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Transistors
The solid state transistor is the single component that has driven the electronics revolution. Initially they were very hard to manufacture and regarded expensive specialized parts. Improved mass production techniques gradually lead to transistors becoming cheaply and easily available. Later on a new wave on innovation was created when it was realised that multiple transistors could be manufactured on a single wafer of silicon – leading to the integrated circuit, microprocessors, microcontrollers and all the advanced toolkit available to modern electronic engineers.
We are going to focus on one type of transistor – the NPN bipolar transistor. A transistor has three terminals and acts as an amplifier. For an NPN transistor the terminals are called the “collector”, “base” and “emitter”. A small current flowing in to the base and out of the emitter controls a much larger current flowing from the collector to the emitter. Think of the current at the base as working to control a tap that varies the flow through the collector. If no current flows in to the base then no current will flow through the collector either.
The base voltage varies between 0V when the transistor is off and about 0.7V when the transistor is on. Like an LED the input to the base normally needs to be protected by a resistor to stop the base being overloaded.
Using transistors we can build many types of circuits including amplifiers, logic circuits, oscillators, filters and power controllers.
The maximum collector current ranges from a few hundred mA to several Amps depending on the transistor. The gain of the transistor is the multiple of the base current the can flow through the collector. This is normally in the range 50-400 and is sometimes written as hFE.
William Bradford Shockley (February 13, 1910 – August 12, 1989)
Leader of the team at Bell Labs that developed the junction transistor. Nobel prize winner. Also described
as “the notorious eugenicist and scientific racist”..
NPN Transistor
Collector
Emitter
Base
Big current can
flow here
Small control current
Transistor Packages
Transistor type 2N3704 pinout