Darlington transistor

The Darlington circuit is an electronic circuit comprising two bipolar transistors, the first, smaller transistor operates as an emitter follower to the base of the second, larger one. It is applied to increase the current amplification factor of a single bipolar transistor. Are both transistors in a single package, one also speaks of Darlington transistor. A similar arrangement of complementary transistors is called Sziklai pair or a complementary Darlington circuit.

History

The Darlington circuit, originally consisting of two individual components, was invented by Sidney Darlington (1906-1997) at the Bell Laboratories. Darlington had the idea to build in two or three identical transistors on a chip and link to patent, but not the use of multiple or complementary transistors, integrated circuits so that any were not affected by this patent.

Application

Power transistors have the advantage over small signal transistors a much lower current gain B (5-10 opposite 100-1000 Small signal transistors) and therefore require high drive currents, which can be reduced accordingly by the Darlington arrangement. Therefore one of the most important applications is the turning on or off a current at a substantially higher power by a control current of low power.

Another application is in the area of ​​amplification of analog signals. This is because, where the control currents are too low to drive the power transistor directly. Furthermore, the temperature dependence and for the setting of the operating point at Darlington transistors is relatively uncritical; through a resistor between base and emitter of the power transistor, the quiescent current between 0.7 V and 1.1 V is approximately linear. This avoids distortion.

Pros and Cons

The advantage of this technique is that, with a constant space a considerably higher current gain can be achieved, and the control currents required are smaller. The overall gain B ( large signal ) corresponds approximately to the product of the gains of the two individual transistors ( B1 and B2):

By analogy, applies to the small-signal current gain

In modern Leistungsdarlingtonschaltungen the current gain is in the range 1000 and higher. For the small-signal current gain even gains are achieved up to 50,000.

The disadvantage, however, is the exposure to a single transistor larger phase shift, so that when negative feedback rather instabilities may occur. Partly for this reason, Darlington are usually not suitable for high frequency applications.

Darlington transistors have to individual transistors slower switching times, in particular when switching off the current collector, because the first transistor is capable of the carriers from the base of the second transistor " remove ". To improve the switching behavior, therefore, a resistance being integrated parallel to the base -emitter junction of the power transistor. Through this resistor, however, a part of the base current flows from the second stage, whereby the overall gain is reduced accordingly.

Finally, the base -emitter voltage of the Darlington doubled compared to the single transistor ( about 1.2 to 1.4 volts in a silicon Darlington ), and the collector -emitter voltage in the on state is increased by the forward voltage of the base-emitter path of the second transistor, that is approximately 0.9 volts at the small signal types ( over 0.2 volts ) or up to about 2 volts at output types.

For efficiency critical switching applications, Darlington hardly suitable because of these disadvantages, there are usually separate driver and power levels used, that is, the collector of the driver transistor is not connected to the power transistor.

Variants

The Darlington circuit can be constructed with both NPN and PNP transistors as a power element. In the graph resistors are built in parallel with the base -emitter paths of the individual transistors, which, as described above, each allowing the outflow of the charge from the base. The additional diode speed this up even further; when voltage to the first driver transistor is equal to the emitter voltage of the power transistor, the charge from all downstream transistors can flow through the diodes.

The use of diodes is to be avoided, when the base voltage may fall below the emitter voltage.

The diode between collector and emitter of the Darlington transistor has a different purpose; it seeks to protect as a freewheeling diode circuit with inductive load.