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From Switches to Logic Gates

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Begin with the simplest symbol

Before combining two or more inputs, start with a device whose logical function is almost embarrassingly simple:

Y = A

The output has the same logical state as the input.

BUFFER A Y A No bubble: output follows input Y = A INVERTER / NOT A Y A Bubble means inversion Y = ~A

The digital buffer

A plain triangle with one input and one output, and no small circle, is a buffer or digital buffer. It may also be called a non-inverting buffer.

The name voltage follower describes the same “output follows input” idea, but that term is used more commonly for an analog op-amp circuit. In digital logic, buffer is the clearer name.

AY = A
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Why use a device that changes nothing?

Logically, it changes nothing. Electrically, a real buffer can be very useful.

It can isolate one stage from the next and provide more drive current so one signal can feed several later logic inputs without the source being overloaded.

It is better to think of this as strengthening or restoring the digital signal than as increasing its voltage. A 1 should remain a valid 1, and a 0 should remain a valid 0.

input A A BUFFER: Y = A logic input #1 logic input #2 logic input #3 logic input #4 One buffer output can drive several following inputs — within its fan-out rating.

The number of standard inputs that one output can reliably drive is called its fan-out.

Add one small circle and we get NOT

The small circle — often called an inversion bubble or simply a bubble — means the logical state is inverted.

So the same triangle becomes a NOT gate, or inverter:

Y = ~A
AY = ~A
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Do not confuse the digital buffer with an op-amp symbol

An operational amplifier is also commonly drawn as a triangle, but it normally has two inputs marked + and −. A basic digital buffer has one logical input and one logical output.

Visual rule: triangle alone → buffer, output follows input.
Triangle with an output bubble → inverter, output is NOT input.

Try these yourself

What is the Boolean function of a non-inverting digital buffer? BUFFER A Y A No bubble: output follows input Y = A INVERTER / NOT A Y A Bubble means inversion Y = ~A
A non-inverting buffer copies the logical state: the output follows the input.
What does the small circle at the output of the triangle mean?
The small circle is an inversion bubble. A triangle plus an output bubble is a NOT gate, or inverter.
Why might a digital buffer be placed between one logic output and several following inputs? input A A BUFFER: Y = A logic input #1 logic input #2 logic input #3 logic input #4 One buffer output can drive several following inputs — within its fan-out rating.
A real buffer can isolate the source and provide enough output drive for several following logic inputs, within its fan-out rating.

The symbol language begins here

The bubble convention is extremely useful. Once it is understood, the same little circle can be placed on the output or input of other logic symbols to show an inverted or active-LOW condition without drawing a separate inverter every time.

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