Transistors! What are they and how do they work? Let’s explore these relatively simple devices with so much potential for greatness.
What are transistors?
Transistors are electronic devices with plenty of potential. They come in various shapes and sizes and have different ways they operate. They make up the internals of a lot of integrated circuits (ICs) that are used in various devices, like computers processors.
A transistor generally has three connections: the collector (or drain) where current flows into, the emitter (or source) where current flows out of and a base (or gate) that controls how much current flows through the transistor. Different transistors have these connections in different orders on the pins and it is best to consult a datasheet to find out how the transistor is configured.
What do transistors do?
Transistors do a few different things but depending on how they are interconnected with other transistors or electronic components they can solve a vast amount of different problems.
Transistors have 3 basic modes of operation: On (called saturation), Off (cut-off state) or somewhere in between (the active or linear state). Two common basic uses of a transistor are to use it as a switch or as an amplifier where a small input signal can result in a large output signal.
Examples of switch-type uses are logic circuits, computing devices, switches turning devices on and off. Examples of amplifier uses include sound amplification and radio receiving and transmitting circuits.
Different types of transistors
Individual transistors come in many different shapes and sizes. Here are a few frequently-found packages:
Apart from the different packages a transistor can be made in, there are also different types of transistor technologies. In an electronics hobby, you are most likely to use Bipolar-Junction Transistors (BJT) and MOSFETs (metal–oxide–semiconductor field-effect transistor).
How do transistors work?
For this blog, I’ll try to not get too technical or go into too much detail regarding how a transistor works. This quick explanation will be for the bipolar junction transistor (BJT).
A transistor is a semiconductor device, that is it’s made up out of semiconducting material. Semiconducting material is able to conduct current but not as good as a conductor. Silicon is one such material and is what transistors are generally made of. A bipolar junction transistor is made up of three layers of silicon, but these layers are not pure silicon. Each layer has an element added to it in a process called doping that gives the layer its semiconducting properties. Two BJT types exist: NPN and PNP. In an NPN transistor, the collector and emitter are made up of an N-type material, it has extra free electrons. The base is made up of a P-type material, it has a shortage of electrons.
There exists a depletion layer between each layer where free electrons “diffuse” across the boundaries. This depletion layer is an insulation barrier when the transistor is in its “cut-off” state. A certain current and voltage applied to the base can reduce the size of this depletion layer allowing current to flow through the collector and emitter. In order to turn the transistor “on”, the base needs to have a higher voltage than the emitter. The maximum voltage difference across the base-emitter junction depends on the material and some other parameters but are usually around 0.7 volts. Current generally will flow through the base and through the emitter.
For a fun explanation of how transistors work at an atomic level, feel free to view this video.
How to use an NPN BJT
As mentioned earlier, the BJT has three states. This state is determined by the voltages of each layer to one another.
In the “cut-off” state, the base voltage is at or lower than the emitter and collector voltages. No current is flowing into the base and therefore, no current can flow through the collector and emitter. The base-emitter and base-collector junctions are said to be reverse-biased; that is, no current is flowing through them.
In the “active” or “linear” state, the base voltage is higher than the emitter voltage but lower than the collector voltage. The base-emitter junction is said to be forward-biased, that is current is flowing forward through the base and emitter. The base-collector junction is said to be reverse-biased; no current is flowing through either the emitter or collector.
In the “saturation” state, both base-emitter and base-collector junctions are forward-biased. This is the state that allows the most current through the collector and emitter.
A transmitter as a physical device has various limitations. Some of these limitations include the maximum allowable current that can flow through the transistor (collector current, denoted IC, and emitter current, IE) and the maximum allowable current that can flow into the base (IB). To prevent damage to the transistor, series current-limiting resistors need to be added to the base input and in many cases to in series with the circuit going through the collector and emitter. The maximum values and many other characteristics are available in datasheets for each type of transistor, usually available online.
Transistor circuit examples
LDR activated LED
This simple circuit switches a LED on when the light drops below a certain level. An LDR (Light Dependent Resistor) is connected in series with a variable resistor in order to create a voltage divider. The voltage output of this divider is fed into the base of the transistor. Darker light levels reduce the resistance of the LDR and this results in a higher voltage on the transistor’s base. When the base-emitter voltage is high enough, the transistor starts to conduct, allowing current to flow through the LED. R2 is a current limiting resistor to prevent damage to the LED and transistor.
The variable resistor is used to adjust the “sweet spot” between the on and off conditions.
Common Emitter Amplifier
This is a theoretical circuit: the various resistor and capacitor values need to be carefully selected to correctly bias the transistor and power the microphone.
The microphone is an electret type. It requires a voltage to work. The capacitors act as buffers that “filters” out the direct current, allowing only alternating current (proportional to the sound waves). The biasing resistors on the transistor’s base make sure the base-emitter voltage allows the transistor to be in it’s active state.
Generally this circuit’s output will feed into another transistor or multi-transistor amplifier stage used to drive a speaker.
Switching a motor on and off
A simple circuit with an important lesson. The transistor is operated either on or off. When the switch is closed, current is allowed to flow into the base (note the current-limiting resistor). This allows current to flow through the DC motor and the transistor making the motor turn. But what’s with the diode?
A motor is an inductive device. When current flows through its coils, a magnetic field is generated. When the motor is stopped, the magnetic field collapses and that creates a spike in reverse current. This reverse current is allowed to flow through diode instead of the transistor, thus protecting the transistor against these spikes.
The flyback or freewheeling diode, as it is called, is also important when using a transistor to switch solenoids or relays.
I hope this little tutorial wasn’t too basic or too difficult to understand and that the basics of the operation of transistors were made clear enough to make you want to further play with them in your electronics endeavours. Please leave a comment below.