![IEEE 315-1975 (1993) 8](https://upload.wikimedia.org/wikipedia/commons/f/f9/IEEE_315-1975_%281993%29_8.6.17.svg)
In
electronics
The field of electronics is a branch of physics and electrical engineering that deals with the emission, behaviour and effects of electrons using electronic devices. Electronics uses active devices to control electron flow by amplification ...
, a multi-transistor configuration called the Darlington configuration (commonly called a Darlington pair) is a circuit consisting of two
bipolar transistor
A bipolar junction transistor (BJT) is a type of transistor that uses both electrons and electron holes as charge carriers. In contrast, a unipolar transistor, such as a field-effect transistor, uses only one kind of charge carrier. A bipolar t ...
s with the
emitter of one transistor connected to the
base of the other, such that the current amplified by the first transistor is amplified further by the second one.
The
collectors of both transistors are connected together. This configuration has a much higher
current gain
In electronics, gain is a measure of the ability of a two-port circuit (often an amplifier) to increase the power or amplitude of a signal from the input to the output port by adding energy converted from some power supply to the signal. It is ...
than each transistor taken separately. It acts like and is often packaged as a single transistor. It was invented in 1953 by
Sidney Darlington
Sidney Darlington (July 18, 1906 – October 31, 1997) was an American electrical engineer and inventor of a transistor configuration in 1953, the Darlington pair. He advanced the state of network theory, developing the insertion-loss synth ...
.
Behavior
![Darlington transistor MJ1000](https://upload.wikimedia.org/wikipedia/commons/d/d9/Darlington_transistor_MJ1000.jpg)
A Darlington pair behaves like a single transistor, meaning it has one base, collector, and emitter. 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).
A general relation between the compound current gain and the individual gains is given by:
:
If ''β
1'' and ''β
2'' are high enough (hundreds), this relation can be approximated with:
:
Advantages
A typical Darlington transistor has a current gain of 1000 or more, so that only a small base current is needed to make the pair switch on much higher switched currents.
[
]
Another advantage involves providing a very high input impedance for the circuit which also translates into an equal decrease in output impedance.
The ease of creating this circuit also provides an advantage. It can be simply made with two separate NPN (or PNP) transistors, and is also available in a variety of single packages.
Disadvantages
One drawback is an approximate doubling of the base–emitter voltage. Since there are two junctions between the base and emitter of the Darlington transistor, the equivalent base–emitter voltage is the sum of both base–emitter voltages:
:
For silicon-based technology, where each V
BEi is about 0.65 V when the device is operating in the active or saturated region, the necessary base–emitter voltage of the pair is 1.3 V.
Another drawback of the Darlington pair is its increased "saturation" voltage. The output transistor is not allowed to saturate (i.e. its base–collector junction must remain reverse-biased) because the first transistor, when saturated, establishes full (100%) parallel negative feedback between the collector and the base of the second transistor.
[ Similarly, an ]emitter follower
In electronics, a common collector amplifier (also known as an emitter follower) is one of three basic single-stage bipolar junction transistor (BJT) amplifier topologies, typically used as a voltage buffer.
In this circuit the base terminal ...
never saturates because of the 100% series negative feedback. Another example is an "active diode" made by a transistor with joined base and collector (e.g., the current-setting part of a current mirror
A current mirror is a circuit designed to copy a current through one active device by controlling the current in another active device of a circuit, keeping the output current constant regardless of loading. The current being "copied" can be, and ...
). Since collector–emitter voltage is equal to the sum of its own base–emitter voltage and the collector-emitter voltage of the first transistor, both positive quantities in normal operation, it always exceeds the base-emitter voltage. (In symbols,
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. For equal collector currents, this drawback translates to an increase in the dissipated power for the Darlington transistor over a single transistor. The increased low output level can cause troubles when TTL logic circuits are driven.
Another 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. To alleviate this, the second transistor often has a resistor of a few hundred ohms connected between its base and emitter terminals.
[ This resistor provides a low-impedance discharge path for the charge accumulated on the base-emitter junction, allowing a faster transistor turn-off.
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).
]
Packaging
Darlington pairs are available as integrated packages or can be made from two discrete transistors; Q1, the left-hand transistor in the diagram, can be a low power type, but normally Q2 (on the right) will need to be high power. The maximum collector current IC(max) of the pair is that of Q2. A typical integrated power device is the 2N6282, which includes a switch-off resistor and has a current gain of 2400 at IC=10 A.
Integrated devices can take less space than two individual transistors because they can use a ''shared'' collector. Integrated Darlington pairs come packaged singly in transistor-like packages or as an array of devices (usually eight) in an integrated circuit.
Darlington triplet
A third transistor can be added to a Darlington pair to give even higher current gain, making a Darlington triplet. The emitter of the second transistor in the pair is connected to the base of the third, as the emitter of first transistor is connected to the base of the second, and the collectors of all three transistors are connected together. This gives current gain approximately equal to the product of the gains of the three transistors. However the increased current gain often does not justify the sensitivity and saturation current problems, so this circuit is seldom used.
Applications
Darlington pairs are often used in the push-pull output stages of the power audio amplifiers that drive most sound systems. In a fully symmetrical push-pull circuit two Darlington pairs are connected as emitter follower
In electronics, a common collector amplifier (also known as an emitter follower) is one of three basic single-stage bipolar junction transistor (BJT) amplifier topologies, typically used as a voltage buffer.
In this circuit the base terminal ...
s driving the output from the positive and negative supply: an NPN Darlington pair connected to the positive rail providing current for positive excursions of the output, and a PNP Darlington pair connected to the negative rail providing current for negative excursions.
Before good quality PNP power transistors were available, the quasi-symmetrical push-pull circuit was used, in which only the two transistors connected to the positive supply rail were an NPN Darlington pair, and the pair from the negative rail were two more NPN transistors connected as common-emitter amplifiers.
Safety
A Darlington pair can be sensitive enough to respond to the current passed by skin contact even at safe zone voltages. Thus it can form a new input stage of a touch-sensitive switch.
Amplification
Darlington transistors can be used in high-current circuits such as the LM1084 voltage regulator. Other High current applications could include those involving computer control of motors or relays, where the current is amplified from a safe low level of the computer output line to the amount needed by the connected device.
See also
*Insulated-gate bipolar transistor
An insulated-gate bipolar transistor (IGBT) is a three-terminal power semiconductor device primarily used as an electronic switch, which, as it was developed, came to combine high efficiency and fast switching. It consists of four alternating lay ...
*ULN2003A
The ULN2003A is an integrated circuit produced by Texas Instruments. It consists of an array of seven NPN Darlington transistors capable of 500 mA, 50 V output. It features common-cathode flyback diodes for switching inductive loads ...
* Sziklai pair, sometimes called the "complementary Darlington", a similar configuration but with transistors of opposite type (one NPN and one PNP)
* Integrated injection logic (I2L)
References
External links
*
* "Semiconductor signal translating device" (Darlington transistor)
A Darlington pair motor speed control circuit
ECE 327: Procedures for Output Filtering Lab
nbsp;– Section 4 ("Power Amplifier") discusses Darlington pairs in the design of a BJT-based class-AB current driver in detail.
{{Electronic component
Multi-stage transistor amplifiers
Transistor types