science

Output impedance

Output impedance is a science 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 Output impedance rather than just read about it. In short: In electrical engineering, the output impedance of an electrical network is the measure of the opposition to current flow (impedance), both static (resistance) and dynamic (reactance), into the load network being connected that is internal to the electrical source. The output impedance is a measure of the source's propensity to drop in voltage when the load draws current, the source network being the portion of the…

Key takeaways

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

Reference excerpt

In electrical engineering, the output impedance of an electrical network is the measure of the opposition to current flow (impedance), both static (resistance) and dynamic (reactance), into the load network being connected that is internal to the electrical source. The output impedance is a measure of the source's propensity to drop in voltage when the load draws current, the source network being the portion of the network that transmits and the load network being the portion of the network that consumes. Because of this the output impedance is sometimes referred to as the source impedance or internal impedance.

Description All devices and connections have non-zero resistance and reactance, and therefore no device can be a perfect source. The output impedance is often used to model the source's response to current flow. Some portion of the device's measured output impedance may not physically exist within the device; some are artifacts that are due to the chemical, thermodynamic, or mechanical properties of the source. This impedance can be imagined as an impedance in series with an ideal voltage source, or in parallel with an ideal current source (see: Series and parallel circuits). Sources are modeled as ideal sources (ideal meaning sources that always keep the desired value) combined with their output impedance. The output impedance is defined as this modeled and/or real impedance in series with an ideal voltage source. Mathematically, current and voltage sources can be converted to each other using Thévenin's theorem and Norton's theorem. In the case of a nonlinear device, such as a transistor, the term "output impedance" usually refers to the effect upon a small-amplitude signal, and will vary with the bias point of the transistor, that is, with the direct current (DC) and voltage applied to the device.

Measurement The source resistance of a purely resistive device can be experimentally determined by increasingly loading the device until the voltage across the load (AC or DC) is one half of the open circuit voltage. At this point, the load resistance and internal resistance are equal. It can more accurately be described by keeping track of the voltage vs current curves for various loads, and calculating the resistance from Ohm's law. (The internal resistance may not be the same for different types of loading or at different frequencies, especially in devices like chemical batteries.) The generalized source impedance for a reactive (inductive or capacitive) source device is more complicated to determine, and is usually measured with specialized instruments, rather than taking many measurements by hand.

Audio amplifiers

The real output impedance (ZS) of a power amplifier is usually less than 0.1 Ω, but this is rarely specified. Instead it is "hidden" within the damping factor parameter, which is:

D F = Z L Z S {\displaystyle DF={\frac {Z_{\mathrm {L} }}{Z_{\mathrm {S} }}}}

Solving for ZS,

Z S = Z L D F {\displaystyle Z_{\mathrm {S} }={\frac {Z_{\mathrm {L} }}{DF}}}

gives the small source impedance (output impedance) of the power amplifier. This can be calculated from the ZL of the loudspeaker (typically 2, 4, or 8 ohms) and the given value of the damping factor. Generally in audio and hifi, the input impedance of components is several times (technically, more than 10) the output impedance of the device connected to them. This is called impedance bridging or voltage bridging. In this case, ZL>> ZS, (in practice:) DF > 10 In video, RF, and other systems, impedances of inputs and outputs are the same. This is called impedance matching or a matched connection. In this case, ZS = ZL, DF = 1/1 = 1 . The actual output impedance for most devices is not the same as the rated output impedance. A power amplifier may have a rated impedance of 8 ohms, but the actual output impedance will vary depending on circuit conditions. The rated output impedance is the impedance into which the amplifier can deliver its maximum amount of power without failing.

Batteries Internal resistance is a concept that helps model the electrical consequences of the complex chemical reactions inside a battery. It is impossible to directly measure the internal resistance of a battery, but it can be calculated from current and voltage data measured from a circuit. When a load is applied to a battery, the internal resistance can be calculated from the following equations:

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Output impedance

Start with the simplest possible case. Write down what Output impedance claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, 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 Output impedance 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 Output impedance 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 Output impedance

In research: Output impedance appears in science 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 Output impedance 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: Output impedance is common in secondary-school and first-year university syllabi. It links to neighbouring topics Audio amplifier specifications, Electrical parameters, so understanding it makes those chapters shorter.
In everyday life: Look for Output impedance 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 Output impedance in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Output impedance 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 Output impedance out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Output impedance in simple terms?

Why does Output impedance matter?

Because it connects several science 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 Output impedance?

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 Output impedance.