NTE941M non-inverting opamp has non-zero output with 0V input

Question

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

_{(source: electronics-tutorials.ws)}

• Vcc: +16V
• \$R_f = 800\text{ k}\Omega\$ Potentiometer
• \$R_2 = 100\text{ k}\Omega\$

I have the potentiometer set to \$\approx 200\text{ k}\Omega\$ 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_{\text{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 op amp, the maximum \$V_{\text{os}}\$ is 5mV, which even when amplified by 3, is nowhere near 6V.

Below is a drawing of my circuit:

Answer 1

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?