... Or should I leave them not connected if I'm not going to trim them?

I think I fried an LM741 when it's output was only connected to a single MOSFET's gate (and the single supply was <12V). The only thing I can figure is it's bad to ground those offset null pins, but I can't tell from its datasheet.

  • \$\begingroup\$ Why use the 741? It's obsolete. \$\endgroup\$ – Leon Heller Dec 10 '16 at 9:02
  • \$\begingroup\$ @LeonHeller The usual reason: because it's on hand \$\endgroup\$ – Matt Thomas Dec 10 '16 at 9:37
  • 1
    \$\begingroup\$ what does the data sheet say? \$\endgroup\$ – Neil_UK Dec 10 '16 at 12:18
  • \$\begingroup\$ @Neil_UK It says nothing about the offset pins under "Absolute Maximum Ratings", although under "Electrical Characteristics" it does give a typical and maximum current for those pins. So does that mean grounding those pins is bad because it'll exceed its "Electrical Characteristics", or that it's okay because nothing is said under "Maximum Ratings"? I can't tell from its datasheet \$\endgroup\$ – Matt Thomas Dec 10 '16 at 14:01

This is what a 741 looks like internally: -

enter image description here

Bottom left are the trim pins and, as can be seen, they connect to the -Ve rail of the op-amp via 1 kohm resistors so don't mess with them, leave them open circuit if not being used or you will upset the balance of the current sources feeding the input transistor emitters.

Normally, the trim pins are used like this: -

enter image description here

As you can see, their recommended connection adds resistance in parallel with the two 1 kohm resistors shown in the top picture. Taking one or both to ground (normally regarded as mid-rail) will bias off those current sources.


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.