Re: 0V in a schematic & virtual ground

Rich wrote:


"krw" <krw@xxxxxxxxxxxxxxxxx> wrote in message news:nak1p45tgkbapiofvtch6d9pboc3rv8r8v@xxxxxxxxxx

On Mon, 9 Feb 2009 14:16:43 -0000, "Rich" <notty@xxxxxxxxxx> wrote:


"Rich" <notty@xxxxxxxxxx> wrote in message
news:6vamfpFj27vcU1@xxxxxxxxxxxxxxxxxxxxx

Pretty stiff opamp, too, since to drive the virtual ground to 0V will
require 2A out of the opamp. ;)

JF


Yes, I later realised the math was wrong. I keep switching between
inverting
amplifier and non inverting amplifier. I should concentrate on each type
of
circuit individually.

That diagram I drew was to show *to myself*, how in principle we obtain a
virtual ground at V- for the inverting amplifier. And why we get Vout as
we
do for a given combination of Rin and Rf.

I never knew what virtual ground was until now. And now I know, that as
long
as one end of a circuit compont is held at 0V with reference to ground by
an
"opposing voltage", for all intents and purposes it's akin to being
connected to GND, or 0v. As long as there is somewhere for the current to
flow.


I mean, current must find it's way back to the source, so, to the source, it
looks like it's terminated in Rin.

V- must be 0V above grund or it would look to the source as if it was
terminated in a different resistance value than Rin. If for instance Vin was
10V, and Rin was 10K, and V- was at a potential of 5V above ground, the
voltage across Rin would be 5V, not 10V. So the source would see an input
resistance of 20K, because current flowing in Rin would be 5V/10K = 0.5mA.,
whereas it ought to be 1mA. I think I'm getting it. :c)


No, if I understand the above, the input impedance is still Rin since
the '-' input is still a voltage source. It's no longer "ground", but
ground is just what *you* choose to call ground.


Well, what I'm saying is that the input source is connected to GND, or a point that we call 0V. The source is connected to one side of Rin. It's essential that the other side of Rin is either connected to GND, or 0V for the source to see it's being connected across Rin. With a virtual ground arrangement, if the "earthy" one side of Rin got to be at 5V, then source will not see Rin, but some other terminating value. That's what I understand.

0 volts found in a circuit normally indicates the circuit's common "COM" reference point to all other nodes of that circuit. It does not
mean how ever, that it would be a common to a remote circuit. when I
say remote, I refer to a device that allows external connection from some other devices.

Hence, the 0V ref is a point to where all other nodes in that circuit
are in common with it.

Keep in mind that the "Common" supplied, for external/remote connection does not have to be this same 0V ref, It could be a virtual
ground derived from this circuit or an isolated unit.

0V ref normally indicates a point of reference (com) for that circuit and does not necessarily mean it's the global common.


And to confuse things more, a Virtual ground could also be the 0V ref
which would also be the common through out. This is normally found where
only a single stand alone rail supply is available, for example, a battery or xformer with only 1 rail output. Both would offer isolation from ground conflicts when connecting to external "common" sources, but
offer a way of creating a dual rail scenario to generate signals of +/-
with respect to ground/common.

assuming the (+)&(-) is from a single battery or Single rail isolated supply.


(+)-------[1k]-------(V-Grnd/0V Ref)-------[1k]--------(-)



The above VG could be used as a common in an op-amp circuit while
the (+) and (-) terminals of our isolated single rail supply creates
the (+) and (-) rails with respect to the VG (virtual Ground) for the
Op-amp's Vcc and Vee supply terminals.


The resulting output of the op-amp could generate a true
+/- signal.

The only limitations of a VG, you need twice the voltage to obtain
the dual rail voltages..

12 Battery would yield 6V +/- for example minus the losses from other things, we'll not get into.

http://webpages.charter.net/jamie_5";

.

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