A Darlington transistor (commonly known as a Darlington pair or Darlington transistor) is a composite structure composed of two bipolar junction transistors (BJTs) cascaded in a specific manner. Its main characteristics can be summarized as follows:
Extremely high current amplification factor:
This is the most core and prominent feature of Darlington tube. The total current amplification factor (β or hFE) is approximately equal to the product of the current amplification factors of the two constituent transistors (β ≈ β ₁× β ₂).
This means that it can drive a very large output collector current (Ic) with a very small input base current (Ib). For example, if the beta of each transistor is 100, the total beta of a Darlington pair can reach 10000. This makes it an ideal choice for driving heavy loads such as relays, motors, high-power LEDs, speakers, etc.
Extremely high input impedance:
Due to the fact that the emitter current of the first transistor (driving transistor) directly serves as the base current of the second transistor (output transistor), the emitter load of the driving transistor is actually the extremely high input impedance of the output transistor (β ₂ Re ₂, where Re ₂ is the emitter resistance of the output transistor).
This configuration significantly improves the input impedance of the entire Darlington from the input end (the base of the driver transistor). High input impedance makes it easier for Darlington transistors to be driven by front-end small signal circuits such as microcontroller GPIO pins, operational amplifiers, etc., reducing the requirement for driving current.
Lower saturation pressure drop:
When the Darlington transistor is fully conductive (saturated), the voltage drop (Vce (sat)) between its collector and emitter is mainly composed of the saturation voltage drop (Vce (sat) ₂) of the output transistor and the base emitter voltage drop (Vbe ₁) of the drive transistor in series, that is, Vce (sat) ≈ Vce (sat) ₂+Vbe ₁.
Although Vce (sat) is higher than that of a single transistor (typically>0.7V, with typical values ranging from 1V to several volts depending on current and device), this voltage drop is usually an acceptable cost for applications that require extremely high current gain. Modern power Darlington transistors improve this parameter through internal optimization, such as integrating acceleration resistors and freewheeling diodes.
Lower on/off speed (slower switching speed):
This is a major drawback of Darlington pipes. In the conducting state, the base region of the output transistor stores a large amount of charge. When the input signal requires the Darlington transistor to turn off, these stored charges need to be removed to cause the output transistor to exit saturation.
Due to high input impedance, the "discharge" path for driving shutdown is limited (usually relying solely on internal or external base pull-down resistors), resulting in slow charge dissipation process. This significantly increases the turn off time, making its switching speed much lower than that of a single transistor or MOSFET. Therefore, Darlington transistors are not suitable for applications that require high-frequency switches.
Higher conductivity threshold and temperature sensitivity:
To make the Darlington transistor fully conductive, the voltage at its input terminal (driving transistor base) needs to reach the sum of at least two base emitter junction voltage drops, that is, Vbe (on) ≈ Vbe ₁+Vbe ₂ ≈ 1.2V-1.4V (silicon transistor). This is higher than a single transistor (about 0.7V).
Due to the presence of two PN junctions, their conduction voltage and characteristics are more significantly affected by temperature than a single transistor. An increase in temperature will result in a decrease in the required conduction voltage and an increase in leakage current.
Essentially, it is a 'super' transistor:
From the behavior of external ports (base B, collector C, emitter E), Darlington is equivalent to a single NPN or PNP transistor with extremely enhanced performance (mainly current gain). It inherits the basic characteristics of BJT as a current controlled device, but enhances its current amplification capability to the extreme.
The core value of Darlington transistors lies in their unparalleled current amplification capability and high input impedance, making them very useful in switching and linear amplification applications that require small currents to control high-power loads (such as low-end switches, lamp drivers, audio power output stages). However, this advantage comes at the cost of slower switching speed (especially slow turn off), higher saturation voltage drop, and higher conduction threshold. Understanding these characteristics is crucial for correctly selecting and applying Darlington transistors in circuit design.