Transistors are Electronic Switches. They Can Amplify Currents, Too. A Tiny Control Signal can Control Huge Loads.

Transistors work similar to a dam, and while a valve wheel in a dam controls how much water can flow through it, the transistor control pin (called base or gate) controls how much current can flow through the remaining two pins.

Depending on transistor type, the control pin that is telling the transistor what to do is driven either by voltage or by current.

Transistors are typically used as switch in DC circuits or to amplify low-current low-frequency AC.
To switch AC, thyristors are used.

Switch or Potentiometer?

A transistor typically acts either as switch (in DC circuits) or amplifier (similar to a potentiometer):

Switch: the transistor is either fully on (fully conductive), or completely off (non-conductive), acting like a physical switch. Rather than you flipping a real switch, the transistor is controlled electronically. In many scenarios power transistors (MosFETs) are used this way to replace slow and power-hungry mechanical relais: tiny low-voltage buttons (or in general, small currents and voltages) can control huge currents and dangerous voltages.

Amplifier: By slowly increasing voltage or current to the control pin, the transistor works like a variable resistor and slowly increases or decreases its resistance between the other two pins. So a transistor can be used like a potentiometer, and the use case of a switch (above) is just an extreme example of this (cranking the potentiometer fully open or closed). Unlike a real potentiometer, the transistor can support significant currents. So a tiny voltage or current change at the control pin can result in a massive current change between the other two pins. This is how a transistor amplifier operates.

Can a transistor really act like a variable resistor? Producing highly accurate resistances would be difficult, considering production differences and linear ranges of transistors. That’s why you still need passive resistors or digitally controllable resistors (ICs) whenever you need precise resistances.

Circuit Symbols

In schematics, you identify transistors by these symbols:

Slight Symbol Variants

You may run across slight variations of these symbols, however for most practical purposes, you can focus on the four different transistor types depicted above and can safely ignore the rest of this paragraph if you are in a rush.

  • Diode: Some FET symbols include a symbol for a diode (like above). This often indicates Power MOSFET transistors that can handle huge amounts of currents.

  • Straight line vs. dashed line: the FET symbol may show a straight line instead of three separated small lines (dashed line as seen in the image above). A dashed line represents an enhancement mode FET whereas a straight line represents a much less common depletion mode FET. Enhancement mode FETs are “normally open” so by default, they are off (non-conductive). Depletion mode FETs are “normally closed” so by default, they are on (conductive).

  • 4 Terminals instead of 3: FET transistors internally use four connections, one on the left side and three on the right side. They only expose three terminals though. So typically, the symbol combines two lines with the source pin (as seen above). Occasionally, you may find symbols with a separate forth exposed terminal called substrate. However, there are practcally no FET transistors available that in fact expose this forth terminal.

Special Purpose Transistors

In schematics, you may come across special purpose transistors in preconfigured setups:

  • Darlington: a Darlington transistor is actually a combination of two transistors coupled together for amplification purposes: by combining two transistors, the gain is much higher than any single transistor could provide. The first transistor amplifies the input voltage and then uses the amplified current to drive a second transistor. That is why Darlington transistors are used when you have very weak input signals and require a high amplification factor. The symbol either identifies a Darlington transistor by a double line at the collector, or the dual transistors are actually depicted. When the symbol also includes a diode, this is representing a Power Darlington transistor, capable of handling high currents.
  • Phototransistor: here, a light-sensitive input replaces the base terminal. The minute current created by the light-sensitive material drives the transistor. In essence, the conductivity of the photo transistor is controlled by the intensity of the incoming light.
  • Optocoupler: in this device, a photo transistor is combined with a light source (LED). This way, one circuit (driving the LED) can control a second circuit (driven by the transistor), so Optocouplers physically separate two circuits, much similar to magnetic Reed Contacts or Relais - except Optocouplers have no movable parts, and they support more than just a simple on and off. Instead, they can couple any signal strength. Optocouplers are often used to connect microcontrollers with other circuitry that runs on a much higher voltage.

Transistor Families: BJT and FET

There are two fundamental transistor types available: BJT and FET:

  • BJT: most commonly used transistors in hobbyist projects. They are used for logic and for switching small loads such as LED. BJT are controlled by current and need a protective resistor at their base because their internal resistance between control pin and the collector is very low. So without a protective resistor or another current-limiting component at their base (i.e. a conductor), they shortcut the circuit and go up in flames (very much like LEDs).
  • MosFET: typically used to switch high current loads such as lamps, Power-LEDs, motors, heaters, etc. MosFET are controlled by voltage (electrical field) and do not necessarily need a protective resistor at their gate because their internal resistance between control pin and the source pin is very high so there is no significant current flow between these two pins under any circumstances anyway. You may want to add a protective resistor anyway when controlling big MosFET via microcontroller GPIOs: such MosFET have a considerable capacitance and can initially draw a high current that might be too much for a GPIO pin.

(from left to right: MOSFET, BJT, and a Power BJT)

Legs and Terminals

Transistors have three connectors (legs):

  • Base or Gate (Control): this is the control input that tells the transistor how much power it should pass between the other two pins.
  • Collector or Source: these remaining two pins carry the load current.
  • Emitter or Drain: these remaining two pins carry the load current.

The load flows between Collector and Emitter (for BJT), respectively between Source and Drain (for MosFET). The direction of current flow depends on the transistor type.

Why do Transistor Pins Have Different Names?

The terms base, collector, and emitter are used with BJT transistors whereas the terms gate, source, and drain are used with FET transistors. Regardless, their purpose is similar.

  • The base and gate pins act as the control pin that tells the transistor what to do.

  • The emitter and drain pins combine the current from the other two pins, so they emit or drain the current that entered the transistor through the other two pins (regardless of current flow direction). The collector and source pins acquire the load current that is to be controlled by the transistor, so they collect or are the source of the load current (again, regardless of current flow direction).

You may ask why both BJT and FET transistors are using different terminology for their pins when the three pins actually behave the same. And the answer is: they are not behaving exactly the same, and BJT and FET transistors internally are completely different devices. So their terminology derives from their internal architecture and design, and electrical engineers applaud that.

That said, for practical aspects and hobbyist level, it is perfectly ok to go by the simple “control pin” and two “load current” pins paradigm.

Identifying Legs and Terminals

Obviously, it is crucial to identify the correct “legs” of a transistor before use.

The only dependable ways of determining the pins is either review the data sheet for the particular transistor type, or to physically test the transistor. Only very few transistors actually show distinct pin markings on their housing.

Many multimeters have built-in transistor test capabilities when you switch the dial to the hFE setting.
You may even want to look into purchasing a dedicated transistor test device. They are available for around EUR 10-20 and tell you exactly not just the pins but also the N- or P-type and many additional useful parameters.

If you have neither a component tester device nor the data sheet, then there are a few rules of thumb though (use at own risk):

Plastic Casing With Flat Side

BJT transistory typically come in a black plastic casing with one flat side which is the front side. To identify the pins, look at the flat side and keep the pins pointing downwards: from left to right, the pins are: Collector, Base, Emitter (remember “CuBE”).

The default pin arrangement is not mandatory, and there are many exceptions to the rule. So always look up the transistor type you are using and verify the pin assignment in its data sheet!

Metal housed “can” type

Pins are arranged circularly. There is a tab in the rim of the housing. Typically, the pin closest to the tab is Emitter, the opposite pin is Collector, and inbetween is Base.

Types: N and P

Transistors come in N and in P types. FET transistors are called n-Channel and p-Channel, whereas BJT transistors are called NPN and PNP. As with the pin terminology, you can simplify and consider just two fundamental types of transistors: N-Type (NPN and N-Channel) and P-Type (PNP and P-Channel):

  • In N-type transistors, the Emitter/Drain (the one with the arrow in the symbol) is negative, the control pin is positive, and the current flows from control pin and Collector/Source to Emitter/Drain.
  • In P-type transistors, the Emitter/Drain (again, the one with the arrow in the symbol) is Positive, the control pin is negative, and the current flows from Emitter/Drain to control pin and Collector/Source.

N-Type, N-Channel, and NPN all are negative at their emitter (the pin marked with the arrow), and require a positive control signal
P-Type, P-Channel, and PNP all are positive at their emitter (the pin marked with the arrow), and require a negative control signal

The arrow in the transistor symbol does not generally indicate the direction of the current. Only for BJT transistors, the arrow points into the direction of current. With FET transistors, the arrow points into the opposite direction however. The reason for this inconsistency again is historic: in FET transistors, the arrow points into the direction of internal electron flow.

Choosing between N-Type and P-Type transistors depends entirely on where you want to place the transistor in your schematics, and whether you want to control them with a positive or negative signal.

Example: BJT Transistor in Action

Both schematics below do the same thing and control a load via a push button. The left schematic uses an NPN transistor, and the right schematic uses a PNP transistor:

  • N-Type (left circuit): the NPN BJT transistor switches the load on when the base is connected to positive voltage. The current flows from collector to emitter. The emitter (the pin with the arrow) is Negative (as in NPN or N-Type). A current limiting resistor makes sure the base current is not excessive.
  • P-Type (right circuit): the PNP transistor switches the load on when the base is connected to GND (0V). The current flows from emitter to collector. The emitter (the pin with the arrow) is Positive (as in PNP or P-Type). A current limiting resistor again makes sure the base current is not excessive.

Note how the transistor in the right circuit is flipped vertically. In the left circuit, the emitter (marked with arrow) is on the bottom and connected to GND. In the right circuit, the emitter (marked with arrow) is on top and connected to the positive side.

Example: MosFET Transistor in Action

The same circuitry can also be designed with FET transistors:

Note how the transistor in the right curcuit is again flipped vertically. In the left circuit, the source (marked with arrow) is on the bottom and connected to GND. In the right circuit, the source (marked with arrow) is on top and connected to the positive side.

  • N-Type (left circuit): the N-Channel FET transistor switches the load on when the gate is connected to positive voltage. The current flows from drain to source. The source (marked with arrow) is Negative (as in NPN or N-Type). Since FET transistors have a high resistance between gate and source, they do not require a current limiting resistor because there is no way a destructive current could flow anyway. Instead, since FETs are controlled by tiny voltages at their gate, the gate must now use a pull down resistor that connects the gate with GND. This keeps the gate from floating in an undefined state. So the pull down resistor safely pulls the gate to ground when there is no control signal.
  • P-Type (right circuit): the P-Channel FET transistor switches the load on when the gate is connected to GND (or a lower voltage than source). The current flows from source to drain. The source (marked with arrow) is Positive (as in PNP or P-Type). Again, the gate does not need a protective current limiting resistor. Instead, a pull up resistor is required to safely keep the base positive when the FET is supposed to be off.
Controlling FETs With Microcontrollers

When you plan to control the gate of a FET transistor by a microprocessor, you must add a current limiting resistor between your digital output pin and gate. FET transistors have a capacitance, so they act like a capacitor. When you turn on the MOSFET, it can momentarily draw a huge current of up to a couple of Amperes that may easily destroy your digital output pin. A current limiting resistor in series protects your microprocessor output (not the FET).

Also remember that FET transistors are voltage-controlled and need a given voltage difference between gate and source. The voltage difference required for a FET to become fully conductive (opened) depends on the FET specs. It is specified as Vgs or Vth in the transistor data sheet.

If you (or your microcontroller output) cannot provide the required voltage, you need to add a driver for the gate - which in its simplest form is another transistor that connects gate with whatever voltage is required.

Logic level FETs are specifically designed to directly work with microcontrollers: their gate-source-voltage is within the voltage range of typical microcontrollers. If you must use FETs with higher voltage requirements, there are specific driver ICs available (i.e. LTC7004) that can drive FETs with gate-source-voltages of up to 60V.

Commonly used NPN transistors are 2N2222, 2N3904, TIP120
Commonly used PNP transistors are 2N2907, 2N3906

The default pin arrangement is not mandatory, and there are many exceptions to the rule. So always look up the transistor type you are using and verify the pin assignment in its data sheet!


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(content created Feb 24, 2024)