About Transistors

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 transistor


PNP transistor

Diode representation

NPN made out of diodes

diode representation of a NPN transistor

PNP made out of diodes

diode representation of a PNP transistor

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

To turn on a transistor you need 0.7v or more on the base of the transistor at witch you will get a base current flow Ib, the current through the Collector emitter junction is equal to current through the Base emitter junction X hfe. Ic = hfe x Ib but not if the transistor is saturated. The emitter current Ie = Ic + Ib. Ic is much larger than Ib, so you can say that Ie = Ic. hfe is the amplification of the current that a transistor can produce, DC current gain. Saturated meant that the resistance between collector and emitter = 0 ohms, when this happens the Vce is 0v and the voltage on the collector is determined from the supply voltage and the external resistance of the other components in the circuit.

Transistor LED driver

LED transistor 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.

Darlington pair

Darlington Pair driver

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.

hfetotal = hfe1 × hfe2

This might sound like a good thing right but with all good things come draw backs doubling the transistors also doubles the base voltage and there fore instead on needing 0.7v to switch the transistor you will now need 1.4v. A darlington pair is so sensitive to current flow, that it can detect the current of you touching the base connector with your finger, this makes it perfect for touch plate switches.

Transistor to switch a large load

switching 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


[Link to source of this circuit]

This is a very simple radio circuit I found on the web using:
1 transistor
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


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