The Digital Logic "NOT" Gate
Definition
The digital Logic NOT Gate is the most basic of all the logical gates and
is sometimes referred to as an Inverting Buffer or simply a Digital Inverter.
It is a single input device which has an output level that is normally at logic level "1" and goes "LOW" to a
logic level "0" when its single input is at logic level "1", in other words it "inverts" (complements) its input
signal. The output from a NOT gate only returns "HIGH" again when its input is at logic level "0"
giving us the Boolean expression of: A = Q.
Then we can define the operation of a single input logic NOT gate as being:
"If A is NOT true, then Q is true"
Transistor NOT Gate
A simple 2-input logic NOT gate can be constructed using a RTL Resistor-transistor switches
as shown below with the input connected directly to the transistor base. The transistor
must be saturated "ON" for an inversed output "OFF" at Q.
Logic NOT Gates are available using digital circuits to produce the desired logical function. The standard NOT gate is given a symbol whose shape is of a triangle pointing to the right with a circle at its end. This circle is known as an "inversion bubble" and is used in NOT, NAND and NOR symbols at their output to represent the logical operation of the NOT function. This bubble denotes a signal inversion (complementation) of the signal and can be present on either or both the output and/or the input terminals.
The Digital Inverter or NOT gate
| Symbol | Truth Table | |
Inverter or NOT Gate
|
A | Q |
| 0 | 1 | |
| 1 | 0 | |
| Boolean Expression Q = not A or A | Read as inverse of A gives Q | |
Logic NOT gates provide the complement of their input signal and are so called because when their input signal is "HIGH" their output state will NOT be "HIGH". Likewise, when their input signal is "LOW" their output state will NOT be "LOW". As they are single input devices, logic NOT gates are not normally classed as "decision" making devices or even as a gate, such as the AND or OR gates which have two or more logic inputs. Commercial available NOT gates IC's are available in either 4 or 6 individual gates within a single i.c. package.
The "bubble" (o) present at the end of the NOT
gate symbol above denotes a signal inversion (complimentation) of the output signal. But this bubble can also be present
at the gates input to indicate an active-LOW input. This inversion of the input signal is not restricted to the
NOT gate only but can be used on any digital circuit or gate as shown with the operation of
inversion being exactly the same whether on the input or output terminal. The easiest way is to think of the bubble as
simply an inverter.
Signal Inversion using Active-low input Bubble
Bubble Notation for Input Inversion
NAND and NOR Gate Equivalents
An Inverter or logic NOT gate can also be made using standard
NAND and NOR gates by connecting together ALL their
inputs to a common input signal for example.
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A very simple inverter can also be made using just a single
stage transistor switching circuit as shown. When the transistors base input at "A" is high, the transistor
conducts and collector current flows producing a voltage drop across the resistor
R thereby connecting the output point at "Q" to ground thus resulting in a zero
voltage output at "Q".
Likewise, when the transistors base input at "A" is low (0v), the transistor now switches
"OFF" and no collector current flows through the resistor resulting in an output voltage at "Q" high at a
value near to +Vcc.
|
Then, with an input voltage at "A" HIGH, the output at "Q" will be LOW and an input voltage
at "A" LOW the resulting output voltage at "Q" is HIGH producing the complement or inversion of the input signal.
Hex Schmitt Inverters
A standard Inverter or Logic NOT Gate, is usually made up from
transistor switching circuits that do not switch from one state to the next instantly, there is some delay.
Also as a transistor is a basic current amplifier, it can also operate in a linear mode and any small variation
to its input level will cause a variation to its output level or may even switch "ON" and "OFF" several times
if there is any noise present in the circuit. One way to overcome these problems is to use a Schmitt Inverter
or Hex Inverter.
We know from the previous pages that all digital gates use only two logic voltage states and
that these are generally referred to as Logic "1" and Logic "0" any TTL voltage input between 2.0v
and 5v is recognised as a logic "1" and any voltage input below 0.8v is recognised as a logic "0" respectively.
A Schmitt Inverter is designed to operate or switch state when its input signal goes above
an "Upper Threshold Voltage" limit in which case the output changes and goes "LOW", and will remain in that
state until the input signal falls below the "Lower Threshold Voltage" level in which case the output signal
goes "HIGH". In other words a Schmitt Inverter has some form of Hysteresis built into its switching
circuit.
This switching action between an upper and lower threshold limit provides a much cleaner and
faster "ON/OFF" switching output signal and makes the Schmitt inverter ideal for switching any slow-rising or
slow-falling input signal either an analogue or digital signal.
Schmitt Inverter
A very useful application of Schmitt inverters is when they are used as oscillators or
sine-to-square wave converters for use as square wave clock signals.
Schmitt Inverter Oscillator & Converter
The first circuit shows a very simple low power RC type oscillator using a Schmitt inverter to generate square waves. Initially the capacitor C is fully discharged so the input to the inverter is "LOW" resulting in an inverted output which is "HIGH". As the output from the inverter is fed back to its input and the capacitor via the resistor R the capacitor begins to charge up.
When the capacitors charging voltage reaches the upper threshold limit of the inverter,
the inverter changes state, the output becomes "LOW" and the capacitor begins to discharge through the resistor
until it reaches the lower threshold level were the inverter changes state again. This switching back and forth
by the inverter produces a square wave output signal with a 33% duty cycle and whose frequency is given as:
ƒ = 680/RC.
The second circuit converts a sine wave input (or any oscillating input for that matter)
into a square wave output. The input to the inverter is connected to the junction of the potential divider
network which is used to set the quiescent point of the circuit. The input capacitor blocks any DC component
present in the input signal only allowing the sine wave signal to pass.
As this signal passes the upper and lower threshold points of the inverter the output also
changes from "HIGH" to "LOW" and so on producing a square wave output waveform. This circuit produces an output
pulse on the positive rising edge of the input waveform, but by connecting a second Schmitt inverter to the
output of the first, the basic circuit can be modified to produce an output pulse on the negative falling edge
of the input signal.
Commonly available logic NOT gate and Inverter IC's include
TTL Logic Types
|
CMOS Logic Types
|
Inverter or NOT Gate 7404
In the next tutorial about Digital Logic Gates, we will look at the digital logic
NAND Gate function as used in both TTL and
CMOS logic circuits as well as its Boolean Algebra definition and truth tables.
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