Applied Electronics - Outcome 1 - Transistors

In Higher we need to know about two configurations of Transistors, one to create a greater amplification of current, and one to allow two different things to happen depending on whether the input is positive or negative.

Darlington Pair:

The Darlington Pair uses two transistors with the emitter from one giving the base of the other (two in a row).  This gives double the current gain, but also needs double the VBE to saturate the pair.

Note: The easiest thing for you to forget is to carry out your transistor calculations with 1.4v instead of 0.7v.  Often this is the value that people most readily forget even in a single transistor!

Push Pull Driver:

The push pull driver uses the two different types of transistor so that when VBE is positive, the NPN transistor will saturate and when VBE is negative, the PNP transistor will saturate.

This allows the current to flow either from positive to 0v or from 0v to negative. If we connect a motor to the 0v (the emitters of each transistor) we can make it turn in both directions.  If we connect to different outputs to the collectors of each transistor, we can get a different output from a positive input signal than an negative one.