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Thread: amplifier using hex inverter ??
face BY : Dan Michaels email (remove spam text)

At 07:30 PM 1/5/01 +0800, you wrote:
>have anybody heard anything about amplifier circuit using hex inverter
>chip and other discrete ? where can i find more info ? thanks in advance

As already mentioned, trying to use a digital gate as an analog
device by hooking up in a basic opamp feedback arrangement probably
doesn't give very good results. If all you are after is rail-to-rail
outputs, then better to just go with a CMOS opamp - LMC660 or
LMC6032 are cheap enough.

Alternatively, I have seen ckts [don't remember where] where
CMOS gates are placed "inside" the feedback loop at the output
node of a regular opamp. The gates can be either inverter or
noninverter - just change the +/- pin connections at the opamp
to compensate.

              _ bipolar
             | \opamp
       o-----|+ \
             |   >----CMOS---+---->
           +-|- /     gate   |
           | |_/             |
           |                 |

Why might you want to do this? To get rail-to-rail at output.
Also, you can simply parallel the 6 gates in the CMOS chip and
increase the output drive - probably to the 50-100 mA range. I
am not sure what the slew rate of this arrangement would be
however - but using newer 74ACxx chips might make it pretty fast.

There is some question - raised by piclisters in the past - about
current draw throught CMOS chips when running in the linear region,
but personally I don't think this is really a problem. They are
made to run at high speeds and drive capacitive loads, where both
channels are mostly open anyways. [also, prototyping & smoke-testing
are half the fun].

hope this helps,
- Dan Michaels
Oricom Technologies

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