Input impedance

Input impedance is how much resistance a circuit presents at its input terminal, measured in ohms. In Intro to Electrical Engineering, it tells you how strongly an amplifier or device loads the source feeding it.

What is the input impedance?

Input impedance is the effective opposition a circuit presents to a signal at its input, usually measured in ohms. In Intro to Electrical Engineering, you use it to predict how a source and the next stage will interact, especially in amplifier circuits and sensor interfaces.

Think of it as the load seen by the signal source. If the input impedance is high, the circuit draws very little current from the source, so the source voltage stays closer to what it was meant to be. If the input impedance is low, the source has to supply more current, and part of the signal can drop across the source resistance instead of reaching the input.

That drop is the loading effect. A simple voltage source with a series resistance feeding an amplifier can behave like a voltage divider, where the amplifier input is one leg of the divider. This is why input impedance is not just a property you label on a schematic, it changes the actual signal level that reaches the device.

For FET amplifiers, the gate draws almost no current, so the input impedance is usually very high, often in the megaohm range. That makes common drain and common source FET stages useful when you want to connect to weak or delicate sources, like a sensor or a previous high-impedance stage.

BJT amplifiers usually have lower input impedance because the base-emitter junction needs current. In common emitter or common base configurations, that can mean the source sees more loading. Designers sometimes add buffer stages or choose a FET front end when they need the next stage to listen without disturbing the one before it.

Why the input impedance matters in Intro to Electrical Engineering

Input impedance shows up anytime you connect one circuit to another and want the first circuit to keep its signal shape. In Intro to Electrical Engineering, that means amplifier design, cascading stages, sensor reading, and even early signal analysis problems where you track why a voltage changes when a new block is attached.

If you ignore input impedance, your calculated voltage can look fine on paper but fail in the real circuit. A source with significant resistance feeding a low-input-impedance amplifier may lose amplitude, distort a measurement, or behave differently from the ideal model. That is a common reason lab results do not match a quick hand calculation.

It also helps you compare transistor types. A FET input usually looks like a very light load, while a BJT input often demands more current. That difference is one reason FETs are popular for input stages and why BJTs may need biasing resistors and careful source matching.

You will also use this idea when deciding whether a buffer stage is needed. If the next block should read a signal without changing it much, a high-input-impedance stage is the safer choice. If the goal is to drive a lower-impedance load, then input and output impedance together become part of the design tradeoff.

How the input impedance connects across the course

Output Impedance

Output impedance is the flip side of input impedance. When one stage drives another, you care about both numbers because the source’s output impedance and the next stage’s input impedance form the interaction that sets signal loss and loading. A good match here is about preserving the signal, not just connecting wires.

Loading Effect

Loading effect is what you see when the next circuit changes the behavior of the one before it. Low input impedance increases loading, which can reduce voltage at the input and make a source act weaker than expected. This is one of the main reasons input impedance gets attention in amplifier problems.

Voltage Divider

A voltage divider is the simplest model for understanding input impedance in a real circuit. If the source resistance and input impedance are both significant, the input voltage is set by their ratio. That makes divider calculations a fast way to estimate whether a stage will preserve or reduce the signal.

common drain amplifier

A common drain amplifier, also called a source follower, is often used as a buffer because it has high input impedance and low output impedance. The point is not large voltage gain, but letting one circuit read another circuit without loading it down too much.

Is the input impedance on the Intro to Electrical Engineering exam?

A quiz or problem-set question will usually ask you to compare two amplifier stages, predict which one loads a source more, or decide whether a signal will be reduced by a voltage divider effect. You may be given a simple source resistance and an input impedance, then asked to find the voltage at the input node.

In lab work, you might measure a sensor output before and after attaching an amplifier, then explain why the reading changed. If you see a FET gate versus a BJT base in a circuit, input impedance is one of the first clues for how the stage will behave. A strong answer ties the number to the circuit consequence: less loading, more loading, or a better choice for buffering.

The input impedance vs Output Impedance

Input impedance is what a circuit presents to the signal coming in, while output impedance is what it presents to the next circuit it drives. They are easy to mix up because both describe resistance-like behavior, but they apply to opposite sides of the device.

Key things to remember about the input impedance

• Input impedance is the effective resistance a circuit presents at its input, and it is usually measured in ohms.

• High input impedance means the circuit draws little current from the source, so the source signal is less likely to be reduced.

• Low input impedance increases loading effect and can create a voltage divider with the source resistance.

• FET amplifiers usually have much higher input impedance than BJT amplifiers.

• When a stage must read a signal without disturbing it, a buffer or a high-input-impedance design is usually the better choice.

Frequently asked questions about the input impedance

What is input impedance in Intro to Electrical Engineering?

It is the effective resistance a circuit presents to a signal entering its input terminal. In this course, you use it to predict whether an amplifier or device will load the source and reduce the voltage that reaches the next stage.

Why do FET amplifiers usually have higher input impedance than BJT amplifiers?

A FET gate draws almost no input current, so the input looks very light to the source. A BJT base needs current to control the transistor, so its input is usually lower impedance and more likely to load the source.

How does input impedance affect a voltage divider?

If the source has its own resistance, the input impedance becomes part of a voltage divider. A lower input impedance means more voltage drops across the source resistance, so less of the original signal reaches the circuit input.

When would you choose a high input impedance stage?

Use one when the source is weak, delicate, or high impedance, such as some sensors or a previous amplifier stage. A buffer stage or a FET input often works well because it lets the signal pass with less loading.