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RTL Inverter


Resistor-Transistor Logic (RTL) is a large step beyond Diode Logic (DL). Basically, RTL replaces the diode switch with a transistor switch. If a +5v signal (logic 1) is applied to the base of the transistor (through an appropriate resistor to limit base-emitter forward voltage and current), the transistor turns fully on and grounds the output signal. If the input is grounded (logic 0), the transistor is off and the output signal is allowed to rise to +5 volts. In this way, the transistor does invert the logic sense of the signal, but it also ensures that the output voltage will always be a valid logic level under all circumstances.

Because of this, RTL circuits can be cascaded indefinitely, where DL circuits cannot be cascaded reliably at all.

RTL Inverter

Schematic Diagram

The basic RTL inverter is actually very similar to the LED driver circuit we examined earlier. The primary difference is that the LED driver includes an LED in series with the transistor collector lead. Resistor values are also adjusted to accommodate the different purpose of the LED driver circuit.

Some years ago, when RTL ICs were the standard logic devices used in both commercial and experimental digital circuits, transistors typically had a forward current gain of about 30. With improved manufacturing techniques, modern transistors show current gains of 100 or more. There is also far less variation between transistors of a given type. As a result, we can tolerate a much lower input current to drive the transistor reliably into saturation. The resistor values in the schematic diagram to the right reflect the capabilities of modern transistors; they are significantly higher than the values used in RTL ICs, allowing working circuits to be built that require far less operating current.

Parts List

To construct and test the RTL inverter circuit on your breadboard, you will need the experimental parts listed below.

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. Use the following images as a guide to forming short jumpers and components leads to fit readily on your breadboard socket.

Circuit Assembly

Start assembly procedure

Starting the Assembly

We'll build our experimental circuit just to the right of the center divider on our breadboard socket. If you still have any components in this area left over from prior experiments, remove them now. Of course, leave everything on the left side of the breadboard socket; you'll need these circuits to demonstrate your experimental circuit.

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

0.3" Black Jumper

Prepare a black 0.3" jumper wire (or re-use a black 0.3" jumper left over from a previous experiment) as shown. Install this jumper as shown in the assembly diagram to the right.

Click on the image of the jumper to move on to the next step.

1K, ¼-Watt Resistor

Locate a 1K, ¼-watt resistor (brown-black-red) and form its leads to 0.5" spacing. Install this resistor on your breadboard socket as shown to the right.

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

15K, ¼-Watt Resistor

Locate a 15K, ¼-watt resistor (brown-green-orange) and form its leads to a spacing of 0.5". Install it on your breadboard as shown to the right.

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

2N4124 NPN Transistor

Locate a 2N4124 NPN transistor or equivalent, and form the leads to permit easy installation on your breadboard socket. Install the transistor as shown to the right, so that the emitter will be grounded through the black jumper and the collector connected to +5 volts through the 1K resistor you installed previously.

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

3" Orange Jumper

Take one of the 3" orange jumper wires you created for earlier experiments and use it to connect the free end of the 15K resistor to logic switch S0, as shown to the right. If you did not perform any of the previous experiments, cut a 3" length of orange hookup wire and remove ¼" of insulation from each end to create the jumper.

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

10" White Jumper

Take the 10" white jumper wire you created during a previous experiment (or cut a 10" length of white hookup wire and remove ¼" of insulation from each end) and use this jumper to connect the transistor collector (and 1K resistor) to L0, the rightmost LED driver circuit.

As before, click on the image of the component 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 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.

Measure the output voltage of this circuit for both output states. How much does the output voltage change if L0 is disconnected from the circuit? Will this circuit maintain a legitimate logic 1 output when a reasonable number of other gate inputs are connected to its output?

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 Voltage
With L0
Without L0
0 V V
1 V V


You should have found that when the input was a logic 0, L0 turned on indicating a logic 1 output from this circuit. When you switched the input to logic 1, L0 turned off to indicate a logic 0 output. Thus, this circuit did indeed function as a logic inverter, reversing the logic sense of whatever signal was applied to its input.

The output voltage of our experimental circuit was 4.70 volts for a logic 1 and 0.03 volt for a logic 0, with L0 connected. Disconnecting L0 had no significant effect on the logic 0 output, but allowed the logic 1 output to rise to 4.88 volts. This shows that we cannot drive an infinite number of gates from this output, but we can drive a reasonable number of inputs without overloading this output circuit.

When you have completed this experiment, leave the experimental components in place on your breadboard socket. You will expand this circuit to prepare for the next experiment.

Prev: Diode Logic Next: 4-Input RTL NOR Gate

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