0
\$\begingroup\$

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

\$\endgroup\$
  • \$\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
0
\$\begingroup\$

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.

|improve this answer|||||
\$\endgroup\$
  • \$\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

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.