A transistor can be thought of as a simple current switch. There are two main transistors NPN and PNP. NPN are the most common transistors. A transistor can be thought of as two diodes sharing the same anode for NPN or cathode for PNP. The base emitter junction is forward biased and the base collector is reversed biased. By applying a small voltage to the base of a transistor you allow a current flow through the transistor from the collector towards the emitter. This is easy to remember as the collector will generally be connected to your supply voltage and the emitter will go towards ground. Also it is important to note that a transistor is a current operated device and not voltage. When a transistor is “switched on” it acts as a conductor and therefor has very low resistance. If you put to much current through a transistor it will get VERY VERY hot and will probably breakdown therefore you should have a current limiting resistor connected in series with Collector Emitter of a transistor as well as a series resistor with the base of the transistor to also limit the current flow at the base emitter junction.
Diagrams of transistors: NPN & PNP
NPN made out of diodes
PNP made out of diodes
A transistor has 3 pins: Base, emitter and Collector the majority current flow is through Collector towards emitter there is a secondary current flow from base to emitter.
Transistor threshold voltage
Transistor LED driver
10kohm resistor limits the current flow or you will blow the transistor. the 470 ohm resistor limits the current through the collector emitter junction as a LED has no resistance and will blow both the LED and transistor if not used. Diode representation.
This is two transistors connected together so that the amplified current from the first transistor is amplified further by the second transistor. The first transistor’s emitter feeds into the second transistor’s base and as a result the input signal is amplified. This circuit acts like a single transistor with the gain = to the product of all the gains of the transistors.
Transistor to switch a large load
When a transistor is used as a switch it must be either fully on or off. If driving a inductive load like a relay or any type of coil you should connect a diode in reverse bias across the load so that back EMF will not flow into the transistor, destroying it.
Single transistor radio
This is a very simple radio circuit I found on the web using:
a 1 meter piece of wire as a aerial
you will need a germanium detector diode for the best results these diodes have very low switching voltage, typically about 0.3V
a 3.5mH inductor that is center taped or 2 x 0.18mH connected back to back
and a variable capacitor plus a few passive components