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TTL Inverter

Introduction

Looking at the DTL input circuit, we note that the two diodes are opposed to each other in direction. That is, their P-type anodes are connected together and to the pull-up resistor, while one cathode is the signal input and the other is connected to the transistor's base. This gives rise to a bit of speculation: could we perhaps replace these two diodes with a single NPN transistor?

If it works, we can also make use of the fact that the amount of space required by a transistor in an IC is essentially the same as the space required by a diode. Thus, we can make the IC smaller by eliminating the space required by one of the two diodes.

In this experiment, we'll construct this transistor-transistor logic circuit and test its operation.



Schematic diagram of a TTL inverter.

Schematic Diagram

The figure to the right shows the proposed change to convert a DTL inverter to a transistor-transistor (TTL) equivalent. The resistor values are unchanged; we have merely used one transistor to replace two diodes.



Parts List

To construct and test the TTL inverter circuit on your breadboard, you will need the following experimental parts:



Constructing the Circuit

Select an area on your breadboard socket that is clear of other circuits. Then refer to the image and text below and install the parts as shown.



Circuit Assembly

Start assembly procedure








Beginning the Assembly

Make sure the right hand side of your breadboard socket is clear of all experimental components. You'll be re-using a number of components here, but with different placement and interconnections.

Click on the `Start' button below to begin the assembly of your experimental circuit.

0.3" Black Jumper

You should have several 0.3" black jumpers left over from previous experiments. If not, prepare a new 0.3" black jumper. Install this jumper in the location shown in the assembly diagram to the right.

Click on the image of the jumper you just installed to continue on to the next assembly step.

0.2" Bare Jumper

It's not generally worthwhile to fuss with insulation for a jumper that's less than 0.3" long. Therefore, simply form a length of bare hookup wire into a jumper with 0.2" spacing. You can use a clipped component lead for this purpose if you have any available. Install this jumper on your breadboard socket as shown to the right.

Again, click on the image of the jumper you just installed to continue.

1K, ¼-Watt Resistor

You should have a 1K, ¼-watt resistor (brown-black-red) left over from previous experiments. If not, locate a new 1K, ¼-watt resistor and form its leads to a spacing of 0.5". Install this resistor in the location indicated in the assembly diagram.

As usual, click on the image of the resistor you just installed to continue on to the next assembly step.

4.7K, ¼-Watt Resistor

You should have a 4.7K, ¼-watt resistor (yellow-violet-red) left over from your DTL experiments. If not, locate a new 4.7K, ¼-watt resistor and form its leads to a spacing of 0.5". Install this resistor as indicated in the assembly diagram.

Again, click on the image of the component you just installed to continue.

2N4124 NPN Transistor

You should have several 2N4124 (or 2N3904 or similar) NPN silicon transistors left over from previous experiments. If not, locate one now and form its leads to a spacing of 0.1" so it will fit readily on your breadboard socket. Install this transistor in the location indicated in the assembly diagram. Be sure to observe the required orientation of the transistor.

Click on the image of the transistor you just installed to continue on to the next assembly step.

2N4124 NPN transistor

Locate a second 2N4124 transistor and, if necessary, form its leads as before. Then, install it in the location indicated to the right. As before, observe the required orientation.

Again, click on the image of the transistor you just installed to go on.

3" Orange Jumper

Locate one of the 3" orange jumpers you have used in most of your experiments. If you don't have one, cut a 3" length of orange hookup wire and remove ¼" of insulation from each end. Connect one end of this jumper to S0, and the other end to the point shown in the assembly diagram.

Click on the image of the jumper you just installed to continue on to the next assembly step.

10" White Jumper

Locate the 10" white jumper you have been using to make connections to L0. If you don't have it, cut a 10" length of white hookup wire and remove ¼" of insulation from each end. Connect one end of this jumper to L0, and the other end to the point shown in the assembly diagram.

As usual, click on the image of the jumper you just installed to continue.

Assembly Complete

This completes the construction of your experimental circuit. Check your assembly carefully against the figure to the right, and correct any errors you might find. Then, proceed with the experiment on the next part of this page.

Restart assembly procedure


Performing the Experiment

Turn on power to your experimental circuit, and set S0 to produce a logic 0. Note the resulting state of your experimental circuit as indicated by L0. Record this value on the appropriate line in the table to the right.

Now move S0 to the logic 1 position and record that result in the table. Turn off the power to your experimental circuit when you have finished recording its behavior.

Look over your results. What function does this circuit perform? Compare your results with the discussion below.

Input Output
S0 L0
0
1


Discussion

You should have found that this circuit performed a logic inversion, or NOT function. With a logic 0 input, it produced a logic 1 output. With a logic 1 input, it produced a logic 0 output. Thus, this circuit is indeed an inverter, as expected.

When you have completed this experiment, make sure power to your experimental circuit is turned off. Leave your experimental circuit in place; you will extend it for the next experiment.


Prev: Diode-Transistor Logic Next: Two-Input TTL NOR Gate

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