# 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

mahak, Nov 4, 2007

2. ### John PopelishGuest

mahak wrote:
> 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.

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.

John Popelish, Nov 4, 2007

3. ### BobWGuest

"Jamie" <> wrote in message
news:Mm8Xi.62\$...
> mahak wrote:
>
>> 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.
>>

> 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 ?
>

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

Bob

BobW, Nov 4, 2007
4. ### Phil AllisonGuest

"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.
>

** 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

Phil Allison, Nov 4, 2007
5. ### EeyoreGuest

Jamie wrote:

> 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.

You do talk the most appalling tripe.

Graham

Eeyore, Nov 4, 2007
6. ### EeyoreGuest

BobW wrote:

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

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

Phil Allison has explained it in a nutshell.

Graham

Eeyore, Nov 4, 2007
7. ### mahakGuest

On Nov 4, 5:26 am, John Popelish <> wrote:
> mahak wrote:
> > 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.

>
> 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.

thanks. its convincing.

mahak, Nov 4, 2007
8. ### JamieGuest

mahak wrote:

> 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.
>

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 ?

--
"I'm never wrong, once i thought i was, but was mistaken"
Real Programmers Do things like this.
http://webpages.charter.net/jamie_5

Jamie, Nov 4, 2007
9. ### BobWGuest

"Eeyore" <> wrote in message
news:...
>
>
> BobW wrote:
>
>> Whew. I feel dizzy. I think it's best that the OP stick with Popelish'
>> explanation.

>
> Ignore Jamie's 'explanation'. He's an utter fuckwit.
>
> Phil Allison has explained it in a nutshell.
>
> Graham
>

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

BobW, Nov 4, 2007
10. ### JamieGuest

BobW wrote:

> "Jamie" <> wrote in message
> news:Mm8Xi.62\$...
>
>>mahak wrote:
>>
>>
>>>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
>>>

>>
>>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 ?
>>

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

Not understanding is the first sign of interest

--
"I'm never wrong, once i thought i was, but was mistaken"
Real Programmers Do things like this.
http://webpages.charter.net/jamie_5

Jamie, Nov 4, 2007
11. ### meGuest

Jamie <> wrote in
news:e79Xi.120\$:

>Eeyore wrote:
>
> You do talk the most appalling tripe.
>>

>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.

He could have said it "nicer", but you're way off.

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

The original question is in reference to an actual book often studied by
EE's, not the NET.

> 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.

No measurements are involved.

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

This is a very specific (and basic) question about the operation of
opamps. His reading skills are fine...

me, Nov 4, 2007
12. ### JamieGuest

Eeyore wrote:

>
> Jamie wrote:
>
>
>> 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.

>
>
> You do talk the most appalling tripe.
>
> Graham
>

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!

--
"I'm never wrong, once i thought i was, but was mistaken"
Real Programmers Do things like this.
http://webpages.charter.net/jamie_5

Jamie, Nov 4, 2007
13. ### BobWGuest

"Jamie" <> wrote in message
news:LCaXi.581\$...
> BobW wrote:
>
>> "Eeyore" <> wrote in message
>> news:...
>>
>>>
>>>BobW wrote:
>>>
>>>
>>>>Whew. I feel dizzy. I think it's best that the OP stick with Popelish'
>>>>explanation.
>>>
>>>Ignore Jamie's 'explanation'. He's an utter fuckwit.
>>>
>>>Phil Allison has explained it in a nutshell.
>>>
>>>Graham
>>>

>>
>>
>> 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
>>
>>

> Excuse me sir?
>

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

Bob

BobW, Nov 4, 2007
14. ### JamieGuest

BobW wrote:

> "Eeyore" <> wrote in message
> news:...
>
>>
>>BobW wrote:
>>
>>
>>>Whew. I feel dizzy. I think it's best that the OP stick with Popelish'
>>>explanation.

>>
>>Ignore Jamie's 'explanation'. He's an utter fuckwit.
>>
>>Phil Allison has explained it in a nutshell.
>>
>>Graham
>>

>
>
> 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
>
>

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.

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.

--
"I'm never wrong, once I thought I was, but was mistaken"
Real Programmers Do things like this.
http://webpages.charter.net/jamie_5

Jamie, Nov 4, 2007
15. ### JamieGuest

BobW wrote:
> "Jamie" <> wrote in message
> news:LCaXi.581\$...
>
>>BobW wrote:
>>
>>
>>>"Eeyore" <> wrote in message
>>>news:...
>>>
>>>
>>>>BobW wrote:
>>>>
>>>>
>>>>
>>>>>Whew. I feel dizzy. I think it's best that the OP stick with Popelish'
>>>>>explanation.
>>>>
>>>>Ignore Jamie's 'explanation'. He's an utter fuckwit.
>>>>
>>>>Phil Allison has explained it in a nutshell.
>>>>
>>>>Graham
>>>>
>>>
>>>
>>>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
>>>
>>>

>>
>>Excuse me sir?
>>

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

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.

--
"I'm never wrong, once I thought I was, but was mistaken"
Real Programmers Do things like this.
http://webpages.charter.net/jamie_5

Jamie, Nov 4, 2007