ExpErimEnt 5: DioDE – transistor Logic (DtL)
nanD gatE
~Arghya Maity, 20MS038
⸎Objective: To understand basic gate operation of Diode-Transistor Logic (DTL).
⸎Overview:
¤Digital Logic States
All digital electronic circuits and microprocessor-based systems contain
hardware elements called Digital Logic Gates that perform the logical operations of
AND, OR and NOT on binary numbers. In digital logic only two voltage levels or
states are allowed and these states are generally referred to as Logic "1" or Logic
"0", High or Low, True or False and which are represented in Boolean Algebra and
Truth Tables by the numbers "1" and "0" respectively. A good example of a digital
logic level is a simple light as it is "ON" or "OFF".
Logic operations can be performed using any non-linear device that has at
least two distinct regions of operation. Obvious choices for the electrical engineer are
the semiconductor diode and the bipolar junction transistor. Particular voltage levels
are assigned to logic levels 0 and 1.
While many voltage level assignments are possible, one common assignment is:
logic 1 (HIGH)---- ~ 5 V
logic 0 (LOW) ---- ~ 0 V.
This is known as "Positive logic" system. There is also a complementary
"Negative Logic" system in which the values and the rules of a logic "0" and a logic
"1" are reversed. But, unless stated otherwise, we shall only refer to the Positive
Logic convention for all the experiments. It is important to note that noise, power
source fluctuations, loading by other circuits, and other factors will cause the logic
level voltages to vary over some range.
¤ Simple Basic Digital Logic Gates:
Simple digital logic gates can be made by combining transistors, diodes and
resistors as discrete components. We will investigate one of such circuits:
Diode-Transistor Logic (DTL), as described below:
» Diode-Transistor Logic (DTL):
The simple 2-input Diode-Resistor gate can be converted into a NAND/NOR
universal gate by the addition of a single transistor inverting (NOT) stage employing
DTL. Diode-Transistor Logic, or DTL, refers to the technology for designing and
fabricating digital circuits wherein logic gates employ diodes in the input stage and
bipolar junction transistors at the output stage. The output BJT switches between its
cut-off and saturation regions to create logic 1 and 0, respectively. The logic level
shift problem of DRL gates is not present in DTL and TTL gates so that gates may
be connected in series indefinitely. If a gate drives several similar gates in parallel
problems may occur: the maximum number of gates that can be driven in parallel is
identified as the "fanout" of a gate. DTL offers better noise margins and greater fanouts than RTL (Resistor- Transistor Logic), but suffers from low speed, especially
in comparison to TTL. Diodes take up far less room than resistors, and can be
constructed easily. In addition, the internal resistance of a diode is small when the
diode is forward biased, thus allowing for faster switching action. As a result, gates
built with diodes in place of most resistors can operate at higher frequencies.
Because of this diode-transistor logic (DTL) rapidly replaced RTL in most digital
applications.
Ø DTL NAND Circuit:
The DTL NAND gate combines the DTL inverter with a simple Diode-Resistor
Logic (DRL) AND gate as shown in its circuit diagram. Thus, any number of inputs
can be added simply by adding input diodes to the circuit. The problem of signal
degradation caused by Diode Logic is overcome by the transistor, which amplifies
the signal while inverting it. This means DTL gates can be cascaded to any required
extent, without losing the digital signal.
If any input is less than VIL = VBE (FA), the transistor is cut off and the output is
VOH = VCC
If all inputs are greater than or equal to VBE (FA), the BJT conducts and if they
reach VIH = VBE(SAT) the output is
VOL = VCE (SAT)
Circuit Components:
1. Resistors (100 Ohm, 1 No.; 47.2 kOhm, 1 No.; 2.15 kOhm, 1 No. )
2. 1N4007 silicon diodes (2 Nos.)
3. BC547 NPN silicon transistor (1 No.)
4. D.C. Power supply (5V)
5. Connecting wires
6. Breadboard
Circuit Diagram:
Realizing NAND Gate using Diode and Transistor:
For simplicity we will show here only two inputs NAND gate circuit by using
diodes and transistors. This NAND gate is called DTL NAND gate or Diode
Transistor Logical NAND Gate.
When both input A and B are given with 0 V, both of the diodes are in forward
biased condition that is in ON condition. Supply voltage will get path to the ground
through diode D1 and D2. Entire supply voltage +5 V will ideally drop across resistor
R and hence base terminal of transistor T will not get enough potential to turn ON the
transistor and hence the transistor will be in OFF condition. As a result supply
voltage +5 V will appear at output terminal X and hence output X will become high or
logical 1. A demo circuit is shown here:
Now if either of diode D1 and D2, is applied with 0 V, the same thing happens
as in these cases also the supply voltage gets a path to the ground either of the
forward biased diode. In those cases also the output will be logical high or 1.
When both of the inputs are given with +5 V that is logical 1, both of the diodes are in
OFF condition and hence supply voltage will appear at the base terminal of the
transistor T which makes it switched ON and supply voltage gets a path to the
ground through thses transistors. Ideally entire supply voltage +5 V will drop across
resistor R’ and output terminal X will get ideally zero volts and hence the output is
considered as logical 0. Hence, the output is only 0 when and only when both inputs
are +5 V or logical 1.
Truth Table:
Observations:
Truth Table:
V_out
Truth
V_A (V) V_B (V) (V)
Value
1
0
0
8.3
1
0
5
8
1
5
0
7.9
0
5
5
0
Hence, it follows the truth table of NAND Gate (Proved).
VTC:
V_out
V_in (V) (V)
0.1
0.501
0.2
0.499
0.3
0.376
0.4
0.095
0.5
0.083
0.6
0.077
0.7
0.073
0.8
0.071
0.9
0.07
1
0.068
2
0.062
3
0.06
4
0.06
5
0.06
From the VTC, the following observations were made:
V_OH (OH = Output High) = 0.501 V
V_OL (OL = Output Low) = 0.06 V
V_IL (IL = Input Low) = 0.2 V
V_IH (IH = Input High) = 0.4 V
⸎ Photos and Videos of my circuit:
Demonstration Video:
https://drive.google.com/drive/folders/1HZs3xhK1nLvPxQlt5eVeOpIE82psbDNE?usp=
sharing
Photos:
⸎ Reference:
i)
Logic gate circuits (Various logic families) – niser.ac.in
ii)
Diode-Transistors Logic (DTL)
Gate – Islamic University of Gaza
iii)
iv)
Chapter 6 Diode Transistor Logic DTL Digital Electronics Slidetodoc.com
Electrical4u.com