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Doubts about the concept of Virtual Ground

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saad

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Hi, I have a confusion regarding the concept of Virtual Ground. Aren't these two contradictory statements about this fundamental concepts.

1. Op amp has infinite input resistance (means inverting and non inverting terminals are open circuited)

2. There is zero potential drop across inverting and non-inverting terminal (the two terminals are short-circuited)

Please some senior member may reply. I do not want to get confused by some in-experienced postings.

I hope thats understandable for everyone.

Many Thanks,
Saad
 

Re: Virtual Ground

One input of the amplifier is grounded. The other input is set to ground by the feedback. It has the same potential as ground, but is not connected to ground.
 

    saad

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Re: Virtual Ground

actully virtual ground is a concept for making vid to be zero , also significance of it is that it neither sinks current nor source the current thus making biasing current to a minimum extent.
 

Re: Virtual Ground

The above posts are correct but the concept of virtual ground is better understood by the investigating the notion of feedback.

Now listen to me...

If u use an op amp without a feedback, then the rules U stated are by no means correct. That is, the two inputs of op amp are treated as a regular high-valued resister whose potential difference can be any amount! But when u use feedback, it is the Element in the feedback path which couses the two potentials to be equal. For instance, consider a simple inverting op amp. As u provide the input with some voltage, the output goes to a value so as to provide a cuurent which is sufficient to make the two input pins of the op amp equipotential.

Hope this helped, and if it did. plz click helped me!

Regards.
 

    saad

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Re: Virtual Ground

Input resistance does not mean resistance between the two input terminals.

It is actually the resistance which determines current flowing out of the input terminals.


and the short cicuit isnt a physical one...
it is supposed to be virtual.
 

Re: Virtual Ground

virtual ground for an opamp should be considered when there is a feedback and the gain of the opamp is infinite. To check this, consider the simple inverting amplifier circuit ,
now apply the millers theorem on the feed back resistor (Rf). You get [Rf/(1-A)] at input side and [Rf/(1-(1/A))] on the output side. Here I have cosidered the feed back condition. Now apply the second condition for virtual ground i.e. A= infinte. that becomes [ Rf/(1-A)] = 0 ;Which Implies 0 resistance (short)between the two input terminals , which is called the virtual short and whatever the input you apply at 1 terminal it appears at the other terminal.

Hope you got this,

subbu.
 

Re: Virtual Ground

you not confused regarding of virtual Gnd

only remember that same potential on the both inputs....... in inverting amplifier the noninverting input is grounded so same is generated at inverting input
 

Virtual Ground

Hi

In the concept of virtual ground of an Inverting op-amp, virtually no current flows into the amplifiers input. In this configuration the positive input is grounded. The circuit
characteristics are determined by values of feedback
resistor and gain resistor. Since no current is flowing into the input pin of the amplifier, all the current that’s coming back from the feedback path is going to go through the gain resistor. So, basically, the input current and the feedback current are going to be equal. Since the noninverting
pin is grounded, the inverting pin is going to be
sitting at virtual ground.
 

Re: Virtual Ground

Hello

In a ideal Operational Ampifier (op amp) we have two concepts for simplify analysis.

Virtual Ground: this concept is used when you conect a wire segment between groung and a op amp input (inverter amplifier and similar circuits) .

Based in the ideal characteristics:

R[in] --> infinity then the current for op amp inputs is zero
A[ol] --> infinity then the voltage in the other input terminal of op amp is zero

Virtual Ground is in few words : A shortcut for voltage and a open circuit for current.


when the op amps inputs aren't conected to ground (not inverter amplifier and similar circuits) we have other concept for simplify analysis:

Virtual Shortcut: because the same ideal characteristics we have:

R[in] --> infinity then both input current are zero
A[ol] --> infinity then v[+] - v[-] is zero

Virtual Shortcut is in few wodrs: A open circuit for voltage and a shortcut for current

I hope this information can help you and aclarate your questions
 

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