The basic concept of how transistor amplified current

Transistor can work as a Switch or an Amplifier.  We take a look at how the transistor can amplify a current.  A tiny electric current flowing through one part of the transistor can make a much bigger current flowing through another part of it.  It doesn’t actually amplify the current flowing though it.  It is actually means a current flow from the base to the emitter “opens” the flow of current from the collector to the emitter.


a small base current controls a larger collector current

c = collector, b = base, e = emitter

Base current * the transistors gain (hFE or hfe) = how much collector current ‘could’ flow.


In the above diagram, you need to apply a small control voltage between the base and the emitter to get the current flow from base to emitter (Blue).  When you’ve done this, it will turn the transistor ON and allow a bigger portion current to flow from collector to emitter (Red).

Notes: The collector resistor determines the maximum current that can flow through the collector. Maximum base current when forward biased depends entirely on heat and thus will change based on ambient conditions.

In other words, it’s a kind of current booster. That comes in really useful in things like hearing aids, one of the first things people used transistors for.


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