LESSON 6:
Diode Gate Design
A
8
X
A
II
X
0
0
0
0
I
0
I
0
I
I
I
I
I
I
0
0
0
I
0
0
0
I
I
I
OR Colt
AND Co le
1·1 ~
F i g,
Truth tebltt tor AND and OR t a les . A 0
r t pt t . . nll
a low voltage level and a I a hlth voltat• level .
-V
Oucput
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Th is lesson will introduce the
student to logic and truth tables
as well as the design of diode
gates. Both AND and OR gates
are covered .
Logic Gates and Truth Tables:
Logic gates are circuits which
have a single output and two or
more inputs the state of the out-
put is a funct ion of a set of rules
concerning the states of the in-
puts. For example, the output of an
AND gate is a high level if and only if
all of the inputs are at a high level.
The output state of an OR gate is a
high level if one or more of the inputs
is at a high level.
F ig . 1 ·2 . A
IC:hemt~tlc
dlatretn of 1 two Input di ode OR ,. ,, ,
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-V
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x.
II• OJ
~
A
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" .,. 9 .,.
..,.
..,.
Fig . l · l . A d iagram ot a diode OR gate with all of the poas lbla
Input stat ...
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-V
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F it . 1--4. A .c.hem1tlc dlattam ot • two Input diode AN D gate.
The studen t shoul d compare th i s circu i t to thet ot the OR ,.,,
In F i t . 1-2.
-V
-V
-V
-V
-V
-V
A«D
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R
0
H
-v·
1
Fit. W . A di . . MI of • diode AND t•t• with all of tM
po.,llllt Input atatat .
PLAY METER, May 1, 1981
Instead of defining a logical
operation in words, an alternative
method is to define a high level as
logic 1 and a low level as logic 0.
Also , a truth table can be drawn . It
will contain all possible input
states and their corresponding
output state. In fig. 6-1 are the
truth tables for two input AND
and OR gates. The inputs are
labled A and B and the outputs
is labled X. Notice that the
defin itions given above are con -
sistent with the truth tables.
Diode OR Gates:
The circuit in fig. 6-2 is a diode
OR gate. The circuit operates as
follows : When both the A and B
inputs are at a logic 0, both diodes
are zero-biased , and no current
flows through the resistor R.
When no current flows , the
voltage drop across R is, of
course, zero. Therefore, the out-
put X is a a logic 0. This condition
isillustrated in fig . 6-3, section 1.
When either output A or B is
at logic 1, a current will flow
through R. When a current flows
through R the output raises to a
voltage V' because of the voltage
dropped across R. Therefore, the
output X is a log ic 1. It is easily
seen in fig. 6-3, section 2, that V' is
VF less than V. Also, notice that
the current through the diode is 1
plus the leakage current (IR) of
the zero-biased diode.
The condition of both A and B
inputs at logic 1 is illustrated in
section 3 of fig . 6-3. Notice that I is
now divided between both diodes
and that output X is at a logic 1.
The student should veri fy that the
operation of the diode OR gate in fig.
6-2 is consistent with the OR gate
truth table in fig. 6-1 . Fig. 6-3 should
be used as an aid for translating the
circuit to the truth table.
Design Procedure:
I V ' I·
STEP 1 : De termin e
VF is generally stated on the manu-
facturers ' data sheets. However, as a
general rule: VF for silicon diodes H
0.6V, and for germanium diodes, 02V
can be used.
STEP 2: F ind I.
I v 'I
I
R
Design Example:
Design a two-input diode OR
gate where R is a 6V, 150-omega
lamp. Use a silicone diode type
1N914. (The circuit diagram is
shown in fig. 6-2.
STEP 1 : Determ ine
6.0V - 0 .6V
=
The
IV ' I .
=
5 .4V
general rule for si licon
diodes is used.
STEP 2: Find I.
I
=
V,
R
5.4V
1500
38m a
Diode OR Gates:
The circuit in fig. 6-4 is a diode
AND gate. The circuit operates as
15