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In the datasheet for the Nexperia HEF4543B, in the logic diagram, there are 2 inverters in series: enter image description here

What is the point of these inverters in series?


marked as duplicate by Greg, Elliot Alderson, Edgar Brown, Warren Hill, Finbarr Feb 7 at 12:11

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Those are just extra logic cells to buffer the inputs from the loading of so many internal gates. Two inverters in a row give you the same logic truth you put in, just with extra buffering. Very common with older CMOS series. They have very little drive current per inverter cell, much less than 1 mA, so series and parallel combinations were common with the CD4000 series.

A 'U' at the end of the part number meant it was unbuffered, so it had little drive current. Those with a 'B' suffix had buffered outputs, which often were just 2 inverters in series. Look up the CD4013B FF to see more examples of buffer gates.

  • \$\begingroup\$ Could you explain what you mean by "buffering" in this context? \$\endgroup\$ – eeze Feb 5 at 0:50
  • \$\begingroup\$ Buffer as in isolate the circuit driving that pin from the internal loading of the IC itself. Each one drives many internal gates, and you want the LE\ and PH pins isolated from that. If they are Schmitt trigger inputs, though that symbol is not there, they can really clean up inputs with mild noise issues. \$\endgroup\$ – Sparky256 Feb 5 at 0:54
  • \$\begingroup\$ To add to this in most cases the transistors making up the second inverter stage will be physically much larger than the first, such that they can source much more current and drive more logic gates \$\endgroup\$ – IC_Eng Feb 5 at 9:48
  • \$\begingroup\$ A buffer is a unity-gain amplifier whose input impedance is much higher than its output impedance. It presents a single "standard load" to the signal driving it, while being able to drive many such loads on its output with only a small penalty in terms of latency. In CMOS, the simplest amplifiers are inverting, so you use two of them in series to create a noninverting buffer. \$\endgroup\$ – Dave Tweed Feb 5 at 13:59

To take ill-conditioned input and make them nice and full-swing and sharp-edged.


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