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Project Notes

#113 TTLBufferDriver

Using an OpAmp as a final buffer stage for TTL or similar circuits

Notes

TTL devices and other logic circuits often need to chain to other circuits. A buffer is generally needed to ensure there is a good impedence match to the following circuit, and also to ensure a consistent signal is passed on.

OpAmp buffers can be used to provide unity gain and isolate the input and output sides.

This circuit uses an LM324 configured as a comparator in order to “clean up” the output of an imperfect Resistor-Transistor Logic AND Gate.

With a 5V power supply, the basic AND gate drives a 3.8V output when both transistors are on, and 0V when both are off. But when the lower transistor is on, there’s gate-emitter leakage resulting in a semi-on state of about 1.9V.

The voltage divider Rd1/Rd2 set the 0/1 threshold for the OpAmp comparator at half-way (2.5V), so the resulting output is a clean ON or OFF - nothing in between.

Here’s a sample trace. The lower two traces are the inputs to the AND gate. The top trace is the output of the OpAmp, and the lower-top trace is the output of the AND gate.

processing trace

The Arduino is not really a core part of the circuit. It only does the following:

  • provides +5V power supply (for convenience)
  • automates the toggling of the driving inputs
  • measures the input and output voltages for plotting

Construction

Breadboard

The Schematic

The Build

Credits and References

About LEAP#113 OpAmpTTL
Project Source on GitHub Return to the LEAP Catalog

This page is a web-friendly rendering of my project notes shared in the LEAP GitHub repository.

LEAP is my personal collection of electronics projects, usually involving an Arduino or other microprocessor in one way or another. Some are full-blown projects, while many are trivial breadboard experiments, intended to learn and explore something interesting (IMHO!).

The projects are usually inspired by things found wild on the net, or ideas from the sources such as:

Feel free to borrow liberally, and if you spot any issues do let me know. See the individual projects for credits where due. There are even now a few projects contributed by others - send your own over in a pull request if you would also like to add to this collection.