Often it is observed that TIP120 is used in some circuits created with Arduino UNO or ESP32. It is a Darlington pair (it is a transistor). In many circuits, we can either use a transistor or a MOSFET. For example, in our previous guide, 5050 LED Strip with Arduino, we have mentioned that we can use either transistors or MOSFET. The sentences are not fully correct – a MOSFET is also a transistor.
Transistors can be divided into two Bipolar Junction Transistors (BJT) and Field Effect Transistors (FET). MOSFETs are FET. BJTs are controlled by current but MOSFETs are controlled by voltage. And the terminals (legs) are also differently named. The three terminals of BJTs are the base, emitter, and collector, while MOSFETs have a source, drain, and gate. MOSFETs are typically used for higher-current applications. All three are indeed silicon-based transistors but BJTs are usually called transistors. This is because in real life Bipolar Junction Transistors, MOSFET, and Darlington transistors are used for different purposes. As they are all transistors, replacing one with another works but the result becomes hundreds of problems.
We feel that it is necessary to discuss the basic of Darlington Transistors (such as TIP120).
The Darlington circuit is an electronic circuit of two bipolar transistors, with the first, smaller transistor acting as an emitter follower to the base of the second, larger one. It is used to increase the current amplification factor of a single bipolar transistor. If both transistors are in a single package, this is also referred to as the Darlington transistor. A similar arrangement of complementary transistors is called a Sziklai pair or a complementary Darlington circuit.
So the Darlington transistor is the combination of two bipolar transistors of the same type (both NPN or both PNP), resulting in a hybrid component that still has transistor characteristics. The current gain of the Darlington is equal to the product of the gains of each transistor.
The advantage of this technology is that a considerably higher current amplification can be achieved with the same space requirement or the necessary control currents are lower. The overall gain is the product of the gains of each of the two transistors (1,000 to 20,000). At collector current, the Darlington makes it possible to increase the input resistance of the assembly compared to a single transistor.
The disadvantage, on the other hand, is the larger phase shift compared to a single transistor, so instabilities are more likely to occur in the event of negative feedback. This is one of the reasons why Darlingtons are usually not suitable for high-frequency applications.
Darlington transistors have slower switching times than single transistors, especially when the collector current is switched off because the first transistor is not able to “clear” the charge carriers from the base of the second transistor. To improve the switching behaviour, a resistor parallel to the base-emitter section of the power transistor is integrated. However, part of the base current for the second stage flows out through this resistance, which reduces the total gain accordingly.
The base-emitter voltage doubles at the Darlington compared to the single transistor (about 1.2 to 1.4 volts at a silicon Darlington). The collector-emitter voltage in the conductive state also increases by the forward voltage of the base-emitter link of the second transistor, i.e. about 0.9 volts for small signal types (compared to 0.2 volts) or up to more than 2 volts for power types. This leads to increased power losses at low voltages.
Because of these disadvantages, Darlington transistors are hardly suitable for efficiency-critical switching applications, where separate driver and power stages are usually used, i.e. the collector of the driver transistor is not connected to that of the power transistor. Advances in other transistor technologies have displaced the Darlington transistor, except for a few niches.
Darlington pairs are used in the power audio amplifiers. So, avoid using TIP120 while building things with Arduino/ESP32 unless you know what you are doing. For example, TIP120 may be suitable for a solenoid.
Usually, you need a correctly chosen MOSFET. In our 5050 LED Strip with Arduino guide, the voltage was only 5V. So we can use either MOSFET or BJT transistors. But still, MOSFET is a better choice.