1
\$\begingroup\$

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.

\$\endgroup\$
  • 2
    \$\begingroup\$ Welcome to EE.SE! Please post a schematic. \$\endgroup\$ – winny Oct 2 '18 at 19:37
  • 1
    \$\begingroup\$ 1. What op-amp are you using? Is it rail-to-rail on both input and output? 2. What are the power supply connections? \$\endgroup\$ – The Photon Oct 2 '18 at 19:50
  • 4
    \$\begingroup\$ Also, does your "LM386" tag actually relate to your question? LM386 is not an op-amp. \$\endgroup\$ – The Photon Oct 2 '18 at 19:52
  • 1
    \$\begingroup\$ An LM386 is not an opamp. You can't wire it up like one and expect it to work properly. \$\endgroup\$ – brhans Oct 2 '18 at 22:47
2
\$\begingroup\$

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.

\$\endgroup\$
  • \$\begingroup\$ You were nice not to say so, but the OP is miss-using the LM386 in terms of its intended function and design. It cannot be treated as an op-amp. Coupling capacitors mean it is intended for a single ended power supply. +1 \$\endgroup\$ – Sparky256 Oct 2 '18 at 23:40

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.