I've just started learning about op-amps and something confused me in one of my lectures (apologies for the low quality, it's a screenshot):

enter image description here

The output voltage is going into a rotational electro-mechanical converter, acting as a motor.

Why is the voltage amplified constantly? Since one side of the AC voltage source is connected to the negative side of the rotational converter, surely the op-amp wouldn't be amplifying the voltage?

Sorry for the noob question, I might just be misunderstanding how an AC source works.

Thanks in advance :)

  • 1
    \$\begingroup\$ The ground is your reference point. Remember in highschool trigonometry that a sine wave goes positive and negative. So too with AC. \$\endgroup\$
    – Kartman
    Nov 15 at 4:38
  • \$\begingroup\$ "one side of the AC voltage source" and "the negative side of the rotational converter" are not connected in the conventional sense. Both are connected to GND. This means that (theoretically) there is no interaction between whatever signals are at those points in the circuit. GND is 0 Vac and 0 Vdc, always. Current can flow into and out of GND, but GND id the reference potential against which all other circuit voltages exist. This in effect isolates the return legs of the two circuit elements. \$\endgroup\$
    – AnalogKid
    Nov 15 at 4:43
  • \$\begingroup\$ Your question needs more detail. What is the sin signal on the left? Where is that point in the circuit? What is the circuit supposed to do? What system is it a part of? \$\endgroup\$
    – AnalogKid
    Nov 15 at 4:44
  • \$\begingroup\$ @AnalogKid The circuit is just supposed to rotate a motor-like component. The sin signal is just a measure of the motion (if I remember correctly it was rpm vs time?). It's not part of a bigger system, it was just an example in class that I don't really understand. Regarding your answer, surely the ground section still conducts just as well so why wouldnt the ac current go through to the motor in that direction? Also if it wasnt connected in the conventional sense, how would that side of the ac voltage source go through to the op amp and not the rotational converter? \$\endgroup\$ Nov 15 at 5:03
  • \$\begingroup\$ The power source to the opamp is invisible in the simulation. The signal source does not supply current to the load, so there are two loops of current. The opamp/motor current loop does not intersect the signal source loop. \$\endgroup\$
    – Kartman
    Nov 15 at 6:58

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