Output impedance

Output impedance is the resistance a circuit presents at its output, measured by how much the output voltage changes when output current changes. In Intro to Electrical Engineering, it tells you how well an amplifier can drive a load.

What is output impedance?

Output impedance is the small-signal resistance you see looking back into a circuit’s output terminals in Intro to Electrical Engineering. If the load tries to draw more current, a circuit with high output impedance lets the output voltage sag more, while a circuit with low output impedance holds the voltage steadier.

A good way to picture it is as the circuit’s internal resistance at the output. Real amplifiers are not ideal voltage sources, so the output is never perfectly fixed. When you connect a load, part of the voltage drops across the amplifier itself and the rest appears across the load, which is why the number matters so much in circuit analysis.

The formal definition is based on slope: output impedance is the change in output voltage divided by the change in output current at the output terminals. In small-signal models, that is usually written as a ratio around the bias point, not as a raw DC resistor value. That is why you often find it in transistor amplifier analysis rather than in simple Ohm’s law problems alone.

In transistor circuits, output impedance depends on the device and the configuration. A common-source FET stage often has a relatively high output impedance, which can be good for voltage gain but not always for driving a heavy load. A common collector or source follower stage has low output impedance, so it behaves more like a buffer and can deliver current without a large voltage drop.

This is also where feedback shows up. Negative feedback usually lowers output impedance, which makes the amplifier behave more like a solid voltage source. That is one reason feedback can improve how a stage handles changing loads, reduce distortion, and make the output less sensitive to the next component in the chain.

A common mistake is mixing up output impedance with load resistance. The load is what you connect to the amplifier, while output impedance is a property of the amplifier itself. The two interact, and that interaction is what determines the actual output voltage you measure in the lab.

Why output impedance matters in Intro to Electrical Engineering

Output impedance shows up anytime you connect one stage of a circuit to another, especially in amplifier chains and buffer design. In Intro to Electrical Engineering, it helps you predict whether a transistor stage will keep its voltage steady or lose signal when the next block draws current.

That matters in the BJT and FET configuration topics you see in class. A common emitter or common source stage may give useful gain, but if its output impedance is too high, the next stage can load it down and change the voltage you expected. A buffer stage fixes that problem by presenting a high input impedance and a low output impedance.

It also connects to design tradeoffs. You may want a high output impedance when you care about current-source behavior or certain gain characteristics, but for driving a speaker, sensor input, or another low-resistance stage, a low output impedance is usually better. That tension between gain, drive strength, and loading is a recurring theme in amplifier design.

If you can reason about output impedance, you can explain why one amplifier stage works well in a simulation but looks different on the breadboard once a load is attached. That is a classic electrical engineering move: compare the ideal source to the real circuit and trace how the load changes the output.

How output impedance connects across the course

input impedance

Input impedance is the flip side of output impedance. Input impedance tells you how hard a circuit is to drive, while output impedance tells you how well it can drive something else. In amplifier chains, you usually want the next stage to have much higher input impedance than the previous stage’s output impedance so the signal is not heavily loaded.

buffer stages

Buffer stages are designed to isolate one part of a circuit from another. They usually have low output impedance, which lets them drive a load without a large voltage drop. If you see a source follower or common collector stage, think of it as a buffer that trades voltage gain for better drive ability.

common drain amplifier

A common drain amplifier, also called a source follower, is a classic low-output-impedance FET stage. It gives little voltage gain, but it is very good at supplying current to a load. That makes it useful when you need to preserve signal shape and avoid loading the previous stage too much.

common collector

A common collector BJT stage, also called an emitter follower, has low output impedance for the same reason a source follower does: the output is taken from a terminal that tracks the input closely. This makes it a natural buffer when you need current drive more than voltage gain.

Is output impedance on the Intro to Electrical Engineering exam?

A quiz question or problem set will usually ask you to compare amplifier stages, predict loading effects, or decide which transistor configuration best drives a given load. You might be shown a common source or common emitter circuit and asked whether the output will hold its voltage well when connected to a resistor, a later stage, or a measurement device.

In a lab, you may measure output voltage with and without a load attached, then explain the drop using output impedance. A strong answer connects the observed voltage change to the amplifier’s internal output resistance, not just to the load value by itself. If feedback is part of the circuit, you may also be asked how it changes the output impedance and why that improves drive performance.

Output impedance vs input impedance

Input impedance describes what the source of a signal sees when it feeds a circuit, while output impedance describes what the load sees looking back into the circuit. They are opposite sides of the same connection, and confusing them can lead to the wrong loading prediction. If the input side is high and the output side is low, the signal usually transfers more cleanly.

Key things to remember about output impedance

• Output impedance tells you how much a circuit’s output voltage changes when the load draws current.

• Low output impedance is usually better for driving loads because the output voltage stays steadier.

• Transistor configuration matters, since common collector and common drain stages are built to have low output impedance.

• A high output impedance stage can still be useful, but it may lose voltage when it is connected to a heavy load.

• Negative feedback often lowers output impedance and makes an amplifier behave more like an ideal voltage source.

Frequently asked questions about output impedance

What is output impedance in Intro to Electrical Engineering?

It is the effective resistance you see looking into a circuit’s output terminals. In an amplifier, it tells you how much the output voltage changes when the load current changes. Low output impedance means the circuit can hold its voltage better under load.

Why is low output impedance usually preferred?

Low output impedance helps the amplifier keep its output voltage from dropping when it has to supply current. That makes it easier to drive the next circuit stage or a lower-resistance load. It also reduces signal distortion caused by loading.

How is output impedance different from input impedance?

Input impedance is what the source feeding a circuit sees, while output impedance is what the load sees coming from the circuit. They affect different sides of a connection, but both matter for signal transfer. A good match usually means high input impedance and low output impedance.

What circuit types have low output impedance?

Buffer stages like common collector BJTs and common drain FETs are classic low-output-impedance circuits. They are built to drive a load without much voltage loss. That is why they show up when a weak signal source needs to feed a heavier load.