DOES A GATE WORK?
On the previous page we saw how the output of a gate goes HIGH
or LOW according to the voltage level on the inputs. But
how is a gate used in a circuit?
To answer this we need to have a circuit that requires a gate.
Suppose we have a circuit producing a constant tone. We
want the output to be a beep - beep - beep. To do this we can
turn the tone on and off or send it through a GATE
that allows the tone to pass to the output amplifier then SHUTS
IT OFF and repeats the action.
For this to work, the amplitude of the tone must be equal to
rail voltage. In other words it must be a DIGITAL SIGNAL.
In the animation below, the tone entering the gate is CONTROLLED
by the action of input A - the GATING LINE. The tone
actually passes through the gate. This may be hard to
visualise but the tone is identical to a switch on input B,
taking the line HIGH, then LOW at a very rapid rate.
If we look at the Truth Table for an AND gate, we see that if
input A is LOW, any signal on line B does not appear on
the output. This is shown in the top half of the Truth
When input A is HIGH, the output reflects the voltage
level on input B. This is shown in the lower Truth Table and
is exactly what we want. In the final version of the circuit,
the switch will be replaced by a low-frequency oscillator to
produce the beep - beep - beep effect.
When the signal is prevented from appearing on the output it is
said to be INHIBITED.
When the signal appears on the output, the gate is said to be
In the diagram above, you will notice the oscillator and AND gate are
not connected to the power rails. This is a BLOCK
DIAGRAM and power connections are never shown. They are
assumed to be connected.
DOES A GATE DO?
This question has
almost been answered above. A gate ENABLES (allows) a signal to
pass from one part of a circuit to another. It also
prevents (INHIBITS) a signal. The action of allowing and
preventing a signal is called GATING.
Suppose the amplitude of the waveform from the oscillator above is
not sufficient to activate the AND gate - in other words the
amplitude is not rail voltage. If this is the case, we cannot
use a "GATE" as it will not be activated.
The solution is to turn the waveform ON and OFF with a GATING
A gating line is slightly different to a GATE. A waveform
passes through a GATE but a Gating Line "KILLS" the
waveform (attenuates it) then "allows it to pass."
The simplest gating line is a diode connected to the output of a low
frequency oscillator as show in the diagram below. You will
notice the output waveform consists of the oscillator frequency
"in bursts." This is the beep - beep - beep sound we
The operation of the
low-frequency oscillator is shown in the animation below: It
simply goes HIGH, the LOW to produce the pulses of tone. The
resistor on the line prevent damage to the oscillator as the
diode "kills" the waveform by acting like a very low
resistance and the and the resistor in the circuit is a
the output of the low-frequency oscillator goes HIGH, the signal
from the oscillator appears on the output.
When the output of
the low-frequency oscillator goes LOW, the signal on the
output is LOW.
To understand how
the diode GATES the oscillator, we have to study the DIODE.
Just like most electronic components, the diode performs a
number of "jobs" according to where it is placed in a
circuit and the value of the surrounding components.
A diode has a number of characteristics (features that cannot be
altered) and the feature used in the gating line is the fact
that a diode passes electricity in ONLY ONE DIRECTION.
A diode is made up
of two different materials that have been fused together to
create a "junction." This junction passes
current in only one direction. If the voltage is applied
in the reverse direction to the junction, a "barrier
layer" or insulating layer develops to block the passage of
current. If the voltage is applied in the forward direction
the barrier layer is very thin and current is allowed to
You can read about the technicalities of how a diode works in
any text book but the essential point to understand is the fact
that current only flows in one direction and the arrow on the
symbol shows the direction.
Diodes come in all
shapes and sizes and two of the most popular types are shown
below. A signal diode is shown at top and a Power diode
below: Basically a signal diode will only pass a small current
and a power diode will pass a large current.
It's fortunate the symbol shows the direction of current flow
and the reason is the symbol was created in the early days
when current (electricity) was considered to flow from positive
This convention is still maintained today and is called
CONVENTIONAL CURRENT FLOW.
When describing our
circuits, it is convenient to use conventional current flow as
the arrows on the diodes, transistors and LEDs all point in the
direction of current flow. (The other convention is called
ELECTRON FLOW and electrons flow from negative to
Normally we talk about the cathode lead of a diode as this is the lead that
is identified on the body of the diode. It can be identified by
a black ring or band at one end, a dimple or cut-out near the
cathode lead or the letter "k."
When the voltage on
the ANODE is higher than the CATHODE, CURRENT
WILL FLOW through the diode.
If the voltage on the cathode is higher than the anode NO
CURRENT WILL FLOW.
We can now go into
more detail and show how the diode in the oscillator circuit
above, BLOCKS the signal when the low-frequency oscillator is
LOW and allows the signal to pass when the output of the
low-frequency oscillator is HIGH.
It is important to
"see" what a diode is doing in a circuit. You must be
able to VISUALISE it operating.
There are two facts to remember:
1. If the ANODE voltage is HIGHER than the CATHODE, the diode is
passing current and the voltage across it is very low. (about
0.6v to 0.7v)
2. If the CATHODE
voltage is HIGHER than the ANODE, no current will
through the diode and it is the same as if it is removed from
So, you can think of
the diode as two things:
In the first case: A length of wire.
In the second case: An open circuit.
Understanding how a
diode operates is very important. There are 4 ways a diode can
be placed on the output line: The following 4 diagrams show you
the effect of the diode.
If a diode is placed
between positive rail and the signal line, the output is a
This is shown below. The voltage on the anode "turns
on" the diode and current flows though it. The voltage on
the cathode is 0.7v less than the anode and this makes the
signal line a constant HIGH.
If the diode is
placed between the signal line and the 0v rail, the output is a
This is shown below. When the signal rises above 0.7v, the
diode "turns on" and conducts. Current flows through
it and the voltage does not rise above 0.7v.
If the diode is
placed as shown below, the anode is never higher than the
cathode and so it is never "turned on." The diode has
NO EFFECT on the signal and it is the same as if the diode is
If the diode is
placed as shown below, it has no effect on the signal. (you
can consider the diode is removed from the circuit).
THE DIODE "GATES" THE SIGNAL
Using the following two facts, we can show how the diode on the output of the low frequency
oscillator CONTROLS the signal:
1. When the output of the low
frequency oscillator is LOW, the output signal is LOW.
2. When the output of the low frequency oscillator is HIGH, the
high-frequency waveform appears on the output. (the diode is effectively "out of
circuit" and it has no effect on the signal).
animation below shows exactly how the diode is creating the
Here is a slowed-down
you can see exactly what is happening:
DIODE OR GATE
Diodes can also be used to create gates, where the signal
passes through the diode.
The diode OR gate is a simple example. The voltage and
current is passed to the output by the incoming signal and a
pull-up resistor is not needed. A pull-down resistor will
prevent the output "floating."
The output will go HIGH when line A OR line B is HIGH
(and also when BOTH inputs are HIGH).
A voltage-drop of 0.6v is lost across the diode but this will
not affect the any circuit using this gate.
For the diode AND gate, BOTH
input A AND input B must be HIGH for the
output to be HIGH.
The diode AND gate works in a slightly different way to the
diode OR gate.
The pull-up resistor delivers the output voltage and
current (in the diode OR gate, the input line(s) deliver
the voltage and current to the output).
The input lines ALLOW the output voltage to rise when BOTH
inputs are HIGH.
The output current of the AND gate is determined by the value of
resistor R. When the output is low, this current is termed
BLEED CURRENT and flows through the diode(s) to the 0v rail.
This current is "waste" current and must be kept to a
minimum for good design.
Name the gate that requires both inputs to be HIGH
for the output to be HIGH.
Ans: The AND gate.
the gate that goes high ONLY when one input is HIGH.
Ans: The XOR gate.
the gate that goes HIGH when one input goes HIGH.
Ans: The OR gate.
OR gate be produced with diodes?
is the voltage drop across a diode?
gate is INHIBITED, does the signal appear on its output?
the voltage on the anode of a diode is greater than the cathode,
does current flow through the diode?
Current flows from ANODE to CATHODE or CATHODE to ANODE, in a
Ans: Anode to Cathode.
does the letter "k" on a diode symbol represent?
Ans: The cathode. (Kathode in German)