Computer Science Principles V11

Clive W. Humphris

LOGIC INTERFACING: Logic Buffer Stage.  


Ordinary CMOS or TTL logic gates have very imprecise switching threshold voltages. This means they cannot be guaranteed to change state at exactly the same input levels. An improvement is to use a gate call a 'Schmitt Trigger'.

These have two precise input states, inputs below 1.5V are considered logic 0 and those above 1.5V are logic 1. The type of gate can be recognised by the hysteresis symbol inside the gate diagram.

Apart from the precise timing that can be achieved, the switch over is considerably faster. The Darlington transistor has high input impedance and so can be connected directly to a logic gate output without drawing excessive current.

This type of transistor output has a low impedance making it suitable for driving high current loads directly. Note, the symbol shows two NPN transistors inside one package.

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