enter image description here When i assemble the attached circuit in breadboard and switch on the power supply, the desired output is obtained for the initial 10seconds after which the output doubles. What may be the reasons ?

  • \$\begingroup\$ Are you trying to measure the thermal conductivity of a gas? \$\endgroup\$
    – Andy aka
    Mar 16, 2019 at 18:36
  • \$\begingroup\$ Increase all the resistors by 10X. \$\endgroup\$ Mar 17, 2019 at 1:57
  • \$\begingroup\$ 10 seconds is about the thermal time constant of a square inch of copper foil. \$\endgroup\$ Mar 17, 2019 at 1:58
  • \$\begingroup\$ It's a bandgap reference \$\endgroup\$
    – user211628
    Mar 17, 2019 at 2:00

1 Answer 1


When you switch the circuit on, currents start to flow. Together with the voltages across the components, this causes them to dissipate power which causes them to warm up.

That is perfectly normal and to be expected.

Read the manual of an accurate voltmeter or reference voltage generator and it will state that you must leave the device on to warm up. Often that needs to be for at least one hour before the specified accuracy is guaranteed.

The resistors in your circuit will not be very sensitive to temperature changes but the transistors will be. The forward voltage of a diode and therefore also the Vbe of a transistor will have a typical temperature coefficient of -2 mV/K meaning Vbe decreases 2 mV for every degree Kelvin (or Celcius) temperature increase.

So it is not the opamp itself that's causing this behavior. In your (Bandgap reference) circuit, the opamp will do "whatever it takes" to make the voltages at its inputs the same. As the transistors change their behavior, the opamp needs to adjust its output voltage.

  • \$\begingroup\$ So you mean to say that the designed output will be obtained only after 1 hour ? \$\endgroup\$
    – user211628
    Mar 16, 2019 at 14:19
  • \$\begingroup\$ If I said "Yes", would you believe that and not question that at all? Where do I write that "the designed output will be obtained only after 1 hour" ? Read carefully what I write and think why that is so. You're not thinking so switch on your brain and think. If you don't think then really, analog circuits isn't going to be "your thing". \$\endgroup\$ Mar 16, 2019 at 18:47

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