Op amp: Find Vo in terms of Vg and A

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Discussion Overview

The discussion revolves around finding the output voltage (Vo) of an operational amplifier (op amp) in terms of the input voltage (Vg) and the open-loop gain (A). Participants explore concepts related to op amp behavior, including the implications of open-loop gain and the configuration of the circuit, specifically inverting amplifiers.

Discussion Character

  • Homework-related
  • Debate/contested
  • Technical explanation

Main Points Raised

  • Some participants express confusion about the meaning of open-loop gain and its implications for the circuit's operation.
  • One participant suggests that the open-loop gain refers to the op amp's gain without feedback or input components.
  • There are conflicting claims regarding the expression for Vo, with one participant stating Vo=5Vg and another challenging this by stating it is incorrect.
  • A later reply asserts that the correct expression for Vo in an inverting amplifier configuration is Vo=-10Vg/2=-5Vg, referencing a textbook for support.
  • Some participants note that the assumption of infinite gain leads to incorrect conclusions about the output voltage.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the correct expression for Vo, with multiple competing views presented regarding the relationship between Vo, Vg, and the open-loop gain A.

Contextual Notes

There are unresolved assumptions about the circuit configuration and the specific values of resistors involved, which may affect the calculations and interpretations of the output voltage.

ElijahRockers
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Homework Statement



eqnk1z.jpg


Find Vo as a function of Vg and the open-loop gain A.

Attempt

I'm not sure what open-loop gain means.

I redrew the circuit so it made more sense to me, but I'm still not getting it. It seems that Vo is the voltage across both RL and the 4k resistor.

The voltage across the 4k resistor is equal the voltages at the input terminals.

Aside from these bits of information I don't really know how this thing operates. Should the current going into the input terminals still be 0? But at the positive terminal the current is 8/50k right? or does this mean there is a -8 voltage source after the 50k resistor?
 
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The diagram shows neither an RL nor a 4k resistor.
 
ElijahRockers said:

Homework Statement



eqnk1z.jpg


Find Vo as a function of Vg and the open-loop gain A.

Attempt

I'm not sure what open-loop gain means.
It probably means the op amp gain by itself (no feedback or input components). Ground one input, apply a signal to the other input, the OL gain = output/input.

Should the current going into the input terminals still be 0?

Yes. The op amp is ideal except for finite gain.
 
Whoops. Sorry guys, wrong picture. But I figured it out with some help, thank you!
 
Vo=5Vg
 
hoangkyem said:
Vo=5Vg

That's incorrect too.
 
rude man said:
That's incorrect too.
Why? Vo=10/2Vg=5Vg
Since it is Inverting amplifier
 
hoangkyem said:
Why? Vo=10/2Vg=5Vg
Since it is Inverting amplifier

Because you have ignored the finite gain of the amplifier (the "open-loop gain A").

Your expression holds only if the amplifier gain is infinite.
 
rude man said:
Because you have ignored the finite gain of the amplifier (the "open-loop gain A").

Your expression holds only if the amplifier gain is infinite.

I think I'm right. It's is inverting amplifier, so the closed-loop gain (Acl) independent of the op amp's internal open-loop gain (Aol)
Acl=-Rf/Ri where Rf is feedback resistor, Ri is input resistor
→Vo=-10Vg/2=-5Vg

I founded on internet a textbook: Electronic Devices, 9th ed, T.Floyd
You can download and refer in chapter 12, page 618.

Ebook Electronic Devices 9th ed, T.Floyd
 

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