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I'm in the process of importing a bunch of old schematics from a 70's era synthesizer into KiCAD. In the process I've come across a symbol that I can't make sense of. It looks like this:

Strange Comparator Symbol

Does anyone have an idea of what part this is? The only other IC used in the schematic is an LM741. Could this just be another LM741? Why does it have FET written inside it?

Edit: The schematics are dated December, 1975. So the parts would have to be older than that.

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  • \$\begingroup\$ Oh, one more thing from closer review of the incomplete board layout files that I have. It might be a 6 pin DIP package \$\endgroup\$ Jun 28 '20 at 20:24
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    \$\begingroup\$ CA3140 probably, but TL071 would be an acceptable substitute, more robust (use static sensitive procedures with the 3140) and easier to find. \$\endgroup\$ Jun 28 '20 at 20:25
  • \$\begingroup\$ More, the .15uF capacitor stated as Mylar definitely calls for a very high input impedance opamp and possibly some guard ring on the PCB layout \$\endgroup\$
    – carloc
    Jun 28 '20 at 20:35
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Given that it's from the 1970s it's more likely to be a TL071 series FET input op-amp. These have much higher input impedance and lower bias currents than the alternatives (such as the LM741) at the time.

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  • \$\begingroup\$ Maybe I should've put a specific date - the schematics say they were drawn up in December, 1975. Looks like that part came out in 1978, at least by the dating on the datasheet. \$\endgroup\$ Jun 28 '20 at 20:17
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I believe the UA740 was a FET_input diff_pair opamp.

Some IC companies sell 15_volt_rail CMOS opamps, just for such purposes; note this circuit is a sample_hold, so low droop is the need for the FET input.

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After some further investigation, I found an image of the board with "CA3140" printed on the side of one of the packages. @Brian Drummond was right on target in his comment. Problem solved!

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