Non-inverting op amp problem

Question

I have problem with an lm386 op amp in an non-inverting configuration. For the resistors, I take two equal resistors so i set a gain of 2. When i put a signal of 3.3 volts on the non-inverting pin, i should get 6.6 volts on the output on the op amp, but i get totaly different measuring. I have also measured the voltage on the inverting and non-inverting pins and i do not get the same voltage but i should. Also when no voltage is connected on the non-inverting pin (when its grounded) i am not getting 0 volts on the output, but i am getting zero point something volts. I have also tried to change the ic or the resistors but it didnt worked. So now i wanted to ask am i makeing a mistake or the mistake is in the ic. Every answer is welcome. Thanks a lot.

Answer 1

Whenever you see a big difference between inverting and non-inverting terminals, one of three things may be happening:

- the open-loop gain is degenerate, so the error voltage cannot be reduced further
- the output is saturated, limited by the supply rails
- the measurement equipment was affecting the circuit

LM386 is an audio amplifier, not an op amp, even though it has some superficial similarity.

According to LM386 datasheet, the open-loop gain Av is only 26dB i.e. 20V/V. This is far from the ideal opamp where Av is considered near infinite.

Consequence of this reduced open-loop gain is that there will be significant difference between the inverting and non-inverting inputs, as you observed.