# virtual ground

Discussion in 'Electronic Design' started by Danny, Feb 9, 2004.

1. ### DannyGuest

Hi

In an opamp (inverting amplifier) how does the virtual ground come about?

Dan

2. ### Jim ThompsonGuest

Virtually ?

...Jim Thompson

3. ### John WoodgateGuest

I read in sci.electronics.design that Danny <[email protected]
The current flowing from the input to the circuit flows through the
feedback resistor to the output. No appreciable current flows into the -
input of the op-amp. The very high open-loop gain of the op-amp means
that almost no voltage difference between the + and - inputs is
necessary to produce the required output signal. The + input is at
ground potential, so the - input must be at the same potential.

4. ### mikemGuest

The "vitrual ground" when referring to an opamp is somewhat of a
misnomer. What really happens when you configure an opamp to have
negative feedback, is that it tries to minimize the voltage difference
between the + and - inputs.

If you "ground" the + input in an inverting amplifier, and assuming that
the bias current is small compared to the currents flowing in the gain
setting resistors, then the voltage on the - input will be "very close"
to zero volts...

MikeM

5. ### Paul Hovnanian P.E.Guest

Drive a virtual ground rod?

6. ### N. ThorntonGuest

Hi

In inverting configuration, with the + input grounded, the opamp will
do its best to keep its - input at ground potential. Hence it is
referred to as a virtual ground.

Consider the 2 key resistors in the above setup:
______ _____
-----|______|------O------|_____|-----
R_in R_feedback

If 1v is fed in thru R_in, and R_feedback is 10x R_in, the output must
swing to -10v in order to keep the -input at 0v - this is what happens
in this config.

Regards, NT

7. ### gwhiteGuest

As a negative feedback amplifier, the inverting input is driven by the output to
be _equal_ to the non-inverting input (it "seeks" to drive the _difference to
zero_).

Since the non-inverting input is at ground, and the inverting input is driven to
the same zero potential, a "virtual" (not real) ground exists. That is, if you
put a voltmeter probe on that inverting node, you will see the same number of
volts as if you put the probe on ground. That "same number of volts" is
specifically zero volts, since the non-inverting input is grounded.

8. ### Fred BloggsGuest

In modern engineering parlance the word "virtual" is used in a multitude
of fields to generally mean "acts like for practical purposes but is not
actually" and can apply to other things besides GND. So a virtual GND
refers to a circuit node that acts like it's grounded without actually
being grounded. This implies that the node potential remains at constant
0V for all circuit conditions to which it is subjected. In the case of
an operational amplifier, this effect is achieved by negative feedback
being applied in such a way to keep the virtual GND node at 0V, it does
this by maintaining a zero sum current at the inverting node. The term
usually has a context, for example whether the virtual GND is a virtual
AC-GND or a virtual DC-GND or both.

9. ### Frank BemelmanGuest

And don't ground the virtual ground

10. ### Fred BloggsGuest

Well- that would make it an actual GND.