain, which may range beyond thousand typically.
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