# Op Amp virtual ground

Discussion in 'Electronic Basics' started by mahak, Nov 4, 2007.

1. ### mahakGuest

Hi,
This is mahak.
can anybody tell me what is virtual ground concept.I refer to Micro
electronics by Sedra & SMith.
There it says Virtual Groung is because of infinite gain of amplifier.
But if that is the case then it should be there for both inverting and
non inverting amplifier. Please clarify.

2. ### John PopelishGuest

An opamp with negative feedback connected between output and
inverting input, tries to produce whatever output that will
cause the inverting input voltage to match what is applied
to the non inverting input. This is because the opamp has a
very high differential voltage gain, so the only way the
output will not be saturated either positive or negative is
if the two inputs have an almost perfect match (the
imperfection in that match providing the signal that is
amplified by the high differential voltage gain to produce
the finite output).

So, when you connect the non inverting voltage to ground,
the feedback forces the inverting input to be an almost
perfect copy of that ground voltage. From the viewpoint of
any signal connected to an input network to the inverting
input, that signal acts as if the input network were
connected to ground, even though it is just balanced there
by gain and feedback. Pretending that the inverting node is
grounded (virtually grounded) makes it a lot simpler than
doing the math that includes the tiny voltages at that node
because the amplifier gain is not actually infinite. It is
a useful approximation.

Of course, if you connect the non inverting input to some
other voltage reference than zero, the inverting input is a
virtual copy of whatever reference voltage you use, instead
of ground.

3. ### BobWGuest

Whew. I feel dizzy. I think it's best that the OP stick with Popelish'
explanation.

Bob

4. ### Phil AllisonGuest

"mahak"

** Virtual ground refers to the situation where the " + " input of an op-amp
is connected to circuit ground and the circuit is an inverting amplifier
stage.

Then the " - " input becomes a "virtual ground" since ( in all normal
operation ) there is only the *tiniest* of AC or DC voltages on that pin.

So, it is "virtually grounded" during operation.

Calculations and analysis can use that simplifying assumption quite safely.

....... Phil

5. ### EeyoreGuest

You do talk the most appalling tripe.

Graham

6. ### EeyoreGuest

Ignore Jamie's 'explanation'. He's an utter fuckwit.

Phil Allison has explained it in a nutshell.

Graham

7. ### mahakGuest

thanks. its convincing.

8. ### JamieGuest

Hmm. I can't find that reference on the NET that you speak of.

In my eyes. A virtual ground is no more than a voltage
divider where the divided network is the ground and the outer of
the network are connected to the + & - of the single source of energy.
In this system, the common would be derived from the center divide of
the network.
I don't see where the gain of an amp comes into play here unless, they
are referring to the open-loop gain? That would be the (+) input
connected to the common/virtual ground tap point, and the (-) being the
input for example.

Taking measurements on this type of output with no input on (-)
should yield 0 volts. This is assuming an ideal amp.
biasing the (-) would result in very high gain but not infinite since
in theory, only ideal op-amps have that and there is no such thing as an
ideal op-amp or amp for that matter.

This seems to be a general theory question at best.
Maybe you read it incorrectly ?

9. ### BobWGuest

Assuming Jamie had ever used an opamp then he would know what "virtual
ground" is and how to (simply) explain it.

It's amazing how people like Jamie will spew the limited knowledge that they
have in an attempt to impress people. It always has the opposite effect, in
the long run.

Jamie just needs to go to school. If he's been to school then he needs to go
back.

Bob

10. ### JamieGuest

Not understanding is the first sign of interest

11. ### meGuest

He could have said it "nicer", but you're way off.
Your entire post is:

The original question is in reference to an actual book often studied by
EE's, not the NET.
get your eyes checked.
No measurements are involved.
This is a very specific (and basic) question about the operation of
opamps. His reading skills are fine...

12. ### JamieGuest

Love how you left out the rest of my post as it did go on with
the op-amp discussion.

But that's typical of you any ways you lifeless turd.

Want to smell my shorts? I haven't changed them in a while! it
should turn you on! I heard that turns you on!

13. ### BobWGuest

I think it's time to play -- SPOT THE LOONEY!

Bob

14. ### JamieGuest

Excuse me sir?

or should it be jerkelator in your case?

Limited knowledge you say? Where did get yours,
the five and dime store?

I think 30+ years in the field is enough.

That also covers putting 2 son's through
college for EE both graduating with a 4.5 grade average, both with
dyslexia and extreme help from their father.

The title on this group reads "BASIC" which means basic in terms
of explanation and laymen descriptions.

It's assholes like your self, parade around
in public trying to impress some one.

Facts are, you can impress those that don't know any better, but
there are many that you will never impress with that kind of approach!

Should I go on? or does it seem that some one might be talking about you?

--------------- Bottom line is.. -----------------

You'd be the wiser to keep the trap shut, you might even learn
something to help that lacking personally you're exhibiting.

Let me drop to your level for a moment.

"Fucking idiot you are! 100 % Grade A snot nose jerk.

I bet you're real popular around the boys with bull
shit like that coming out of that urinal.

If I didn't know any better, I think you're in bed

with Mr. Graham, or is that Ham.

You two were meant for each other.. "

Now you know how you look on the other side.

Btw..
My employer just signed a 3 year contract with me, I'm above
the 135k mark now. I've been with this employer for years, along
with doing some of my own side adventures in robotics capacitor
assemblers that you, you're self may have actually had in your hands,
components made by automative equipment that I designed.

Scare's the hell out of you doesn't it?

I think if my skills were suffering, there would be some issues
with that?

Have a good fucking day and may your shorts and faults be many!.

btw, I've turn down guys many times better than you in interviews
for EE/Electronics E. Don't take it personally.

15. ### JamieGuest

If that video is appealing to you, I can understand your problem.

I might had enjoyed watching something like that when I was around 10
or so how ever, I find that video very poorly acted out and boring to
say the least. A very poor dictation of english humor, but then again,
it's not much different than a couple of British posters I see here
dumping their vile.

Have a good day.

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