Darlington Transistor Pair

Title:Darlington Transistor Pair

The Darlington Transistor named after its inventor, Sidney Darlington is a special arrangement of two standard NPN or PNP bipolar junction transistors (BJT) connected together. The Emitter of one transistor is connected to the Base of the other to produce a more sensitive transistor with a much larger current gain being useful in applications where current amplification or switching is required.

Darlington Transistor pairs can be made from two individually connected Bipolar Transistors or a one single device commercially made in a single package with the standard: Base, Emitter and Collector connecting leads and are available in a wide variety of case styles and voltage (and current) ratings in both NPN and PNP versions.

As we saw in our Transistor As A Switch tutorial, as well as being used as an amplifier, the bipolar junction transistor, (BJT) can be made to operate as an ON-OFF switch as shown.

Bipolar Transistor as a Switch

When the base of the NPN transistor is grounded (0 volts) and no base current, Ib flows, no current flows from the emitter to the collector and the transistor is therefore switched “OFF”. If the base is forward biased by more than 0.7 volts, a current will flow from the emitter to the collector and the transistor is said to be switched “ON”. When operated in these two modes, the transistor operates as a switch.

The problem here is that the transistors base needs to be switched between zero and some large, positive value for the transistor to become saturated at which point an increased base current, Ibflows into the device resulting in collector current Ic becoming large while Vce is small. Then we can see that a small current on the base can control a much larger current flowing between the collector and the emitter.

The ratio of collector current to base current ( β ) is known as the current gain of the transistor. A typical value of β for a standard bipolar transistor may be in the range of 50 to 200 and varies even between transistors of the same part number. In some cases where the current gain of a single transistor is too low to directly drive a load, one way to increase the gain is to use a Darlington pair.

A Darlington Transistor configuration, also known as a “Darlington pair” or “super-alpha circuit”, consist of two NPN or PNP transistors connected together so that the emitter current of the first transistor TR1 becomes the base current of the second transistor TR2. Then transistor TR1 is connected as an emitter follower and TR2 as a common emitter amplifier as shown below.

Also note that in this Darlington pair configuration, the collector current of the slave or control transistor, TR1 is “in-phase” with that of the master switching transistor TR2.

Basic Darlington Transistor Configuration

Using the NPN Darlington pair as the example, the collectors of two transistors are connected together, and the emitter of TR1 drives the base of TR2. This configuration achieves β multiplication because for a base current i_b, the collector current is β.i_b where the current gain is greater than one, or unity and this is defined as:

But the base current, I_B2 is equal to transistor TR1 emitter current, I_E1 as the emitter of TR1 is connected to the base of TR2. Therefore:

Then substituting in the first equation:

Where β₁ and β₂ are the gains of the individual transistors.

This means that the overall current gain, β is given by the gain of the first transistor multiplied by the gain of the second transistor as the current gains of the two transistors multiply. In other words, a pair of bipolar transistors combined together to make a single Darlington transistor pair can be regarded as a single transistor with a very high value of β and consequently a high input resistance.

Darlington Transistor Example No1

Two NPN transistors are connected together in the form of a Darlington Pair to switch a 12V 75W halogen lamp. If the forward current gain of the first transistor is 25 and the forward current gain of the second transistor is 80. Ignoring any voltage drops across the two transistors, calculate the maximum base current required to switch the lamp fully-ON.

Firstly, the current drawn by the lamp will be equal to the collector current of the second transistor, then:

Using the equation above, the base current is given as:

Then we can see that a very small base current of only 3.0mA, such as that supplied by a digital logic gate or the output port of a micro-controller, can be used to switch the 75 Watt lamp “ON” and “OFF”.

If two identical bipolar transistors are used to make a single Darlington device then β₁ is equal to β₂and the overall current gain will be given as:

However, if the value of β² is much greater than that of 2β, this can be ignored and the final equation for two identical transistors used as a Darlington pair is given as:

Identical Darlington Transistors

Then we can see that for two identical transistors, β² is used instead of β acting like one big transistor. Darlington transistor pairs with current gains of more than a thousand with maximum collector currents of several amperes are available.

The advantage of using an arrangement such as this, is that the switching transistor is much more sensitive as only a tiny base current is required to switch a much larger load current as the typical gain of a Darlington configuration can be over 1,000 whereas normally a single transistor stage produces a gain of about 50 to 200.

Then we can see that a darlington pair with a gain of 1,000:1, could switch an output current of 1 ampere in the collector-emitter circuit with an input base current of just 1mA. This then makes darlington transistors ideal for interfacing with relays, lamps and motors to low power microcontroller, computer or logic controllers as shown.

Darlington Transistor Applications

The base of the Darlington transistor is sufficiently sensitive to respond to any small input current from a switch or directly from a TTL or 5V CMOS logic gate. The maximum collector current Ic(max) for any Darlington pair is the same as that for the main switching transistor, TR2 so can be used to operate relays, DC motors, solenoids and lamps, etc.

One of the main disadvantage of a Darlington transistor pair is the minimum voltage drop between the base and emitter when fully saturated. Unlike a single transistor which has a saturated voltage drop of between 0.3v and 0.7v when fully-ON, a Darlington device has twice the base-emitter voltage drop (1.2 V instead of 0.6 V) as the base-emitter voltage drop is the sum of the base-emitter diode drops of the two individual transistors which can be between 0.6v to 1.5v depending on the current through the transistor.

This high base-emitter voltage drop means that the Darlington transistor can get hotter than a normal bipolar transistor for a given load current and therefore requires good heat sinking. Also, Darlington transistors have slower ON-OFF response times as it takes longer for the slave transistorTR1 to turn the master transistor TR1 either fully-ON or fully-OFF.

To overcome the slow response, increased voltage drop and thermal disadvantages of a standardDarlington Transistor device, complementary NPN and PNP transistors can be used in the same cascaded arrangement to produce another type of Darlington transistor called a Sziklai Configuration.

Sziklai Transistor Pair

The Sziklai Darlington Pair, named after its Hungarian inventor George Sziklai, is a complementary or compound Darlington device that consists of separate NPN and PNP complementary transistors connected together as shown below.

This cascaded combination of NPN and PNP transistors has the advantage that the Sziklai pair performs the same basic function of a Darlington pair except that it only requires 0.6v for it to turn-ON and like the standard Darlington configuration, the current gain is equal to β² for equally matched transistors or is given by the product of the two current gains for unmatched individual transistors.

Sziklai Darlington Transistor Configuration

We can see that the base-emitter voltage drop of the Sziklai device is equal to the diode drop of a single transistor in the signal path. However, the Sziklai configuration can not saturate to less than one whole diode drop, i.e. 0.7v instead of the usual 0.2v.

Also, as with the Darlington pair, the Sziklai pair have slower response times than a single transistor. Sziklai pair complementary transistors are commonly used in push-pull and class AB audio amplifier output stages allowing for one polarity of output transistor only. Both the Darlington and Sziklai transistor pairs are available in both NPN and PNP configurations.

Darlington Transistor IC’s

In most electronics applications it is sufficient for the controlling circuit to switch a DC output voltage or current “ON” or “OFF” directly as some output devices such as LED’s or displays only require a few milliamps to operate at low DC voltages and can therefore be driven directly by the output of a standard logic gate.

However as we have seen above, sometimes more power is required to operate the output device such as a DC motor than can be supplied by an ordinary logic gate or micro-controller. If the digital logic device cannot supply sufficient current then additional circuitry will be required to drive the device.

One such commonly used Darlington transistor chip is the ULN2003 array. The family of darlington arrays consist of the ULN2002A, ULN2003A and the ULN2004A which are all high voltage, high current darlington arrays each containing seven open collector darlington pairs within a single IC package.

Each channel of the array is rated at 500mA and can withstand peak currents of up to 600mA making it ideal for controlling small motors or lamps or the gates and bases of high power semiconductors. Additional suppression diodes are included for inductive load driving and the inputs are pinned opposite the outputs to simplify the connections and board layout.

The ULN2003A Darlington Transistor Array

The ULN2003A is a inexpensive unipolar darlington transistor array with high efficiency and low power consumption making it useful for driving a wide range of loads including solenoids, relays DC Motor’s and LED displays or filament lamps. The ULN2003A contains seven darlington transistor pairs each with an input pin on the left and an output pin opposite it on the right as shown.

ULN2003A Darlington Transistor Array

The ULN2003A Darlington driver has an extremely high input impedance and current gain which can be driven directly from either a TTL or +5V CMOS logic gate. For +15V CMOS logic use the ULN2004A and for higher switching voltages up to 100V it is better to use the SN75468 Darlington array.

When an input (pins 1 to 7) is driven “HIGH” the corresponding output will switch “LOW” sinking current. Likewise, when the input is driven “LOW” the corresponding output switches to a high impedance state. This high impedance “OFF” state blocks load current and reduces leakage current through the device improving efficiency.

Pin 8, (GND) is connected to the loads ground or 0 volts, while pin 9 (Vcc) connects to the loads supply. Then any load needs to be connected between +Vcc and an output pin, pins 10 to 16. For inductive loads such as motors, relays and solenoids, etc, pin 9 should always be connected to Vcc.

The ULN2003A is capable of switching 500mA (0.5A) per channel but if more switching current capability is required then both the Darlington pairs inputs and outputs can be paralleled together for higher current capability. For example, input pins 1 and 2 connected together and output pins 16 and 15 connected together to switch the load.

Darlington Transistor Summary

The Darlington Transistor is a high power semiconductor device with individual current and voltage ratings many times higher than a conventional small signal junction transistors.

The DC current gain values for standard high power NPN or PNP transistors are relatively low, as low as 20 or even less, compared to small signal switching transistors. This means that large base currents are required to switch a given load.

The Darlington arrangement uses two transistors back to back, one of which is the main current carrying transistor, while the other being a much smaller “switching” transistor provides the base current to drive the main transistor. As a result, a smaller base current can be used to switch a much larger load current as the DC current gains of the two transistors are multiplied together. Then the two transistor combination can be regarded as one single transistor with a very high value of β and consequently a high input resistance.

As well as standard PNP and NPN Darlington transistor pairs, complementary Sziklai Darlington transistors are also available which consist of separate matching NPN and PNP complementary transistors connected together within the same Darlington pair to improve efficiency. Also Darlington arrays such as the ULN2003A are available which allow high power or inductive loads such as lamps, solenoids and motors to be safely driven by microprocessor and micro-controller devices in robotic and mechatronic type applications.

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