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TL082: Why the amplifier will be self-heating when it rail out？

Chengshang Yang
Prodigy 30 points
Part Number: TL082

1.Why the amplifier will be consuming more current than normal and self-heating when it rail out？as is saying in《Properly terminating an unused op amp》Fig.2(a)

（http://www.electronicproducts.com/Analog_Mixed_Signal_ICs/Amplifiers/Properly_terminating_an_unused_op_amp.aspx）

"The circuit in Fig. 2a connects both inputs together and to GND, but it ignores the effects of input offset voltage (VIO) between the inputs. Depending on the polarity of VIO, the small voltage between the inputs causes the output to drive to one supply rail or the other. Since this circuit has large open loop gain, the output saturates and the device consumes extra current. This implementation should be avoided."

if the ampilfier is of JFET-input,  its input current is as low as nA, and output current is 0A, why the OP can consume much current, and where is the current loop? 

  • 0
    TI__Guru**** 157236 points
    Chengshang,

    I would have answer yesterday, but the e2e post was invisible. I don't know why however.

    I don't believe the TL082 would consume extra current in this case. Have you verified extra ICC for TL082?
  • 0 in reply to Ron Michallick
    TI__Guru**** 157236 points
    Chengshang,

    I tested a sample device.
    When TL082 output is railed high, ICC decreases by 48%
    When TL082 output is railed low, ICC increases by 2%

    This op amp does not have a sensitivity to open loop operation.
  • 0 in reply to Ron Michallick
    Prodigy 30 points

    I'm sorry, the OP number TL082 maybe confuse you, it only just a example.

    what I want to know is the description of ”Properly terminating an unused op amp”，why OP will consume extra power when it rail out?

    "The circuit in Fig. 2a connects both inputs together and to GND, but it ignores the effects of input offset voltage (VIO) between the inputs. Depending on the polarity of VIO, the small voltage between the inputs causes the output to drive to one supply rail or the other. Since this circuit has large open loop gain, the output saturates and the device consumes extra current. This implementation should be avoided."

    in the following site:

    http://www.electronicproducts.com/Analog_Mixed_Signal_ICs/Amplifiers/Properly_terminating_an_unused_op_amp.aspx

    can you open it?

    in anohter book,  “OP Amps for Everyone(3rd edition)”, similar discription is also found.

  • 0 in reply to Chengshang Yang
    TI__Guru**** 157236 points
    Chengshang,

    For a theoretical op amp that has a NPN collector sink driver output (emitter to GND). Due to VIO of -5mV (for example) the loop will try to get an output of -5mV which is not possible with GND being lowest supply voltage. So op amp gives base of NPN a lot of current trying to get output lower. Let's say this current can be 30mA due to a really poor design. This base current comes from VCC which could be 30V. This give a power dissipation of 900mW. Let's say the package is small and has a 200 C/W Tja. Temperature rise is 180C which will soon cause permanent damage.

    If you want a actual real device example, Connect with Todd on e2e. Maybe he knows.
  • 0 in reply to Ron Michallick
    Prodigy 30 points
    Ron,

    Thanks for your answer and patient. I kown the extra current comes from interior of OP amps.