Download DTL Inverter Introduction One of the problems with RTL circuits is

DTL Inverter
Introduction
One of the problems with RTL circuits is that the input resistor for each
input has the effect of slowing down the rate at which the internal capacitance
of the transistor can charge or discharge. This limits the frequency at which
RTL gates can switch states. A second problem is that resistors take up a
significant amount of space, or "real estate," on the silicon chip that comprises
the integrated circuit. High-value resistors generally require more space than
lower-value resistors, and in any case the shape of the resistor on the chip
depends very much on the required resistance value. Real estate is very
precious, and every effort is made to develop and use techniques that will
minimize the amount of space required for any circuit.
Diodes take up far less room than resistors, and can be constructed to all be
the same. 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.
Schematic Diagram
The DTL inverter uses the same transistor as the RTL inverter, and the same
1K collector load resistor. However, in place of an input resistor in series with
the base lead, we use a pair of diodes as shown. The diode connected directly to
the transistor base serves to raise the input voltage required to turn the
transistor on to about 1.3 to 1.4 volts. Any input voltage below this threshold
will hold the transistor off. Thus, we can be sure that another circuit of this
same type will have no trouble properly controlling the state of this circuit.
The base resistor is considerably smaller than 15K to turn the transistor on
more quickly when the input rises to a logic 1. A 15K resistor would be enough
to drive the transistor into saturation, but would limit the operating frequency
of the gate just as it does with RTL. By reducing the value of this pull-up
resistor, we allow the transistor to turn both on and off more rapidly, thus
enabling higher switching speeds than are possible with RTL.
Parts List
To construct and test the DTL inverter circuit on your breadboard, you will
need the following experimental parts:
 (1) 1K, ¼-watt (brown-black-red) resistor.
 (1) 4.7K, ¼-watt (yellow-violet-red) resistor.
 (2) 1N914 silicon diodes.
 (1) 2N4124 NPN silicon transistor.
 (1) 0.3" black jumper.
 (1) 3" orange jumper.
 (1) 10" white jumper.
Constructing the Circuit
Select an area on your breadboard socket that is clear of other circuits. Our
construction procedure places this circuit just to the right of the center divider
of the breadboard socket, as shown in the construction image below. Refer to
this image and the step-by step instructions as you install the experimental parts
for this circuit.
Three-Input DTL NAND Gate
Introduction
One of the limitations of RTL circuits was that while NOR gates are easy to
construct, it is difficult to obtain a NAND function in a single gate.
Theoretically we could stack multiple transistors in series, but this leads to a
range of problems.
With DTL, however, we can simply add more input diodes to the inverter
circuit. This effectively combines a Diode Logic AND gate with an inverter
circuit. The use of the inverter re-amplifies the signal and thus overcomes the
limitations we saw in the plain DL AND gate.
In this experiment, we will extend our DTL inverter according to this idea,
and see how well it works.
Schematic Diagram
The DTL NAND gate combines the DTL inverter with a simple Diode
Logic (DL) AND gate. 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.
In addition, the use of diodes in this configuration permits the construction
of NAND gates; something that was not practical with RTL because there was
no practical way to allow any single logic 0 input to override multiple logic 1
inputs. Therefore, DTL offers more possible configurations as well as better
performance than RTL.
Parts List
To construct and test the three-input DTL NAND gate circuit on your
breadboard, you will need the DTL inverter you constructed in the previous
experiment, plus the following experimental parts:
 (2) 1N914 silicon signal
 (2) 3" orange jumpers.
diodes.
Constructing the Circuit
You should still have the DTL inverter circuit in place from the previous
experiment. If you did not perform this experiment, or if you removed the parts,
go back now and reassemble the inverter circuit. You will expand on it for this
experiment.
Performing the Experiment
Turn on power to your experimental circuit, and
set switches S0, S1, and S2 all to logic 0. Observe the
output state of your experimental circuit, and record
this logic state on the top row of the table to the right.
Inputs
Output
S2
S1
S0
L0
Continue by setting the three input switches to
each of the eight possible input combinations. For
each input combination, note the state of the output as
indicated by L0. Record your results in the table to the
right.
0
0
0
?
0
0
1
?
0
1
0
?
When you have recorded your results for all input
combinations, look over your results and decide what
0
1
1
?
kind of logical function is actually performed by this
circuit. Is it really a NAND gate?
When you have made this determination, turn off
the power to your experimental circuit and compare
your results with the discussion below.
1
0
0
?
1
0
1
?
1
1
0
?
1
1
1
?
Discussion
You should have found that this gate produces a logic 1 output as long as
any of the inputs is at logic 0. Only when all inputs are at logic 1 will the
transistor be allowed to turn on and produce a logic 0 output. This is an
inverted AND, or NAND function, so this circuit does indeed behave as a 3input NAND gate. The number of inputs can be increased to any desired extent
by simply adding more input diodes, and the circuit will still exhibit NAND
behavior.
When you have completed this experiment, make sure power is turned off,
but leave your experimental circuit intact; you will modify it for your next
experiment.
Performing the Experiment
Turn on power to your experimental circuit, and move
switch S0 back and forth between logic 1 input and logic 0
input. What is the output state of this circuit for each of the
possible input states? Record your results in the table to the
right.
Does this circuit correctly invert the logic sense of the
input signal?
When you have made this determination, turn off the
power to your experimental circuit and compare your results
with the discussion below.
Input
Output
S0
L0
0
?
1
?
Discussion
You should have found that this circuit does indeed invert the input signal,
just as the RTL inverter did. After all, it's the same transistor circuit, except that
we have modified the input circuit to remove the resistor in series with the
input signal. Thus, a logic 1 input produces a logic 0 output, while a logic 0
input produces a logic 1 output.
The main difference in behavior between RTL and DTL circuits is that with
RTL, an open input connection provides no drive to the transistor, so it is read
as a logic 0. This also makes the open RTL input susceptible to certain kinds of
noise signals, so that an open RTL input can introduce spurious signals under
some conditions. With DTL, the pull-up resistor causes the open input line to
act as a logic 1. In addition, any noise signal will have to pull the input down
nearly to ground in order to affect the transistor. As a result, the DTL circuit is
much less susceptible to noise than the RTL circuit. This is one of many
reasons why RTL circuits are not generally used any more.
When you have completed this experiment, make sure power is turned off
and continue on to the next experiment when you are ready. Do not remove any
components or jumpers from your breadboard socket. You will expand on this
circuit in the next experiment.
Two-Input DTL NOR Gate
Introduction
In the previous experiment, we found that we could combine a Diode Logic
AND gate with a DTL inverter, and thereby create a DTL NAND gate. This
leads us to the question, "Can we combine a DL OR gate with a DTL inverter
to get a NOR gate?"
We could do that, but we might run into a bit of trouble. Both the inverter
and the NAND gate use a pull-up resistor in the base circuit of the transistor,
and the preceding gate must actively pull the input down to logic 0 in order to
override that behavior. However, the DL OR gate would require the active
drive current for the transistor to come from the previous circuit. This led to
trouble with Diode Logic, since it changed all the references around. We would
prefer to keep the input circuit for DTL gates operating in the same way for all
gates.
We can accomplish this by using the same technique that we used with
RTL. That is, we can combine the outputs of multiple inverters to provide a
NOR function. We'll explore this idea in this experiment.
Schematic Diagram
The DTL NOR gate combines multiple DTL inverters with a common
output as shown in the schematic diagram to the right. This is exactly the same
as the method used to combine RTL inverters to form a NOR gate. Any number
of inverters may be combined in this fashion to allow the required number of
inputs to the NOR gate.
It would have been possible to construct the NOR gate by using a Diode
Logic OR gate followed by a transistor inverter, just as we used a DL AND
gate as the input to the DTL NAND gate. However, if we did that, the input
circuitry for the NOR gate would have very different characteristics than the
input circuitry for the NAND gate, and this might well lead to some serious
complications in the design of any circuit involving both types of gates. By
keeping the input circuitry and behavior of all gates the same, we ensure that
the input characteristics of all gates will be consistent and easy to work with.
Parts List
To construct and test the two-input DTL NOR gate circuit on your
breadboard, you will need the DTL NAND gate left over from the previous
experiment, plus the following experimental parts:
 (1) 0.3" black jumper.
 (1) 4.7K, ¼-watt resistor (yellow-violet-red).
 (2) 1N914 silicon signal diodes.
 (1) 2N4124 NPN silicon transistor.
 (1) 6" orange jumper.
Constructing the Circuit
You should still have the DTL three-input NAND gate circuit left over from
the previous experiment in place on your breadboard socket. If you removed
these components or if you did not perform the previous DTL circuit
experiments, go back and complete them now. Experiments for each logic
family build on previous circuits. You will be removing some components and
adding others to convert this circuit to a NOR gate.
Performing the Experiment
Turn on power to your experimental circuit, and move
switches S0 and S7 to logic 0 input. Record the output state
of this circuit, as indicated by L0, on the top row of the
table to the right.
Inputs
S7
S0
Output
L0
Continue by setting S0 and S7 to each of the four
possible combinations of two logic switches in turn, and
record your results in the table to the right. When you have
recorded the output state for all possible input
combinations, look over your results. Do you recognize this
logic function? Is it a NOR function?
When you have made this determination, turn off the
power to your experimental circuit and compare your
results with the discussion below.
0
0
?
0
1
?
1
0
?
1
1
?
Discussion
You should have found that when both inputs are at logic 0, the output was a
logic 1. However, when either input became a logic 1, that transistor turned on
and forced the output to logic 0. Setting both inputs to logic 1 did not change
this.
This is the normal behavior for a NOR gate, which is just an OR gate with
an inverted output. Thus, this circuit does indeed perform a NOR function, as
intended.
When you have completed this experiment, make sure power is turned off.
Leave your experimental circuit intact; you will use it in your next experiment.
Two, Two-Input DTL AOI Gate
Introduction
Because we haven't changed the inputs to our DTL inverters when forming a
NOR gate, it is possible for us to combine the DL AND function with the DTL
NOR function and obtain an AND-NOR combination. Since an AND-OR
sequence is very often in demand, this combination is useful in some
applications.
In addition, it is a very easy function to obtain; we need only add the input
diodes for the AND functions, just as we did for the three-input NAND gate in
a previous experiment.
In this experiment, you will expand your NOR gate to perform an ANDNOR function. This combination is also known commonly as an AND-ORInvert function.
Schematic Diagram
The DTL AND-OR-Invert gate simply adds input diodes to each transistor
in the basic NOR gate. Thus, we have DL AND gates connected to a DTL
NOR gate. This arrangement should work, in view of our success with the DTL
NAND gate. Here, we are simply combining AND and NOR functions in a
single gate.
Of course, this function is not always what is wanted in a given application.
However, where such a function is desirable, this combination serves the need
in a very compact and efficient form.
Parts List
To construct and test the DTL AND-OR-Invert gate on your breadboard,
you will need the DTL NOR gate from the previous experiment plus the
following experimental parts:
 (2) 1N914 silicon signal
 (1) 3" orange jumper.
 (1) 6" orange jumper.
diodes.
Constructing the Circuit
Select an area on your breadboard socket that is clear of other circuits.
You'll need two adjacent sets of five bus contacts for this project. Then refer to
the image and text below and install the parts as shown.
Performing the Experiment
Set switches S0, S1, S6, and S7 all to logic 0.
Then, turn on power to your experimental
circuit, and record the output as indicated by L0
in the first row of the table to the right. Continue
to set your logic switches to each possible
combination of four input signals, and record the
circuit output for each possible combination of
inputs. What is the logic function performed by
this circuit?
When you have entered your results for all 16
input combinations, turn off power and then
compare your results with the discussion below.
Inputs
Output
S7
S6
S1
S0
L0
0
0
0
0
?
0
0
0
1
?
0
0
1
0
?
0
0
1
1
?
0
1
0
0
?
0
1
0
1
?
0
1
1
0
?
0
1
1
1
?
1
0
0
0
?
1
0
0
1
?
1
0
1
0
?
1
0
1
1
?
1
1
0
0
?
1
1
0
1
?
1
1
1
0
?
1
1
1
1
?
Discussion
In performing this experiment, you should have found that switches S0 and
S1 operate as a pair, while S6 and S7 operate as another pair. If both switches
in either pair were at logic 1, L0 was forced to a logic 0 state. However, so long
as one switch in each pair was at logic 0, the output shown by L0 was a logic 1.
Thus, this circuit first performs a logical AND between the switches of each
pair, then performs a NOR operation between those two results. That is exactly
what we were looking for from this circuit.
This is the final experiment with DTL gates. Make sure power is off; then
remove all experimental components from the right hand side of your
breadboard socket and set them aside. You will need many of them for your
next set of experiments.