AoE question

The first 'HC04 is biased with the 1Meg resistor to the "sweet spot" (threshold) so that it is sensitive to small input signals.

...Jim Thompson

-- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona Voice:(480)460-2350 | | | E-mail Address at Website Fax:(480)460-2142 | Brass Rat | |

| 1962 | I love to cook with wine. Sometimes I even put it in the food.

Hello Gary,

The first one is used in "semi-analog" fashion with it's input biased at threshold. So there was probably concern that the output signal wouldn't look quite so kosher and it can't hurt to further square it up.

BTW, if you take the Ref input signal away the HC04 will become a bit toasty because the first stage in the schematic will be sitting at threshold and thus will be drawing quite some quiescent current. It's not advised to leave them in that condition (meaning without ref input) when that HC04 is supplied near its max rated VCC.

--
Regards, Joerg

http://www.analogconsultants.com

I'd use an 'HCU04 myself ;-)

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

Hello Jim,

Yeah, most folks these days won't even know what unbuffered logic is. In this case the designers probably wanted all the gain they could get. Logic inverters of the buffered and unbuffered kind are probably still the lowest cost analog amps money can buy if you don't count "time shares" on an ASIC.

I remember when I was a kid and needed a frequency counter it took me days to figure out how to build a nice transistor amp. Wasn't allowed to the library because I had some kind of contagious flu. Then another few days, cuts and bruises to scavenge AF139 transistors out of old TV sets. Had I only known this trick of using inverters back then. But at least I learned something by building that transistor pre-amp.

--
Regards, Joerg

http://www.analogconsultants.com

Why? In both places?

'HCU04 is STABLE with DC feedback... use the remainder (as needed) of the package to square-up.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.