The Darlington transistor is among the most desired electronic part, because its design consists of 2 bipolar transistors with which has been designed to amplify the current in a subsequent pattern. As the current passes through the first transistor it already gets amplified, but it is even further amplified as it passes through the second transistor. This clever design allows for a much higher current output compared to a single bipolar transistor, also when it comes to integrated circuit configuration, the Darlington transistor is a more efficient choice since it will significantly save some space rather than having two transistors.
Darlington transistors in integrated circuits are packaged in array (usually eight) transistor-like pairs. In 1953 engineer Sidney Darlington of Bell Laboratories invented the Darlington configuration. He was the one who made the bi-transistor/tri-transistor packed in a single chip having a common collector for his patent. Another design resembling the Darlington transistor is called the Sziklai pair in which it has a configuration of opposite type NPN and PNP with it. It is often referred to as the “complementary Darlington.”
Behavior
Even though the Darlington transistor was designed to have a high output current gain it still behaves like a normal single transistor. This is proven by the fact that the Darlington transistor has the same number of leads as normal transistors have, which are Base, Collector and Emitter.
The individual gains and the compound current gain can be equated as:
If β1 and β2 have higher outputs than in the normal range, then the relation between the two gains can be said as:
Modern transistors can have a current gain as high as 1000 dB or even higher, which only the minimal amount of base current is required to turn on the pair. But this configuration doesn’t come neatly because it has several drawbacks that makes the entire circuit complicated.
The Basics of a Darlington Pair
Using a single transistor, which in this case is an emitter-follower, in a designated circuit, can limit the input impedance and the current gain level. The total gain of the two independent transistors combined is the resultant gain accumulated by the Darlington transistor.
Current gaintotal = HFE1 x HFE2
In order to form the circuit of a Darlington transistor, the input transistor’s emitter junction has to be connected to the base of the second transistor, wherein the emitter of the first transistor will drive the current of the base in the second transistor and both collector junctions will have to be connected together. The newly formed Darlington pair transistor circuit can now be treated as a single transistor in any circuit design; however, its function as an emitter-follower is its primary role.
Basic Configuration of a Darlington Transistor
Electronic engineers, circuit designers and many experts considers the Darlington pair as an individual transistor that has a significantly high output gain, even in schematic diagrams it is shown more often than not as an independent component of the circuit.
Darlington Electronic Schematic Symbol
Even though the Darlington transistor can be treated as a single component on its own, however, it still does have relative differences when compared to a regular transistor diode. One obvious difference is that the Darlington transistor has a significantly higher voltage gain around the emitter and base sectors that is stemming from the input transistor’s base all the way through the output transistor’s emitter junction.
VBE = VBE1 + VBE2
Since this is the case the Darlington transistor will therefore require two times the average 0.7 volts of electricity in the base-emitter region to jump start the device. However, the saturation voltage is approximately 0.7 volts indeed when we talk about the Darlington transistor configuration. That’s 3 and ½ times higher than the normal saturation voltage of single transistors which is 0.2 volts, where the saturation voltage is needed to trigger the device. Exercise caution though because too much voltage saturation in the Darlington transistor might result in some power dissipation throughout the device, naturally some applications may require for higher current to pass through the block.
It is clear, however, that most Darlington transistors does not operate on the same speed as those of single transistors. The reason for this is that the primary transistor in the Darlington pair is unable to turn off the current of the base in the second pair on its own. As a side effect the overall performance of a Darlington transistor is significantly slower than regular transistors, this is to prevent a reduced flow of current or the whole system shutting down. A resistor had to be connected between the base junction and the emitter junction in order to fix this problem. The resistor acts like a current magnet and stopper at the same time because it will stop current leakage in the system so as not to accidentally trigger the output transistor in the circuit.
The leakage current is measured to be at least several nano-amperes (for a low output transistor) and sometimes it reaches to several hundred micro-amperes if it is a high output transistor. The emitter resistor has been precisely selected to have ideal values that will allow just the right amount of current to exit to the output transistor’s base, yet at the same time it will keep the current leakage at bay so that it will not cause a voltage drop which approximates the amount of voltage that activates the output transistor in the circuit. The rated range of resistors used in a Darlington pair are between several hundred to a couple of thousand ohms depending on the type of transistor is going to be included in integrated circuits.