science

Input impedance

Input 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 Input impedance rather than just read about it. In short: In electrical engineering, the input impedance of an electrical network is the measure of the opposition to current (impedance), both static (resistance) and dynamic (reactance), into a load network or circuit that is external to the electrical source network. The input admittance (the reciprocal of impedance) is a measure of the load network's propensity to draw current.

Key takeaways

Input 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 Input impedance to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Input impedance from memory before moving on to harder problems.

Reference excerpt

In electrical engineering, the input impedance of an electrical network is the measure of the opposition to current (impedance), both static (resistance) and dynamic (reactance), into a load network or circuit that is external to the electrical source network. The input admittance (the reciprocal of impedance) is a measure of the load network's propensity to draw current. The source network is the portion of the network that transmits power, and the load network is the portion of the network that consumes power. For an electrical property measurement instrument like an oscilloscope, the instrument is a load circuit to an electrical circuit (source circuit) to be measured, so the input impedance is the impedance of the instrument seen by the circuit to be measured.

Input impedance If the load network were replaced by a device with an output impedance equal to the input impedance of the load network (equivalent circuit), the characteristics of the source-load network would be the same from the perspective of the connection point. So, the voltage across and the current through the input terminals would be identical to the chosen load network. Therefore, the input impedance of the load and the output impedance of the source determine how the source current and voltage change. The Thévenin's equivalent circuit of the electrical network uses the concept of input impedance to determine the impedance of the equivalent circuit.

Calculation If one were to create a circuit with equivalent properties across the input terminals by placing the input impedance across the load of the circuit and the output impedance in series with the signal source, Ohm's law could be used to calculate the transfer function.

Electrical efficiency The values of the input and output impedance are often used to evaluate the electrical efficiency of networks by breaking them up into multiple stages and evaluating the efficiency of the interaction between each stage independently. To minimize electrical losses, the output impedance of the signal should be insignificant in comparison to the input impedance of the network being connected, as the gain is equivalent to the ratio of the input impedance to the total impedance (input impedance + output impedance). In this case,

Z i n ≫ Z o u t {\displaystyle Z_{in}\gg Z_{out}} (or Z L ≫ Z S {\displaystyle Z_{L}\gg Z_{S}} ) The input impedance of the driven stage (load) is much larger than the output impedance of the drive stage (source).

Power factor In AC circuits carrying power, the losses of energy in conductors due to the reactive component of the impedance can be significant. These losses manifest themselves in a phenomenon called phase imbalance, where the current is out of phase (lagging behind or ahead) with the voltage. Therefore, the product of the current and the voltage is less than what it would be if the current and voltage were in phase. With DC sources, reactive circuits have no impact, therefore power factor correction is not necessary. For a circuit to be modelled with an ideal source, output impedance, and input impedance; the circuit's input reactance can be sized to be the negative of the output reactance at the source. In this scenario, the reactive component of the input impedance cancels the reactive component of the output impedance at the source. The resulting equivalent circuit is purely resistive in nature, and there are no losses due to phase imbalance in the source or the load.

Z i n = X − j Im ⁡ ( Z o u t ) {\displaystyle {\begin{aligned}Z_{in}&=X-j\operatorname {Im} (Z_{out})\\\end{aligned}}}

Power transfer The condition of maximum power transfer states that for a given source maximum power will be transferred when the resistance of the source is equal to the resistance of the load and the power factor is corrected by canceling out the reactance. When this occurs the circuit is said to be complex conjugate matched to the signals impedance. Note this only maximizes the power transfer, not the efficiency of the circuit. When the power transfer is optimized the circuit only runs at 50% efficiency. The formula for complex conjugate matched is

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Input impedance

Start with the simplest possible case. Write down what Input 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 Input 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 Input 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 Input impedance

In research: Input 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 Input 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: Input 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 Input 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 Input impedance in 20 minutes

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

Frequently asked questions

What is Input impedance in simple terms?

Why does Input 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 Input 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 Input impedance.