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Tip120: Basics on TIP120 Transistor

Introduction to TIP120 Transistor

The TIP120 is a popular NPN (Negative-Positive-Negative) Darlington transistor widely used in electronic circuits for switching and amplification purposes. It is known for its high current gain, high power dissipation, and ability to handle large currents, making it suitable for a wide range of applications such as motor control, relay driving, and power regulation.

Key Features of TIP120 Transistor

  • High current gain (hFE) of 1000 minimum
  • Maximum collector current (IC) of 5A
  • Maximum collector-emitter voltage (VCEO) of 60V
  • Maximum power dissipation of 65W
  • Darlington configuration for enhanced current gain
  • TO-220 package for efficient heat dissipation

Understanding the Darlington Configuration

The TIP120 transistor utilizes a Darlington configuration, which consists of two bipolar transistors connected in a cascade manner. The first transistor’s collector is directly connected to the second transistor’s emitter, and both emitters are connected together. This configuration provides a much higher current gain compared to a single transistor.

Advantages of Darlington Configuration

  1. High current gain: The Darlington configuration multiplies the current gain of the individual transistors, resulting in a very high overall current gain. This allows the TIP120 to control large currents with a small input current.

  2. Simplified biasing: The Darlington configuration simplifies the biasing requirements of the transistor. The base-emitter voltage drop of the first transistor is sufficient to bias the second transistor, eliminating the need for separate biasing circuits.

  3. Increased input impedance: The Darlington configuration increases the input impedance of the transistor, making it easier to drive with low-current signals.

Disadvantages of Darlington Configuration

  1. Higher voltage drop: Due to the series connection of two base-emitter junctions, the Darlington configuration exhibits a higher voltage drop (around 1.4V) compared to a single transistor. This can result in increased power dissipation and reduced efficiency in some applications.

  2. Slower switching speed: The Darlington configuration has a slower switching speed compared to a single transistor due to the increased capacitance and charge storage effects. This may limit its use in high-frequency switching applications.

TIP120 Pinout and Package

The TIP120 transistor comes in a TO-220 package, which is a common package for power transistors. The package provides good heat dissipation capabilities and allows for easy mounting on heatsinks.

Pin Symbol Description
1 Base Base terminal of the transistor
2 Collector Collector terminal of the transistor
3 Emitter Emitter terminal of the transistor

The TO-220 package also includes a metal tab that is electrically connected to the collector terminal. This tab allows for efficient heat transfer to a heatsink when required.

Electrical Characteristics

The following table summarizes the key electrical characteristics of the TIP120 transistor:

Parameter Symbol Value
Collector-Emitter Voltage VCEO 60V
Collector-Base Voltage VCBO 60V
Emitter-Base Voltage VEBO 5V
Collector Current (Continuous) IC 5A
Base Current (Continuous) IB 120mA
Power Dissipation PD 65W
Current Gain (hFE) hFE 1000 (minimum)

It’s important to operate the TIP120 within these specified limits to ensure reliable operation and prevent damage to the device.

TIP120 Applications

The TIP120 transistor finds applications in various electronic circuits where high current gain and power handling capabilities are required. Some common applications include:

  1. Motor control: The TIP120 can be used as a switch to control the current flow in DC motors. It can handle the high currents required by motors and provide efficient switching control.

  2. Relay driving: The high current gain of the TIP120 makes it suitable for driving relays. It can amplify the small control signal to provide sufficient current to energize the relay coil.

  3. Power regulation: The TIP120 can be used in power regulation circuits, such as voltage regulators and power supplies. It can handle the high currents required by these circuits and provide efficient regulation.

  4. LED driving: The TIP120 can be used as a current sink to control high-power LEDs. It can regulate the current flowing through the LEDs and ensure stable operation.

  5. Audio amplification: In audio amplifier circuits, the TIP120 can be used as a power amplifier stage to drive speakers or other load devices.

Basic TIP120 Circuit

Here’s a basic circuit diagram illustrating the use of a TIP120 transistor as a switch:

        |                     |
        |                     |
        |      +---------+    |
        |      |         |    |
        |     B|  TIP120 |    |
        |      |         |    |
        |      +---------+    |
        |       |    |        |
        |     R/   C|        |
        |       |    |        |
        |      E|    |        |
        |       |    |        |
        |       |    |        |
        |       |    |        |
        |      +|----|--------+
        |          |
       GND        Load

In this circuit, the TIP120 transistor is used as a switch to control the current flow through the load. The base terminal is connected to a resistor (R), which limits the base current. When a sufficient voltage is applied to the base (above the base-emitter threshold voltage), the transistor turns on, allowing current to flow from the collector to the emitter and through the load. The load can be a motor, relay, LED, or any other device that requires high current.

TIP120 Transistor Datasheet

For detailed information about the TIP120 transistor, it’s essential to refer to the manufacturer’s datasheet. The datasheet provides comprehensive information on the device’s electrical characteristics, maximum ratings, packaging, and application guidelines.

Some key parameters to note from the datasheet include:

  • Maximum ratings: The datasheet specifies the maximum values for various parameters such as voltage, current, and power dissipation. Operating the transistor beyond these limits can cause damage to the device.

  • Thermal characteristics: The datasheet provides information on the thermal resistance and maximum junction temperature of the transistor. This information is crucial for designing proper heat dissipation and preventing thermal runaway.

  • Graphs and charts: The datasheet often includes graphs and charts that show the relationship between different parameters, such as collector current vs. base current, collector-emitter saturation voltage vs. collector current, and current gain vs. collector current. These graphs help in understanding the behavior of the transistor under different operating conditions.

It’s recommended to thoroughly study the datasheet before using the TIP120 transistor in a design to ensure proper operation and reliability.

TIP120 Transistor Substitutes

In case the TIP120 transistor is not readily available or does not meet the specific requirements of a project, there are several substitutes that can be considered. Some common substitutes for the TIP120 include:

  1. TIP122: The TIP122 is a similar NPN Darlington transistor with slightly higher voltage and current ratings. It has a maximum collector-emitter voltage of 100V and a continuous collector current of 5A.

  2. TIP127: The TIP127 is a PNP complement to the TIP120. It has similar electrical characteristics but with opposite polarity. It is useful when a high-current PNP transistor is required.

  3. BD679: The BD679 is another NPN Darlington transistor with comparable specifications to the TIP120. It has a maximum collector-emitter voltage of 80V and a continuous collector current of 4A.

  4. MJE15032: The MJE15032 is a high-voltage NPN transistor that can be used as an alternative to the TIP120 in applications requiring higher voltage handling capability. It has a maximum collector-emitter voltage of 250V.

When selecting a substitute, it’s important to carefully compare the electrical specifications and package dimensions to ensure compatibility with the circuit design.

Frequently Asked Questions (FAQ)

  1. What is the maximum current that the TIP120 transistor can handle?
  2. The TIP120 transistor can handle a maximum continuous collector current of 5A.

  3. What is the purpose of the resistor connected to the base of the TIP120 in a typical circuit?

  4. The resistor connected to the base of the TIP120 serves to limit the base current and protect the transistor from excessive current. It also helps to control the transistor’s switching speed.

  5. Can the TIP120 transistor be used for high-frequency switching applications?

  6. The TIP120 transistor has a relatively slow switching speed due to its Darlington configuration. It may not be suitable for high-frequency switching applications above a few kilohertz.

  7. How can I ensure proper heat dissipation when using the TIP120 transistor?

  8. To ensure proper heat dissipation, it’s important to mount the TIP120 transistor on a suitable heatsink. The heatsink should have sufficient thermal mass and surface area to effectively dissipate the generated heat. Proper thermal compound should be applied between the transistor and heatsink for optimal thermal transfer.

  9. Can the TIP120 transistor be used as a linear amplifier?

  10. While the TIP120 transistor is primarily designed for switching applications, it can be used as a linear amplifier in certain cases. However, due to its high current gain and Darlington configuration, it may introduce distortion and have limited bandwidth in linear amplification applications.


The TIP120 transistor is a versatile and widely used NPN Darlington transistor known for its high current gain and power handling capabilities. Its Darlington configuration provides enhanced current gain, simplified biasing, and increased input impedance, making it suitable for a wide range of switching and amplification applications.

When using the TIP120 transistor, it’s crucial to operate it within the specified electrical limits and ensure proper heat dissipation to maintain reliable operation. Referring to the manufacturer’s datasheet is essential for detailed information on the device’s characteristics and application guidelines.

By understanding the basics of the TIP120 transistor, its pinout, electrical characteristics, and common applications, designers can effectively utilize this device in their electronic projects. Whether it’s controlling motors, driving relays, regulating power, or amplifying signals, the TIP120 transistor provides a robust and efficient solution.