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About Virtual ground in OPAMP with negative feedback

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subharpe

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About Virtual ground

In OPAMP with negative feedback, the inverting terminal comes to the same voltage as the non-inverting terminal. If the non-inverting terminal is grounded the inverting terminal voltage also comes to 0. If the non-inverting terminal is at some other level, inverting terminal coes to that level.

Then in stead of calling it "virtual short" why is it called "virtual ground".

I am not really looking for a bookish answer to the term "virtual" here. I am confused about the term ground.
 

Re: About Virtual ground

Quote from the book Op Amps - Design, application and troubleshooting, from David Terrel
Since very little current flows in or out of the input terminals of the op amp,
we saw that there was essentially no voltage drop across RB which caused the (+)
input terminal to remain at ground potential. Since % is always near 0 as long as
the amplifier remains unsaturated, this means that the (-) input terminal must
also remain very near to ground potential. This is an important concept. Although
the (-) input is not actually grounded, it remains very near ground potential. We
commonly refer to this point in the circuit as virtual ground.

Hope that helped.
 

Re: About Virtual ground

Thank you, but not really helped. I did not ask about the term virtual. I wanted to ask about the term ground. It needs not be grounded always. Isn't it?
 

Re: About Virtual ground

I'm not an Op Amp specialist, but I think the term "virtual ground" only has significance, when your refer to the potential of one of the inputs when the other is connected to the ground.

Anyway, if you consider the high input impedance of the Op Amp, there is always a negligible current flowing between it's terminals, which is a behaviour more of an open circuit that of a short (remember that one way to think of an open circuit is replacing it by an infinite impedance).
 

Re: About Virtual ground

subharpe said:
Thank you, but not really helped. I did not ask about the term virtual. I wanted to ask about the term ground. It needs not be grounded always. Isn't it?

You are completely right: when the +terminal is at +5V the neg. terminal also will be at app. +5V provided that a neg. feedback path exist. Thus, the term "virtual ground" is misleading. I think the reason is a a pure historical background. It´s quite normal in some cases that a term is defined in the early days of a circuit or a system which - some years later - seems not 100% correct under all circumstances.
 

About Virtual ground

The term ground means the reference voltage, which is this case is 0v.
So, if the non-inverting terminal is grounded. By virtual short, the inverting terminal is at same potential as of the non-inverting terminal. But it is not physically connected the ground.
hence to conclude.
ground=node connected to gnd, havung reference voltage(0v)

virtual ground=node not connected physically to gnd, but has reference voltage.

Hope that helps.
 

Re: About Virtual ground

subharpe,
If you consider the virtual ground as being an AC (dynamic) ground the term makes more sense in that the voltage on the inverting input is independent of the current flowing into the node, since the feedback causes an equal and opposite current to flow out of the node.
Regards,
Kral
 

Re: About Virtual ground

Kral said:
subharpe,
If you consider the virtual ground as being an AC (dynamic) ground the term makes more sense in that the voltage on the inverting input is independent of the current flowing into the node, since the feedback causes an equal and opposite current to flow out of the node.
Regards,
Kral

This may serve as an additional explanation, however, there are opamp applications in which both input terminals are neither on dc nor on ac ground. In these cases the term "virtual ground" is really misleading.
 

About Virtual ground

Friends,

I was under the impression that virtual ground meant that one input was grounded with respect to the other input (potential difference of 0V)... no direct connection so virtual... what do you think???
 

Re: About Virtual ground

May be, you expect too much from the term virtual ground. I think, the term isn't intended as a general explanation of OP circuit operation. It's just a handy expression for the behaviour observed with a class of OP circuits and mainly a method to calculate node voltages. It's a simplification, cause it assumes infinite gain.

It's obviously originated from dual-supply circuits with grounded N.I. input. The suggested term virtual short may be used to extend the concept to other circuits. But as it doesn't actually explain their behaviour, it can't replace an exact analysis. Thus I doubt, if it's helpful anyhow.
 

Re: About Virtual ground

Yes, that´s right. The term "virtual ground" not only is misleading - it is not helpful at all neither for understanding opamp circuits nor by designing them.
Thus, forget it !!
 

I just wanted to know an extension of this. If say I have V1 (500mV) applied at negative terminal and V2(300mv) applied at positive terminal then what happens to virtual ground theory. Does both the terminals virtually become 500mV? Or does it become virtually 300mV?

Could anyone explain?
 

I think i have an answer... Not to the previous question though..

Lets go to the basics... if you have a direct connection between terminal A and B then whatever there is in terminal A gets replicated to B... this is called the "short" condition, Isn't it? More specifically it is said to be the 'Mechanical Short'

Now Between the two input terminals of Op amp.. There is a very high resistance (3M Ω).. So this cant be the condition of 'mechanical short'

Looking at the figure.. since the internal resistance of the op amp is v large (ideally taken as infinite) thus the current entering it must be zero. as shown I=0.

since there is no current flowing in op amp... you can consider R*I (3MΩ * 0 A=0V)
Thus the voltage drop is ideally zero for the internal resistance.. Since there is no voltage drop between two points- it is considered as equipotential. Only coz of a resistor in between it is not said 'mechanical' short but 'virtually short'

Virtual short has a gr8 significance.. It remains short for voltage but open for current ( current cannot pass in). This is indicated (or rather reminded to all the ppl) by drawing a DOTTED GROUND ON THE OTHER SIDE so never draw solid grnd on the other side.
 

NOW TALKING ABOUT VIRTUAL GROUND
If one of the terminal is at 500mV then other is at the same level. This is virtual SHORT.
If one is at ground (0 V level) naturally the other also gets grounded. This is a special case of virtually short analysis and is said to be VIRTUALLY GROUND.

Added after 8 minutes:

And the question about different inputs at the same time... I think the op amp will work in a very unusual way.. Obviously the voltage levels would not compromise with each other and and settle up to one middle voltage... But a very very small current 0.75µA will flow between the terminals... This is too negligible, thanks to the high Input resistance of the op amp.

But with different connections and feed backs one can make the op amp work as an COMPARATOR. an v useful application.

a very very small current 0.75µA will flow between the terminals

This leads to another significance of virtual short as explained by Albert Malvino that the concept is imaginary (as said by the word virtual) but this approximation leads to simplification of the analysis and calculations for the op amp circuits... as we consider the current too small and neglect it we can consider that I(i) = I(f) thus leading to simplicity in calculations...

Thanks,
Akshay
 

May I clarify something?

The "virtual ground" exist only in an inverting opamp with feedback.
It is NOT a result of a (nearly) infinite input resistance. Instead, the "virtual ground principle" results from the open loop gain - if it as high as 100k---1000k.
In this case the negative feedback action forces the negative input to have a voltage in the mikro-volt region - which can be approximated to nearly zero volts (virtual ground). That`s all!
 

By "virutal ground", I think of it as meaning there is a virtual short circuit between the inverting and non-inverting terminals. It does not mean a "ground" to 0V DC ground plane!

And it is just a rule of thumb. There are plenty of conditions, especially transient ones, where the rule is not true.
 

biff44 said:
............
And it is just a rule of thumb. There are plenty of conditions, especially transient ones, where the rule is not true.

Yes, that`s correct, it can be applied for linear operations only.
 

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