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explanation required(virtual ground)

Thread starter #1
hi,

recently in uni i've been dealing alot with op-amps and most of the circuits i am required to build have virtual grounds. while knowing how to create a virtual ground, i do not really understand the concept of a virtual ground. all i know is that it is a point in the circuit where the voltage is held constant and is the substitute of a real ground. my lecturer's explanation is confusing and wikipedia's article is too deep. can u guys help out? i want to know how a virtual ground really works and when do we need one. thx in advance
 

Nigel Goodwin

Super Moderator
Most Helpful Member
#2
Depends really what you're actually talking about?.

Opamps generally need a split supply (it gives many advantages), and if you only have a single supply you can 'fake' a split supply with two same value resistors across the supply (a potential divider), with a decouling capacitor. This is sometimes known as a 'virtual earth'.

Or for an opamp adder, this is called in audio circles a 'virtual earth mixer', because the mixng point at the inverting input has no signal on it, so is considered to be 'virtually at earth potential'.
 
#3
A virtual ground is a result of an opamp trying to keep its two input terminals at the same potential when used in a feedback circuit.

Take a look at a standard inverting opamp amplifier configuration.
We know that (ideally) current does not flow into an opamp input terminal.
So when the (+) terminal is held at say ground (or some other DC), the opamp will do what it needs to to also make its (-) input at the same potential. If this potential happens to be ground (common in many cases) then input current will flow towards that "ground" potential but not go into the opamp input terminals since they do not sink current. So a ground potential which actually cannot sink current is called a "virtual" ground.
 
Thread starter #4
i think i'm starting to get a rough idea. so virtual grounds are only truly required when working with op-amps? and also my project this semester involves rotating a dc motor cw and ccw quickly. been reading up on H-bridges but do you guys have any online links where i can refer to? the budget im given does not allow for a motor driver. thanks alot!
 
#5
Virtual ground is usefull for other circuits, too. Not just op-amps. Search google for H-bridge? or have a look here Motor control This came from this forum just a few questions away.
 
Last edited:
#7
The big problem with virtual grounds is they're not 'real' grounds, so they're subject to the circuit that creates the voltage refrence, which means it will drift depending on what's going on in the rest of the circuit. It's a very easy way for noise to get in. In high precision or otherwise important applications it's usually a better idea to use real split supply.
 

Roff

Well-Known Member
#8
I believe that the original definition of "virtual ground" was only the zero volts which appears at the negative input pin of an op amp whose noninverting input is connected (perhaps through a resistor) to GND, and has DC feedback to the inverting input.
The term has been used more recently to denote the voltage divider which is used in single-supply op amp circuits, and serves as the bias voltage for the noninverting pin(s) on those op amps. This is not really, IMHO, "virtual ground". It's a bias voltage. The summing vodes of the op amps could be called, I suppose, "virtual bias voltage", or "virtual AC ground". :D
 
#9
Roff said:
I believe that the original definition of "virtual ground" was only the zero volts which appears at the negative input pin of an op amp whose noninverting input is connected (perhaps through a resistor) to GND, and has DC feedback to the inverting input.
The term has been used more recently to denote the voltage divider which is used in single-supply op amp circuits, and serves as the bias voltage for the noninverting pin(s) on those op amps. This is not really, IMHO, "virtual ground". It's a bias voltage. The summing vodes of the op amps could be called, I suppose, "virtual bias voltage", or "virtual AC ground". :D
But if you connect that "virtual ground" to a real Rack chassis that is connected to AC ground, it becomes real.
I have used a 12VDC wall wart, used a 10K/10K series resistor voltage divider to bias the non-inverting input of a TL074 op amp wired as a voltage follower. I then connected the output of the op-amp to chassis ground, making the previously floating output of the wall wart +/-6V with respect to real ground.

I works as long as the ground current does not exceed the 20mA output limitation of the op-amp.

Regards
Bob
 

Roff

Well-Known Member
#12
Nigel Goodwin said:
Well Wikipedia is hardly an authority on anything! :D

I support your view, as I sort of mentioned in my first post.
Yeah, I realize that Wikipedia entries may be no more than some hunyucker's opinion.
ElectronicDefinitions.com supports the narrower definition, but a search for "virtual ground" yields a lot of hits that appear to refer to the broader definition.
 
#13
Roff said:
Yeah, I realize that Wikipedia entries may be no more than some hunyucker's opinion.
I love this place. I see words I have to look up all the time:

Hunyucker: Essentially a Northern version of a Redneck.
Can be either urban blue-collar or rural. Can be from any area of North America north of the 42nd Parallel, but especially concentrated in the Dakotas, Minnesota, Wisconsin, the lower peninsula of Michigan (Yoopers are a separate breed altogether), and Upstate New York. Especially fond of 1970's & 80's arena rock music. Wisconsin hunyucks are particularly rabid fans of the Green Bay Packers (not all Packer fans are hunyucks, but all Wisconsin hunyucks are Packer fans, with the exception of those living in the St. Croix River Valley, who own reversible jackets with Packers on one side, and Minnesota Vikings on the other). Primary consumers of Calvin Peeing and "Ditch the *****, Let's Go Snowmobiling" stickers in the northern tier of states. Older hunyucks are responsible for the majority of polka music produced in the United States.

The English english slang is fascinating too.
 

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