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I generate a sine wave (100-500 Hz) and amplify it using an LM741 op-amp with a gain of 3. When I measure the output everything is fine; I see a pure sine wave of the same frequency as that of the input and amplified by the mentioned gain.

However, as soon as I connect the haptic vibration motor, the signal turns into something strange. I have attached the photos.

When I change the input frequency, I feel that the frequency of the output vibrations is also changing. Unfortunately, these are just my finger sensing and not the actual data on the oscilloscope.

Can you please explain why this is happening? Is it possible to visualize the vibrations from the motor on the oscilloscope and see a sine wave as in the output without the motor?

Op-amp circuit: OpAmp Circuit

Op-amp output, the motor is NOT connected: enter image description here

The motor is connected to the output of the op-amp: enter image description here

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    \$\begingroup\$ Your schematic shows feedback connected to the non-inverting input, rather than to the inverting input. Does that represent your actual circuit? Or is the schematic incorrect? \$\endgroup\$ Commented Mar 18, 2023 at 10:39
  • \$\begingroup\$ yes, I am indeed using a non-inverting configuration \$\endgroup\$
    – mangozu
    Commented Mar 18, 2023 at 10:45
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    \$\begingroup\$ The motor data sheet recommends driving the motor with an audio amplifier. Consider using something like a LM386 and connect it as for driving a loudspeaker. \$\endgroup\$
    – user131342
    Commented Mar 18, 2023 at 10:51
  • \$\begingroup\$ For either a non-inverting amplifier, or an inverting amplifier, the feedback is connected to the inverting input. That is, you should have negative feedback. The circuit in your schematic shows positive feedback, and will not work if intended as a linear amplifier. You should not expect a sine wave output from that circuit. \$\endgroup\$ Commented Mar 18, 2023 at 11:22

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as soon as I connect the haptic vibration motor, the signal turns into something strange.

Connecting the motor to virtually any op-amp (let alone the trashy 741) is like trying to run a real automobile on a slot-car track. The op-amp will be totally over-powered by the loading effect of the motor.

Can you please explain why this is happening?

The motor is specified as having an impedance of 5.5 Ω but op-amps are more suitable to loads that are hundreds or thousands of times less onerous (higher in impedance).

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  • \$\begingroup\$ Oh.. thank you! So, would you advise using a different, more powerful amplifier? What do you think about MAX9744? Could you please check it? I would appreciate that very much! \$\endgroup\$
    – mangozu
    Commented Mar 18, 2023 at 10:48
  • \$\begingroup\$ Yes, use a more powerful amplifier. You can get power op-amps of course but, I can't remember part numbers and neither can I assess the device you mentioned @BroBro because that is steering too far away from your original question and this site isn't a forum. \$\endgroup\$
    – Andy aka
    Commented Mar 18, 2023 at 10:50
  • \$\begingroup\$ @Audioguru that is irrelevant; the load is 5.5 ohms and that is beyond the capability of 99% of op-amps. \$\endgroup\$
    – Andy aka
    Commented Mar 18, 2023 at 15:00
  • \$\begingroup\$ Andy, he asked if he could use a 20W power amplifier which would blow up the tiny vibrator motor. \$\endgroup\$
    – Audioguru
    Commented Mar 18, 2023 at 15:42
  • \$\begingroup\$ @Audioguru then properly mark your comment like this @BroBro and be clear about what you mean to avoid ambiguity. \$\endgroup\$
    – Andy aka
    Commented Mar 18, 2023 at 16:41

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