Switching Loads And Amplifying Currents

Transistors are versatile semi-conductors that can act both as an electronic switch or as an amplifier.

Picking A Transistor

There is a vast variety of transistors available: they exist in different technologies (BJT, MOSFET, and more) and configurations (P- and N-type). It is not trivial to pick the right transistor for a given purpose, especially if you are just starting.

This section is designed to assist you in picking the best transistor for a given purpose. It also helps you find alternative (replacement) transistor types if you work with a circuit design but don’t have the particular transistor types at hand that the original circuit design uses or can’t order these particular transistor types.

[NOTE:] If you are fundamentally interested in how transistors work and what their different technologies are, please visit the Fundamentals Section.

Use Cases

Transistors are very versatile and can be used for many different use cases. Each use case focuses on specific features of a transistor. To find the best and most suitable transistor (or find a replacement type with similar specs), it all starts with your intended use case.

Switching Loads

One of the most common use cases is using a transistor to switch big(ger) loads. Here, the transistor acts like an electronically controllable switch. Like with a real switch, there are technical specs to watch out for:

  • Voltage: what is the maximum voltage you can switch? What is the maximum isolation voltage (the reverse voltage)?
  • Current: how much electrical power do you want to switch? What are the maximum currents the switch needs to sustain?
  • On-Resistance: when in on state, is the transistor really acting like a closed switch with next-to-zero resistance, or is there still a small on-resistance that may cause a voltage drop and produce heat?
  • Off-Resistance: when in off state, is the transistor really acting like an opened switch with no resistance, or is there still a small leaking current that might cause a battery power supply to slowly drain?
  • Frequency: in cases where you want to use the switch not just as a replacement for a manual switch (occassionally turning it on and off), the maximum frequency may become important as well: when you use the switch as part of a PWM (pulse width modulation) dimmer or inside a switching DC-DC-regulator, the switch is turned on and off many thousand times per second. Here, it is important to check that the transistor can actually switch this fast.


Another common use case is amplification: you need more than just on and off and what to use the transistor like a potentiometer? Maybe you want to control the brightness of a lamp, or the speed of a motor.

When you amplify, the transistor continues to act like a switch: all considerations above continue to apply. In addition - since now the transistor can also have a variable resistance - these considerations become important, too:

  • Heat: When a transistor is not fully open, it has a resistance, and when current flows through a resistance, heat is produced: how much heat does the transistor tolerate, and how much heat can it dissipate? In essence: how much current can you amplify without frying the transistor by excessive heat?


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(content created Apr 27, 2024)