I am trying to amplify a low voltage AC input with a gain close to 5. It has a 0 DC offset and about 300 mV of AC amplitude, at 250 kHz. This is my first venture into electrical engineering and I’m having a good time, but alas, my first circuit does not work.

After doing some research on op-amps, I bought a NJM072D because it has enough gain bandwidth product & slew rate. But for some reason, after placing it in a non-inverting configuration with a 0-5 V supply, I’m getting a constant 3.4 V at the output.

I saw this post, so I decided to put a capacitor & resistor at the input, but the problem still persists.

So then I went back to you guys and found this post. And I think I’m having a similar problem?

The common mode voltage is +/- 10V on the data sheet and it was supplied +/-15V, so am I correct in saying that this op-amp will only accept inputs within 5 V of either side of the power supply? Why does this requirement exist? I have a 0-30 V power supply, so can I simply use that to power then op-amp and then offset the input to be above +5V and below +25V?

Here is my schematic: enter image description here

  • \$\begingroup\$ You Don't need capacitor. Besides what's the input voltage rage?? Like 0-2 volt, or -1 to +1 \$\endgroup\$ – Sadat Rafi Mar 30 at 17:08

The minimum operating voltage for the op-amp you chose is 8 volts. You are running on 5 volt. Looking at other parameters in the data sheet that device is totally unsuited for what you want and even on a supply as low as 8 volts you would have no guaranteed input common mode voltage range.

Looking at your schematic just in case, and I see you have the +Vin pin biased at 0 volts - at the very least, for amplification of an AC signal this would need to be biased midrail. Choose a better op-amp suited for low voltage supplies and bias +Vin correctly.

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  • \$\begingroup\$ Thanks for the advice. Can you tell me how you came up with 8 V as the minimum operating voltage? I’m not seeing how you got that from the data sheet. \$\endgroup\$ – Claxton May 25 '18 at 21:36
  • \$\begingroup\$ From the first page - "Features: supply voltage +/- 4V to +/- 18V." \$\endgroup\$ – WhatRoughBeast May 26 '18 at 0:13
  • \$\begingroup\$ I see. So it needs at least an 8 V voltage difference to operate. \$\endgroup\$ – Claxton May 26 '18 at 12:42

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