engineering

Virtual ground

Virtual ground is a engineering topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Virtual ground rather than just read about it. In short: In electronics, a virtual ground (or virtual earth) is a node of a circuit that is maintained at a steady reference potential, without being connected directly to the reference potential. In some cases the reference potential is considered to be that of the surface of the earth, and the reference node is called "ground" or "earth" as a consequence.

Key takeaways

Virtual ground belongs to engineering; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Virtual ground to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Virtual ground from memory before moving on to harder problems.

Reference excerpt

In electronics, a virtual ground (or virtual earth) is a node of a circuit that is maintained at a steady reference potential, without being connected directly to the reference potential. In some cases the reference potential is considered to be that of the surface of the earth, and the reference node is called "ground" or "earth" as a consequence. The virtual ground concept aids circuit analysis in operational amplifiers and other circuits and provides useful practical circuit effects that would be difficult to achieve in other ways. In circuit theory, a node may have any value of current or voltage but physical implementations of a virtual ground will have limitations in terms of current handling ability and a non-zero impedance which may have practical side effects.

Construction A voltage divider, using two resistors, can be used to create a virtual ground node. If two voltage sources are connected in series with two resistors, it can be shown that the midpoint becomes a virtual ground if

V out V in = − R f R in {\displaystyle {\frac {V_{\text{out}}}{V_{\text{in}}}}=-{\frac {R_{\text{f}}}{R_{\text{in}}}}}

An active virtual ground circuit is sometimes called a rail splitter. Such a circuit uses an op-amp or some other circuit element that has gain. Since an operational amplifier has very high open-loop gain, the potential difference between its inputs tends to zero when a feedback network is implemented. This means that the output supplies the inverting input (via the feedback network) with enough voltage to reduce the potential difference between the inputs to microvolts. More precisely, it can be shown that the output voltage of the amplifier in the figure is approximately equal to − R f R i n V i n {\displaystyle -{\frac {R_{f}}{R_{in}}}V_{in}} . Thus, as far as the amplifier is working in its linear region (output not saturated, frequencies inside the range of the opamp), the voltage at the inverting input terminal remains constant with respect to the real ground, and independent from the loads to which the output may be connected. This property is characterized a "virtual ground".

Applications Voltage is a differential quantity, which appears between two points. In order to deal only with a voltage (an electrical potential) of a single point, the second point has to be connected to a reference point (ground). Usually, the power supply terminals serve as steady grounds; when the internal points of compound power sources are accessible, they can also serve as real grounds. If there are no accessible source internal points, external circuit points with steady voltage relative to the source terminals can serve as artificial virtual grounds. Such a point has to have steady potential, which does not vary when a load is attached.

See also Voltage-to-current converter and current-to-voltage converter show some typical virtual ground applications Miller theorem applications

References

External links Create a Virtual Ground with the LT1118-2.5 Sink/Source Voltage Regulator Rail Splitter, from Abraham Lincoln to Virtual Ground Application note on creating an artificial virtual ground as a reference voltage. Creating a Virtual Power Supply Ground Inverting configuration shows the application of the virtual ground concept in an inverting amplifier (Archived)

Worked examples

Example 1 — a first encounter with Virtual ground

Start with the simplest possible case. Write down what Virtual ground claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In engineering, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Virtual ground before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Virtual ground ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Virtual ground

In research: Virtual ground appears in engineering research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Virtual ground in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Virtual ground is common in secondary-school and first-year university syllabi. It links to neighbouring topics Electrical circuits, Electricity concepts, so understanding it makes those chapters shorter.
In everyday life: Look for Virtual ground outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Virtual ground in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Virtual ground means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Virtual ground out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Virtual ground in simple terms?

Why does Virtual ground matter?

Because it connects several engineering ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Virtual ground?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Virtual ground.