Below schematic is a voltage regulator circuit minus the compensations/filters parts (capacitors etc):


simulate this circuit – Schematic created using CircuitLab

The amount of current the op amp is sinking is not more than ~2.6 mA but for some unknown reason the op amp is getting a little warm (~42-45°C).

What causing the op amp to heat up?


  • \$\begingroup\$ Does the regulator work as intended? \$\endgroup\$ Commented May 15, 2020 at 16:10
  • \$\begingroup\$ @StefanWyss Yes it does work and it's not hot, just warm. I'm concerned because the heat may affect the input voltage offset... \$\endgroup\$ Commented May 15, 2020 at 16:11
  • \$\begingroup\$ Did you measure the 2.6mA? \$\endgroup\$ Commented May 15, 2020 at 16:18
  • \$\begingroup\$ @StefanWyss Yes I measured the LM317 output current before connecting it to the op amp. \$\endgroup\$ Commented May 15, 2020 at 16:20
  • \$\begingroup\$ But the Opamp has a -5V supply. So it could also drive to -5V. Why dont you measure the actual current through the diode? \$\endgroup\$ Commented May 15, 2020 at 16:21

1 Answer 1


Ok, lets do some calculation: Supply 30V with 1.5mA load and 5mA quiescent current, total 6.5mA. This is 200mW. With Tja 100deg/W this will be a delta T of 20deg C. So your amp will be at 44 deg C. You seem to have a good feeling for temperature ;-)

  • \$\begingroup\$ The junction will be at 44C, but the package may not. Since there's no thermal impedance info in the linked datasheet and we don't know which package the OP is using it's hard to say. \$\endgroup\$
    – John D
    Commented May 15, 2020 at 17:20
  • \$\begingroup\$ Ok, junction is not case temp, fair enough, But on a breadboard, Tja could be much higher than 100deg/W. I would expect it to be 200 deg/W. \$\endgroup\$ Commented May 15, 2020 at 17:25
  • 3
    \$\begingroup\$ Stefan, you can use HTML entities &Omega;, &mu;, &deg;, &times;, etc. as well as <sup>...</sup> and <sub>...</sub> in the posts (but they don't work in the comments). SI standard recommends a space between the number and units for legibility (and normal writing: "99 bananas" and not "99bananas"). \$\endgroup\$
    – Transistor
    Commented May 15, 2020 at 19:04
  • \$\begingroup\$ I forgot to ask; where in the datasheet the quiescent current is mentioned? \$\endgroup\$ Commented May 16, 2020 at 4:42
  • 2
    \$\begingroup\$ Page 3, bottom, Power supply current \$\endgroup\$ Commented May 16, 2020 at 7:55

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