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I am using a non inverting amplifier (shown below). For the opamp, I am using TL084, and I am using 12V single supply. I am getting an output of 11.20Vdc even though the input is 1.06V sine wave. I don't know what the issue is; any help.

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  • \$\begingroup\$ hey, I understand that I need to bias. Does that mean that I need to bias on the input of the non-inverting amplifier or bias the gnd of the negatvie pin input? \$\endgroup\$
    – Sam
    Jan 29, 2022 at 1:04

2 Answers 2

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What is the input DC offset? The TL084 requires the signal to be V- +4 to V+ -4. So in your case, between 4 and 8 volts with a 12V supply. If your signal isn’t in that range the TL084 won’t follow it.

If you need JFET input with close-to-the rails capability and want single supply, the AD822 or similar would be better. Datasheet: https://www.analog.com/media/en/technical-documentation/data-sheets/AD822.pdf

If you’re looking for general, not-fussy amplification for single supply, the LM324 or its later improved cousins could work.

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  • \$\begingroup\$ The Input DC offset is set to zero. The TL084 takes in wide range of voltage supply including single supply of 4.5V to 40V. I chose TL084 because it has high slew rate, low THD but I don't need to use a JFET opamp. From reading your post, what I understand is that I need to add a DC offset to the input signal. \$\endgroup\$
    – Sam
    Jan 28, 2022 at 18:34
  • \$\begingroup\$ Would a general opamp such as LM324 need a DC offset with a single supply? \$\endgroup\$
    – Sam
    Jan 29, 2022 at 1:03
  • \$\begingroup\$ Yes, any op-amp will. \$\endgroup\$
    – hacktastical
    Jan 29, 2022 at 2:24
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For a 12V single supply the DC operational point should be app. at Vout=6V. In this case, the output can swing relatively symmetric around this point. Hence, it is necessary to bias the device properly with +3V at the non-inv. input (due to the DC gain of 2).

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  • \$\begingroup\$ I don't understand. Can u please give me slight more explanation. \$\endgroup\$
    – Sam
    Jan 28, 2022 at 18:36
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    \$\begingroup\$ Before you can amplify a sinusoidal signal you must give the amplifier (this applies to AA kinds of amplifiers) a suitable DC operational point in the midddle between the max and the min. output voltage: In your case in the middle between 0 and 12V. For this purpose a DC voltage of 3volts is applied to the non-inv. input which will be amplified with (1+R2/R1)=2. Hence, the DC output now is at +6V. Around this value the output can swing - thus representing the amplified input signal which, however, must be coupled via a capacitor. \$\endgroup\$
    – LvW
    Jan 28, 2022 at 20:49
  • \$\begingroup\$ Coupled via a capacitor on the output of the opamp? I guess how do I decide on what type of opamp to buy for something like my situation where I just want to amplify but don't want to add DC offset? \$\endgroup\$
    – Sam
    Jan 28, 2022 at 22:01
  • \$\begingroup\$ I repeat: The INPUT signal has to be coupled using a capacitor (because the input node carries a DC voltage 0f 3 volts). \$\endgroup\$
    – LvW
    Jan 29, 2022 at 9:41

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