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.


Below is a drawing of my circuit. amplifier circuit

  • \$\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

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?

  • \$\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

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.