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Author Topic: HEF4001 NOR gate as an amp?  (Read 242 times)
2E0ILY
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Posts: 137




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« on: December 16, 2014, 09:07:22 AM »

I am building this circuit http://www.gatesgarth.com/amp.jpg

IC1a seems to be being used as an amp, yet it's a NOR gate. I have not come across this usage of a gate, is it unusual and what are the pros and cons of using one like this please? thanks.
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Best regards, Chris Wilson.
KD0REQ
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Posts: 1050




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« Reply #1 on: December 16, 2014, 02:54:21 PM »

see that 1M feedback resistor?  bias trim.

if you sort of let your eyes drift out of focus and look at an older datasheet that has the CMOS internal workings illustrated of this 4001 gate (like my old RCA or NatSemi databooks,) you may see it looks a darn sight like a FET-input op amp.  indeed, it sorta is.  you in fact have some extra steering FETs on the inputs that allow using both inputs as +VSS to 0 VDD logic, and it's good up to about 10 MHz on a really good day, 5 MHz or so worst case.  I first saw this in a mid-70s frequency counter input schematic, but apparently been in use for a while.

put a crystal in that loop, you have an oscillator, which is a much more common look at a 4001 gate to a ham.  in order to get anything to oscillate, there has to be some gain and positive feedback.  selected 455 KHz ceramic resonators above and below 455 switched into that loop, you have USB and LSB for a simple receiver.  it's decently stable.  I've been meaning to drop one into a Zenith TransOceanic for quite a while, but I keep getting called away...

the circuit has been around long enough to get into many, many little publications.
« Last Edit: December 16, 2014, 02:59:42 PM by KD0REQ » Logged
G3RZP
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Posts: 4957




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« Reply #2 on: December 17, 2014, 01:14:47 AM »

This was being done back in the days of RTL in the early 1960s. With two gates, you can arrange it as a Schmitt trigger, so a small AC input gives logic levels at the output. Add a capacitor and you can even get a monostable multivibrator.
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