Virtual ground and reference ground - what is the differenace?

[Evy]

Hello!
I have stumbled upon different "grounds" in my book and wounder if anyone could explain the difference to me?

I have done some exercises with "signal grounds" where I could reset DC sources in the circuitry. I could replace DC voltage source with a short circuit and a DC current source with an open circuit. Is that a virtual ground? Could virtual grounds be in other places than reference grounds?

Sorry if you find this question to be to broad.

[Zero999]

Yes this question is a little vague.

Can you please post some schematics showing examples of what you're referring to?

Generally it's better to have all the power supply rails connected together, with a large copper plane on the PCB and keep digital and analogue circuitry, away from one another.

[Evy]

I am working with amplifiers wight now so its all analog. The circuitry can be seen in this thread

I realized that that type of signal ground I was talking about is when you move from bias circuit to a signal graph.

[Zero999]

So you mean a virtual ground, in the context of a differential amplifier?

It actually isn't 0V, just very close to 0V because the amplifier has a very high gain and uses negative feedback, to keep its inputs at the same voltage, which is normally 0V. In reality, the voltage at the virtual ground is simply the amplifier's output voltage divided by the gain.

[Evy]

Oh, so you can call the input terminal of a differential amplifier a virtual ground then since we assume that its 0V?

[julian1]

Dave explains the concept of a virtual ground in this video,

https://youtu.be/7FYHt5XviKc?t=17m15s

In the example, the non-inverting input (ie. +) is tied to reference ground. Then, given a negative feedback configuration, a virtual ground is created on the inverting input (ie. -)  because the op-amp changes it's output in order to try and make the two input voltages equal. Note, that the op-amp has both +ve and -ve supplies, so ground (reference and virtual) is the common supply voltage - eg. 0V.

[bson]

A virtual ground is a point in a circuit that wants to be at the same potential as ground, without an actual connection.  You often find them in feedback circuits and they add a known quantity that can help with circuit analysis.