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Op amp output when one supply is not connected

Hithesh
Genius 3290 points
Other Parts Discussed in Thread: TLE2061, OPA140

What is the output of the op amp TL2061 when either Vcc or Vee is not connected. We simulated this and got Vsat. +Vsat when Vee was missing and -Vsat when Vcc was missing. But I don't know why the output saturates to +Vsat, even in an inverting configuration. Shouldn't it go to ground, since the output equation is Vout = -Rf/R1 (Vin) Output is same as above when Vin is zero. Can you tell me from the equivalent diagram in the datasheet, which transistors will be turned ON at the output stage? I am guessing 2 transistors should be ON, to get a total drop of around 1.4v.

  • 0
    TI__Guru 64530 points

    Hithesh,

    First, I assume you refer your question to TLE2061 (not TL2061).   When you disconnect one of the rails, the part will not have a return path for the current to flow and it will stop working as an op amp; thus no close-loop gain equation is applicable under such condition.  Depending on which supply gets disconnected, the current will flow to or from the supply rail through the ESD protection diodes into the input terminal causing a large differential voltage to develop between the inputs (no virtual short).  How much differential voltage develops between the inputs depends on the precise circuit topology of the op amp front end.  In an inverting configuration, an additional current will flow from/to one of the output transistors through a feedback and input  resistors  to/from the ground, respectively.

    To illustrate the problem, please see the details of what happens using a similar JFET input part, OPA140.

     

     

     

     

  • 0 in reply to Marek Lis
    Genius 3290 points

    Marek, thanks.

    Fig 1,2 are buffers. Current flows from output terminal to the input thru the feedback loop and create a voltage drop across the input terminals.

    In this case the output doesn't latch to saturation.

     

    In Figs 3,4 inverting config why is the output 2x input?

    Is the current flowing from supply to input via the ESD protection diodes?

    Why would the current flow from supply to input terminal thru the ESD diodes.

     

    Yes, I meant TLE2061.

  • 0 in reply to Hithesh
    TI__Guru 64530 points

    Hithesh,

    In the buffer configuration, with one supply floating, the IQ current has no other path to take but to flow from the remaining live supply through the ESD to the input terminal - it does NOT flow from the output terminal (see below) - 500fA is just the input bias current.

     

     

    In the inverting configuration, the main IQ current (1.8mA) flows the same way as in a follower configuration case but an additional current (454uA) coming out of the Vcc (2.3mA) flows out of output terminal through the gain setting resistors to the ground - it flows this way because that's the only path it can take after removal of Vee supply (see below).

    In inverting configuration, the negative input voltage (454mV) is a result of is a simple voltage divider between the output (906mV) and the ground (neglecting IB).