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I have a non-inverting, single-supply, variable gain op-amp circuit configured in accordance with this diagram:

non-inverting opamp

  • Vcc: +16V
  • Rf: 800k Ohm Potentiometer
  • R2: 100k Ohm

I have the potentiometer set to ~200k Ohm for a gain of:

$$ A=(1+R_f/R_2)=(1+200k/100k)=3 $$

I have confirmed that this works experimentally (A=3.07). However, what puzzles me is that when \$V_{in}\$ is left floating I get an output of 6V instead of approx. 0V. Consequently, any input below approx. 2V yields no change in output.

Per the datasheet for this OpAmp, the maximum \$V_{os}\$ is 5mV, which even when amplified by 3, is nowhere near 6V.

Edit:

Below is a drawing of my circuit. amplifier circuit

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  • \$\begingroup\$ The op amp is only specified for dual voltage operation. How are you providing the bias voltage for the inputs with a single supply? With the input floating, there is no path for the op amp bias current. Thus the output cannot be determined. Note that a floating input is not the same as a 0 volt input. You need to provide a complete circuit so that we can better comment on its operation. \$\endgroup\$ – Barry Mar 21 '18 at 0:30
  • \$\begingroup\$ @Barry Thank you for the reply. I did not know that this opamp was dual supply only. Would a dual-supply allow the output to "rest" at 0V? For the bias current, I think I understand what you mean because the way that I tested the gain was by connecting points a and b together which would provide current to the non-inverting input. \$\endgroup\$ – user3578834 Mar 21 '18 at 1:04
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Extracts from the NTE941M data sheet: -

  • Common Mode Input Voltage Range is typically ±12 V on a ±15 V supply.

This means that if one supply of the op-amp is ground (0 volts) then you must have your input signal greater than typically 3 volts above ground or all bets are off.

  • Output Voltage Swing (for RL ≥ 10kΩ) is typically ±14 V on a ±15 V supply.

This means that you cannot expect the output to get any closer to the 0 volt rail than typically +1 volt above it.

any input below approx. 2V yields no change in output

Do you understand why now?

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  • \$\begingroup\$ I think so. I'm going to need a more precise amp that gets closer to the 0V rail. \$\endgroup\$ – user3578834 Mar 21 '18 at 17:03
  • \$\begingroup\$ @user3578834 you will find quite a few op-amps that can have their inputs work below the negative rail but you won't find any op-amps whose output goes below the negative rail and it usually falls short by a few tens of millivolts and even more when loaded. \$\endgroup\$ – Andy aka Mar 21 '18 at 17:08

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