Op amp output when supply is missing

AI Thread Summary
When Vcc or Vee is disconnected from an op amp, the output saturates to +Vsat if Vee is missing and -Vsat if Vcc is absent. The output equation Vout = -Rf/R1 (Vin) only applies when both power supplies are connected, as the op amp floats at Vcc without Vee. The output stage remains in a high impedance state, unable to source current, which explains the saturation behavior. Understanding the internal components, such as back-to-back diode junctions in the TLE2061, is crucial for predicting output behavior when power supplies are missing. The discussion highlights the complexity of op amp operation under these conditions and the need for deeper analysis of the internal circuitry.
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What is the output of the op amp when either Vcc or Vee is not connected.
I 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 a inverting configuration. Shouldn't it go to ground, since the output equation is Vout = -Rf/R1 (Vin)
Same is try when Vin is zero. Op amp in question is TLE2061
 
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Your formula Vout = -Rf/R1 (Vin) is valid only if both Vcc and Vee are connected and Vcc is positive with respect to Vee. Without Vee, the whole opamp floats at Vcc. There is no way the output could be anything else. Most likely in spite of it being at Vcc the output would be high impedance and unable to source appreciable current.
 
Why does it saturate at +Vsat when Vee is absent and -Vsat when Vcc is absent.
How is the output stage turned on when the input is zero.
 
The output stage is neither saturated nor turned on. If you replaced the opamp with a resistor and connected one end to Vcc, the other side of the resistor would also be at Vcc because there is no current flowing through it and no voltage drop across it.
 
That's a simplified explanation. It's nice though.
But why should I replace the opamp with a resistor. What exactly is happening at the output of the opamp?
 
Really, you would have to look at the internal components to know what the behavior or a particular op amp would be like. In the case of the TLE2061, the differential inputs are are several reverse diode junctions away and should have little effect. Without biasing the IC, the inputs have little effect on the output.

On the other hand, Vcc and the output are separated by two back-to-back diode junctions. You are effectively looking at a high impedance two terminal device where the return path is supplied by the current through your DVM.
 
Back to back diode junctions? I can't find them. Can you tell me the transistor designators - Q39, Q42?
 

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