Why op amp behave like that?

Question

This is a non-inverting first order low pass filter.

The cut of frequency= 1591.5 Hz

The input frequency = 264.2 Hz

The input voltage = 1.5 Vp-p Sinewave

^{simulate this circuit – Schematic created using CircuitLab}

According to this equation:

Af = 1+ R2 / R1

Af = 2

voltage gain Av at 264.2 Hz and 1.5 volt is 1.97 volts.

Output voltage = 1.5 * 1.97 = 2.95 volts.

I tried to measure thes values: The output voltage is only 1.431 volts!! and a distortion happened to the wave form. Why does that happen?

Thank you very much,

Answer 1

The bottom distortion is trivial to explain - you appear to be running the op-amp between 8+V and ground, you have not created a virtual ground at +4V, and so the signal is naturally clipping where the op-amp has no power to drive it.

For a simple approach, add a second battery to provide a -8V rail, if using 8V batteries, and power the negative power pin of the op-amp from that. Ground stays where it is, the + of the new battery connects to it, and the - of the new battery connects to the op-amp's negative power pin (not the inverting input - the one you currently show going to ground.)

The op amp is probably also feeling somewhat abused by having the inputs driven below the negative power rail in your current circuit - most (nearly all) op-amps are not fond of having an input that exceeds the power rails.

Answer 2

Your input voltage is an AC waveform which oscillates above and below the zero volt / ground line. It should peak at about +0.75 V and -0.75 V.

Your op-amp has its negative terminal connected to ground. The lowest voltage it can possibly output is 0 V and most will only get within a volt or two of that.

The fix

Power your op-amp from a dual supply, +8 V and -8 V or similar.

^{simulate this circuit – Schematic created using CircuitLab}

Figure 1. Dual rail supply.