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I built a simple non inverting op amp amplifier with 2 resistors and a TS972IN powered between 0-5V but my output was always set to 5V. I tried several others Op Amp, others resitors and I also moved my circuit elsewhere (I'm working with a breadboard) and none of this changed anything. In order to solve my issue, I decided to build something even simpler, a voltage follower but I have the same behavior, the output stays to 5V...

My circuit with the pinout I use:

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ that's an OPamp that's explicitly deprecated by its manufacturer; while certainly fine for learning and prototyping, be aware that it might not be possible to get any quantity of it in the future. \$\endgroup\$ – Marcus Müller Dec 31 '19 at 15:55
  • \$\begingroup\$ According to the datasheet pl.mouser.com/datasheet/2/389/ts971-957361.pdf, The allowed input voltage range lies between 1.35V...3.65V. \$\endgroup\$ – G36 Dec 31 '19 at 15:59
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Data sheet, page 5:

Vicm Common mode input voltage range VDD + 1.15 to VCC - 1.15 V

You're trying to make it work VDD + 0 to VCC - 0 V; that won't do. This is just what is known as Latch-up behaviour.

You have a rail-to-rail output opamp. What you'd need to have is a rail-to-rail input and output (RRIO) opamp for this to work.

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  • \$\begingroup\$ Many thanks I thought I was going insane, I'll pay attention to this for future circuits ! You mentionned this part is deprecated ,can you advise me an RRIO opamp to play with ? (I'm trying to amplify a 162KHz signal received by a tuned ferite rod) \$\endgroup\$ – Robert Dubois Dec 31 '19 at 16:18
  • \$\begingroup\$ why would you need the full 0 to 5V input range for that? Just bias your input to say 2.5 V? \$\endgroup\$ – Marcus Müller Dec 31 '19 at 16:25
  • \$\begingroup\$ Ok I see I can keep the TS972 for experimenting by working around VCC/2, to respect VDD + 1.15 to VCC - 1.15 V range. \$\endgroup\$ – Robert Dubois Dec 31 '19 at 16:27
  • \$\begingroup\$ exactly! Just use a rather high-ohmic voltage dividers (maybe 2× 1 MΩ) to put your antenna-originating signal up to 2.5 V (voltage divider: VCC-1MΩ-(x)-1MΩ-GND, couple in the antenna via a capacitor to (x), connect your non-inverting opamp input to (x). \$\endgroup\$ – Marcus Müller Dec 31 '19 at 16:33

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