Op amp: Find Vo in terms of Vg and A

In summary, the conversation discusses finding Vo as a function of Vg and the open-loop gain A in a circuit. The participants initially have difficulty understanding what open-loop gain means, but eventually come to the conclusion that it refers to the op amp's gain without feedback or input components. They also discuss the operation of the circuit and the importance of considering the finite gain of the amplifier. Finally, the conversation ends with a recommendation for a textbook as a reference.
  • #1
ElijahRockers
Gold Member
270
10

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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  • #2
The diagram shows neither an RL nor a 4k resistor.
 
  • #3
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.
 
  • #4
Whoops. Sorry guys, wrong picture. But I figured it out with some help, thank you!
 
  • #5
Vo=5Vg
 
  • #6
hoangkyem said:
Vo=5Vg

That's incorrect too.
 
  • #7
rude man said:
That's incorrect too.
Why? Vo=10/2Vg=5Vg
Since it is Inverting amplifier
 
  • #8
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.
 
  • #9
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
 

1. What is an op amp?

An op amp, short for operational amplifier, is an electronic circuit component that amplifies the difference between two input signals. It typically has a very high gain and is used in a variety of applications, such as signal conditioning, filtering, and mathematical operations.

2. How does an op amp work?

An op amp consists of a differential amplifier followed by a high-gain amplifier stage. The differential amplifier amplifies the difference between the two input signals, and the high-gain amplifier stage amplifies this difference even further. The output of an op amp is dependent on its inputs, as well as its gain and other circuit components.

3. What is Vg and A in the equation for finding Vo?

Vg is the input voltage to the op amp, and A is the open-loop gain of the op amp. The equation assumes that the op amp is operating in an open-loop configuration, meaning there is no feedback from the output to the input.

4. Can Vo be negative in terms of Vg and A?

Yes, Vo can be either positive or negative, depending on the values of Vg and A. If Vg is positive and A is greater than 1, the output will be positive. If Vg is negative and A is greater than 1, the output will be negative. If A is less than 1, the output will be opposite in polarity to Vg.

5. What are some practical applications of the op amp equation for finding Vo?

The equation for finding Vo in terms of Vg and A is commonly used in electronic circuit design, particularly in applications where a precise output voltage is needed. It is also useful in signal processing and control systems, where the input and output signals may need to be amplified or manipulated.

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