Using an OpAmp as a final buffer stage for TTL or similar circuits
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.
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
- LM324 Datasheet
- 2N3904 datasheet - the NPN used in this circuit
- Resistor-Transistor Logic AND Gate - the basic AND gate project without buffer