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I have created a circuit for which I would like a rail to rail output of 0 - 15V, using an OPA548 with the driving waveform generated by a microcontroller DAC powered by 3.3V.

The datasheet for OPA548 is given here: OPA548 High-Voltage, High-Current Operational Amplifier

The circuit is shown below: enter image description here

The selected gain is about 4.5 V/V, such that the output of the op-amp saturates when the input waveform is at 3.3V.

Using an oscilloscope, when providing a 0-3.3V sine wave on the 'DAC_OUT' pin, the 'PULSE_OUT' pin will always saturate to 0V or 15V (similar to a square wave), and never follows the curve of the sine wave. Originally, I believed that this was an input bias current issue on the op-amp input pin, for which I added a 2.2KOhm resistor in series between the 'DAC_OUT' signal and op-amp inverting input, however this did not fix the issue.

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    \$\begingroup\$ If you are trying to make a non-inverting amplifier, you have the inputs switched around. The OPA548 is also not a rail-to-rail output amplifier. \$\endgroup\$
    – vir
    Commented Aug 12, 2022 at 16:58
  • \$\begingroup\$ Yes, Ideally I would like a non-inverting amplifier, but the current version has the inputs switched, which I plan to fix in the next version of my board. Would the switched inputs cause the op-amp to only work output saturation and not in-between? Also, your comment is about the OPA548 not being rail-to-rail is noted, and I do now realize that the output saturates to 13.4 V and not 15V, which might be okay for my application, as I would like to use the DAC to create outputs near the middle of the range, and can increase the supply voltage at will. \$\endgroup\$
    – md-raz
    Commented Aug 12, 2022 at 17:03
  • \$\begingroup\$ You also need to connect something to the current limit pin. Else it defaults to zero current. \$\endgroup\$
    – Mattman944
    Commented Aug 12, 2022 at 17:03
  • \$\begingroup\$ I did connect the current limiting pin (3) to V- (GND) as the datasheet specifies "connecting ILIM directly to V– programs the maximum output current limit, typically 5 A." \$\endgroup\$
    – md-raz
    Commented Aug 12, 2022 at 17:05
  • \$\begingroup\$ @vir, thank you for your quick reply, although I was skeptical that the inversion of the inputs was the reason the op-amp kept saturating, switching the input pins has fixed the issue for me. The op-amp output now follows the input multiplied by the gain. \$\endgroup\$
    – md-raz
    Commented Aug 12, 2022 at 17:33

1 Answer 1

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This sounds like a show-stopper: -

enter image description here

Also, from the data sheet read this: -

enter image description here

Typically, if your power rail is 15 volt and ground, you should be able to produce about 12 volts p-p on the output.

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    \$\begingroup\$ As I said in an above comment, I was initially skeptical that the inversion of the inputs was the reason the op-amp kept saturating, switching the input pins has fixed the issue for me. The op-amp output now follows the input multiplied by the gain. Thanks for the quick reply. \$\endgroup\$
    – md-raz
    Commented Aug 12, 2022 at 17:34
  • \$\begingroup\$ @md-raz The inversion of the inputs will always do that, because the negative feedback the op-amp needs to provide controlled gain is turned into positive feedback that just pushes the output towards either rail. \$\endgroup\$ Commented Aug 12, 2022 at 23:14

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