Electronic – Gain of Non-Inverting Amplifier Does Not vary with Resistor Values

amplifiergainnon-invertingoperational-amplifierpotentiometer

I am trying to build a non-inverting amplifier using a LM7171 op amp, and I am implementing it with potentiometers as Rf and R2 so that I can smoothly control the gain of the amplifier (and easily check if its working correctly). I am applying +/- 15 V to the op amp. When I adjust the potentiometer's resistance the gain does not change as expected, or noticeably at all. Setup is shown below with traces when the resistances are Rf=Ri=540 Ohm and Rf=1.8 kOhm, Ri=540 Ohm, and as can be seen the gain/amplitude is the same for the different resistances used. The yellow output on the oscilloscope is the output of the amplifier, and the green is directly from a t-connector to the function generator's output.

I have used two different op amps and got the same behavior, so it is not broken, and all connections seem good. I measure the values of the potentiometer so am confident they are as described. Any ideas what might cause this? Thank you in advance for your help.

enter image description here

enter image description here

enter image description here

enter image description here

enter image description here

100 kHz sinusoids.
enter image description here

enter image description here

Setup as voltage follower

enter image description here

enter image description here

enter image description here

Best Answer

The phase shift is the give-away. You are bandwidth limited. You need a faster op-amp, or a lower frequency.