Buffer
A buffer copies its input to its output without inversion, but with stronger drive current and faster slew. The 74HC125 is a typical 4-channel three-state buffer, while the 74HC244 is a popular 8-bit octal driver for memory and bus expansion.
A non-inverting buffer is simply two CMOS inverters in series — the first restores the signal cleanly, the second sizes up the output transistors so they can drive heavy capacitive loads (long traces, multiple inputs, LEDs) without slowing the edge. Three-state variants add an output-enable that puts the final stage into high-Z, useful on shared busses.
In plain terms
A megaphone for a whisper: it doesn't change what's being said, it just makes the same word loud enough to carry across a stadium.
Why designers use it
- Drive a long PCB trace whose capacitance would slow the source IC's edges.
- Fan out one signal to many inputs without overloading the source's drive capability.
- Provide a three-state interface to a shared bus where multiple talkers must take turns.
- Translate a weak open-drain signal up to a full CMOS rail when paired with a pull-up resistor.
Best for
- Bus drivers
- Fan-out
- Trace driving
Key specifications
- Operating limits: Check the exact manufacturer's datasheet (A family name does not establish voltage, current, temperature or timing limits.)
- Pin assignment: Match the complete part and package code (A similar name or function does not guarantee the same wiring.)
- Mechanical fit: Use the exact package drawing (Check pad layout, dimensions and viewing direction before building.)
When not to use it
- When a level shift is needed — a buffer doesn't change voltage, so use a level translator if rails differ.
- For galvanic isolation — a buffer is purely electrical; reach for a digital isolator if you need barrier protection.
Common mistakes
- Leaving a three-state output enabled when another driver on the bus is also active, causing contention that overheats both devices.
- Driving a 50 Ω transmission line directly from a buffer with no series termination, generating reflections that ring on every edge.
Where you will find it
- An IBM PC/XT bus card uses a 74LS244 octal buffer to drive the 8-bit data bus from a peripheral chip onto the ISA backplane: the buffer's three-state outputs only enable when the card is selected, letting up to eight cards share the same 62 wires of the slot connector without bus contention.
- A Game Boy Advance link cable uses 74HC125 three-state buffers as a tiny multi-master bus: each handheld drives the line only when sending and tristates while listening, allowing up to four units to share two wires for multiplayer games — the same circuit principle as RS-485 but on a 5-pin connector.
- A logic-analyser probe pod uses 74AC541 buffers to capture a 100 MHz signal from a target board with negligible loading: the buffer's high input impedance means the analyser doesn't slow the target's edges, while its strong output drives the long ribbon cable to the instrument without distortion.
A short history
A logic buffer reproduces its input logic state without inversion. Its output stage can help a signal drive a load that the original source could not drive directly. It is not an unlimited power amplifier: the chosen buffer still has specified input thresholds, output-current limits and propagation delay.