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Discussion Overview

The discussion revolves around the operational amplifier (op-amp) behavior in the absence of supply voltages, particularly focusing on the implications for current flow and output characteristics in amplifiers without feedback. Participants explore various aspects of op-amp design and functionality.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants assert that without supply voltage, the output transistors of an op-amp will be off, leading to high input impedance and no current flow.
  • Others question whether it is possible to have any current at all without supply voltages, suggesting that this depends on the specific chip design.
  • One participant notes that active electronic circuits, such as op-amps, are designed to operate only when power supplies are connected, indicating that the question is largely hypothetical.
  • Another participant emphasizes that all standard op-amps require power and mentions historical context regarding user inquiries about neglecting to power circuits, referencing National Semiconductor's AN31 publication.
  • It is mentioned that in an unpowered op-amp, there is no reliable feedforward, and thus no usable output can be generated.

Areas of Agreement / Disagreement

Participants generally agree that supply voltage is necessary for op-amps to function, but there is some debate regarding the specifics of current flow and the implications of chip design. The discussion remains unresolved regarding the nuances of current behavior in unpowered op-amps.

Contextual Notes

Participants reference specific publications and historical context, indicating a reliance on older materials for understanding modern op-amp behavior. There are also mentions of special cases where certain op-amps might draw power differently, which could complicate the generalization of claims made.

Femme_physics
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Question at topic title.
 
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Supply voltage is needed to bias the output transistors. Without it the OPAMP output transistors will be all off and the output node will be have high input impedance. i.e. Whatever voltage the outside circuit tries puts on the output node it will keep that voltage.
 


So no supply voltages = no current at all?

I'm talking about an amplifier with no feedback btw
 


Femme_physics said:
So no supply voltages = no current at all?

That depends entirely on the chip design, but it's a hypothetical question, because active electronic circuits (e.g. op amps) are only designed to be used when the power supplies ARE connected.
 
Just assume all opamps have to be powered. Any that draw their power from input pins instead of V+ and V- pins are special cases.

When National Semiconductor published their AN31, OP Amp Collection, it didn't show the power pins. They got so many inquiries from newbie experimenters who had neglected to power their circuits they put out a special bulletin to that effect.

AN31 is really old now but those guys were geniuses and the design of their circuits is quite instructive.
It's a great introductory learning tool, as is TI's "OpAmps for Everyone".
One can use modern opamps in these old circuits.

AN31: http://www.ti.com/general/docs/lit/getliterature.tsp?svadoc=an-31&reg=en

OPAmps for Everyone: http://www.google.com/url?sa=t&rct=...QxYdkL4XemccjuNYQ&sig2=SsJrqlAmpgf3c7JV7uzt3w

or search on "opamps for everyone" slod006

It's a 464 page textbook in pdf that should be printed and kept in one's technical library.
 


Femme_physics said:
So no supply voltages = no current at all?

I'm talking about an amplifier with no feedback btw
Another puzzle from the questioning mind of Femme_physics! :smile:

There is no feedforward in an unpowered op-amp (well, none that can be relied upon, anyway!)[/color] So the output cannot be powered by the input in the manner you are picturing.

No supply voltage/s = no usable output
 


Another puzzle from the questioning mind of Femme_physics!

:) hehe

No supply voltage/s = no usable output

Gotcha! Thanks.

And also Jimhardy and alephzero--thanks for the "feedbacks":) (my jokes used to be better)..
 

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