# NTE941M Non-inverting OpAmp Has Non-zero Output with 0V Input

I have a non-inverting, single-supply, variable gain op-amp circuit configured in accordance with this diagram: • 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. • 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. – Barry Mar 21 '18 at 0:30
• @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. – user3578834 Mar 21 '18 at 1:04

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?

• I think so. I'm going to need a more precise amp that gets closer to the 0V rail. – user3578834 Mar 21 '18 at 17:03
• @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. – Andy aka Mar 21 '18 at 17:08