what is the functionality of following circuit? It's a LM324 opamp and a sin or ramp signal get into it's inverting input and non-inverting input grounded. It's power is 12 and -12 and output of op amp with a 10k resistor connected to transistor (2n6660) ?


  • \$\begingroup\$ You might want to show a picture of the actual circuit or use the schematic entry tool to draw your own. Personally I am too lazy to imagine the circuit from a text description. \$\endgroup\$ May 4 '18 at 14:25
  • \$\begingroup\$ I add circuit pic \$\endgroup\$ May 4 '18 at 14:38
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    \$\begingroup\$ It's a voltage comparator. Go google. \$\endgroup\$
    – Andy aka
    May 4 '18 at 14:39
  • \$\begingroup\$ How about resiator, what is Its functionality ? \$\endgroup\$ May 4 '18 at 15:36

Assuming this is the whole circuit, we can conclude from the lack of feedback that it is used as a comparator.

The input is connected to the negative input, with ground on the positive. If the input is positive, the output will be as negative as the OP-amp can make it. If the input is negative, the OP-amp output will be as positive as it can make it.

This signal turns an transistor on or off, but we can't say much more about that.

It is possible that the output is connected with a larger feedback loop back to the input, in which case this could have a different function. Without the whole circuit it's hard to tell.

  • \$\begingroup\$ It's interesting that there are current limiting resistors on the opamp supply lines, I haven't seen that before. Is that common? \$\endgroup\$
    – BeB00
    May 4 '18 at 15:22
  • \$\begingroup\$ @BeB00 It's a power supply filter, it's common. \$\endgroup\$
    – Dorian
    May 4 '18 at 15:25
  • \$\begingroup\$ 56 ohms is the biggest I've ever seen though. I guess if your only load is a FET gate and the gate capacitance is much lower than the bypass capacitor value, it'll work fine. \$\endgroup\$
    – The Photon
    May 4 '18 at 16:08
  • \$\begingroup\$ @ThePhoton: The power filtering resistors will have a negligible effect on switching speed, given the size of the associated capacitors and especially the 10k series resistor on the output. \$\endgroup\$
    – Dave Tweed
    May 4 '18 at 16:18
  • \$\begingroup\$ @DaveTweed - But they do (or can, depending on things like layout) have a critical impact on behavior for inputs near ground. If switching the load changes the supply voltage, and this changes the amp input characteristics, the circuit may oscillate. It used to be called motorboating, due to the sound of the oscillation in an audio amp. \$\endgroup\$ May 4 '18 at 16:53

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