--- title: "Output Impedance | Electrical Circuits and Systems I" description: "Output impedance is a circuit's resistance to delivering current at its output, shaping voltage stability in op-amp and load-driven circuit analysis." canonical: "https://fiveable.me/electrical-circuits-systems-i/key-terms/output-impedance" type: "key-term" subject: "Electrical Circuits and Systems I" unit: "Unit 5" --- # Output Impedance | Electrical Circuits and Systems I ## Definition Output impedance is the effective resistance a circuit presents at its output, which affects how much its voltage changes when you connect a load. In Electrical Circuits and Systems I, it matters most in op-amp and buffer circuits. ## What It Is Output impedance in Electrical Circuits and Systems I is the effective opposition a circuit presents at its output terminal when it has to supply current to a load. If the load tries to draw more current, a circuit with higher output impedance will let the output voltage sag more than a circuit with lower output impedance. You can think of it as the output side's built-in resistance, even though it is often a combination of resistive and reactive effects, not just one literal resistor. In ideal circuit models, the output impedance of an op-amp is treated as zero, which means the output behaves like a perfect voltage source. Real devices are never perfect, so the actual output impedance is small but not zero. This idea shows up fast when you connect an op-amp to a load resistance. If the load is large, the output voltage stays close to the intended value. If the load is small, the circuit has to deliver more current, and any nonzero output impedance creates a voltage drop inside the source itself. That is why two circuits with the same open-circuit voltage can behave very differently once you attach a real load. Negative feedback is the main reason practical op-amp circuits can act like low-output-impedance sources. Feedback senses the output and adjusts the input side of the amplifier so the output stays close to the target value even when the load changes. In a voltage follower, this is exactly what you want: the output should copy the input voltage while being strong enough to drive the next stage. The term is not just about DC behavior. In AC steady-state, output impedance can vary with frequency, which matters when you are driving capacitive loads or when a later stage has frequency-dependent input behavior. So when you see output impedance in this course, think about how faithfully a circuit can hold its output voltage under real loading conditions, not just about a number on a datasheet. ## Why It Matters Output impedance is one of the main reasons practical op-amp circuits are useful instead of just theoretical. In a circuit chain, the source stage has to deliver signal to the next stage without the voltage collapsing, and low output impedance is what makes that possible. This term connects directly to voltage followers, buffer design, and load resistance problems. A voltage follower is built to give you nearly the same voltage at the output as at the input, but with much better ability to drive a load. If you do not account for output impedance, you can predict the wrong output voltage when the load is not very large. It also helps you reason about distortion and signal loss. A source with too much output impedance can form an unwanted voltage divider with the load, which changes the amplitude seen by the next block. In lab work or homework, this often shows up as a circuit that looks fine on paper but behaves differently once a real resistor, speaker, sensor, or next amplifier stage is attached. When you move into feedback and frequency response, output impedance becomes part of the bigger picture of how stable and accurate a circuit feels. It is one of the practical limits that separates an ideal amplifier from a useful one. ## Connections ### Load Resistance Output impedance only matters when something is connected to the output, so the load resistance is the other half of the story. A large load resistance draws less current and causes less voltage drop across the source's output impedance. A small load resistance stresses the circuit more and makes the output voltage shift farther from its ideal value. ### [Negative Feedback](/electrical-circuits-systems-i/key-terms/negative-feedback) Negative feedback is the main tool that lowers effective output impedance in many op-amp circuits. The circuit measures its own output and corrects errors, which helps the output stay steady as the load changes. If you see a circuit with feedback, you should expect better voltage regulation than an open-loop stage. ### [Voltage Follower](/electrical-circuits-systems-i/key-terms/voltage-follower) A voltage follower is the clearest example of why low output impedance matters. It gives you about the same voltage at the output as at the input, but with a much stronger ability to drive the next stage. That makes it a buffer, not a gain stage. ### Input Impedance Input impedance and output impedance work as a pair in two-stage circuits. You want the input impedance of the next stage to be high, so it does not load the previous stage, and you want the previous stage's output impedance to be low, so it can deliver the signal cleanly. Together, they control how much of the signal gets preserved across stages. ## On the AP Exam A quiz or problem set will usually ask you to predict what happens when an op-amp output drives a particular load resistance. You might compare two circuits, one with a buffer and one without, and decide which one keeps the output voltage closer to the intended value. Another common move is to use a voltage-divider style model to see how finite output impedance changes the measured output. In lab questions, you may be asked to explain why the output drops when the load gets smaller or why negative feedback improves the result. The skill is not memorizing a single number, it is tracing how the source and load interact. ## output impedance vs Input Impedance Input impedance describes how much a circuit resists current coming into its input, while output impedance describes how much resistance appears at the output when the circuit tries to drive a load. They are opposites in function, but both affect how well one stage connects to the next. High input impedance and low output impedance is usually the pairing you want. ## Key Takeaways - Output impedance is the effective resistance seen at a circuit's output when it drives a load. - A lower output impedance lets the circuit hold its output voltage more steadily as load current changes. - In op-amp circuits, negative feedback usually reduces effective output impedance. - A voltage follower is a common buffer because it gives low output impedance with about unity gain. - If output impedance is too high, the output can sag, distort, or lose amplitude when the load is small. ## FAQs ### What is output impedance in Electrical Circuits and Systems I? Output impedance is the effective resistance a circuit presents at its output when it supplies current to a load. In op-amp work, it tells you how well the circuit can keep its output voltage from changing when the load changes. Lower output impedance means better voltage stability. ### Why does low output impedance matter in op-amp circuits? Low output impedance means the op-amp can drive a load without losing much voltage across its own output stage. That is what makes buffers and voltage followers useful when one stage has to feed another stage. If the load is heavy, high output impedance can make the output droop. ### How does negative feedback affect output impedance? Negative feedback usually lowers the effective output impedance of an op-amp circuit. The circuit senses output error and corrects it, which helps the output act more like a stiff voltage source. That is one reason feedback improves real circuit performance. ### Is output impedance the same as load resistance? No. Load resistance belongs to the device or stage being driven, while output impedance belongs to the source that is driving it. They interact to determine the actual output voltage. A load can be fixed and still produce different results depending on the source's output impedance. ## Related Study Guides - [5.4 Practical Op-Amp Circuits and Applications](/electrical-circuits-systems-i/unit-5/practical-op-amp-circuits-applications/study-guide/OxYVH2hNey16k90Y) ## About This Document Canonical Fiveable pages are available as Markdown at the same path plus `.md`. - [llms.txt](https://fiveable.me/llms.txt): index of Fiveable's sections and URL patterns - [llms-full.txt](https://fiveable.me/llms-full.txt): complete subject and unit listing - [MCP server](https://fiveable.me/mcp): call Fiveable as tools instead of fetching pages (`https://fiveable.me/api/mcp`) - [MCP server for AP teachers](https://fiveable.me/mcp/teachers): a teacher's classes, assignments and AP-rubric grading (`https://fiveable.me/api/mcp/teacher`) ## Structured Data ```json {"@context":"https://schema.org","@graph":[{"@type":"LearningResource","@id":"https://fiveable.me/electrical-circuits-systems-i/key-terms/output-impedance#resource","name":"Output Impedance | Electrical Circuits and Systems I","url":"https://fiveable.me/electrical-circuits-systems-i/key-terms/output-impedance","learningResourceType":"Concept explainer","educationalLevel":"AP® / High School","about":{"@id":"https://fiveable.me/electrical-circuits-systems-i/key-terms/output-impedance#term"},"audience":{"@type":"EducationalAudience","educationalRole":"student"},"dateModified":"2026-07-03T02:21:25.273Z","isPartOf":{"@type":"Collection","name":"Electrical Circuits and Systems I Key Terms","url":"https://fiveable.me/electrical-circuits-systems-i/key-terms"},"author":{"@type":"Organization","@id":"https://fiveable.me/editorial#content-team","name":"Fiveable Content Team","url":"https://fiveable.me/editorial","parentOrganization":{"@id":"https://fiveable.me/#organization"},"member":[{"@type":"Person","name":"Amanda DoAmaral","sameAs":["https://www.linkedin.com/in/adoamaral"]},{"@type":"Person","name":"Becky Bahr","sameAs":["https://www.linkedin.com/in/becky-bahr-b52743132"]}]},"publisher":{"@type":"Organization","name":"Fiveable","url":"https://fiveable.me"}},{"@type":"DefinedTerm","@id":"https://fiveable.me/electrical-circuits-systems-i/key-terms/output-impedance#term","name":"output impedance","description":"Output impedance is the effective resistance a circuit presents at its output, which affects how much its voltage changes when you connect a load. 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