Input and output impedance – A Level Electronics WJEC Revision

Understanding Input and Output Impedance

In the context of electronics, impedance is the overall opposition a circuit presents to the flow of current at a particular frequency.
It comprises both resistance and reactance, and its unit of measurement is Ohms (Ω).
Input impedance refers to the impedance that an electronic circuit presents to the connected devices at its input terminals.
Similarly, output impedance is the impedance the circuit presents at its output terminals.
A circuit with a high input impedance draws very little current from the source, preserving the original signal, making it an ideal input characteristic in amplifiers.
In contrast, a low output impedance is desirable in amplifiers because it ensures maximum power transfer to the load and minimises signal distortion.

Impact of Impedance on Signal Integrity

Impedance mismatching can lead to significant problems, such as signal reflections, which can significantly degrade signal integrity.
It’s important that the output impedance of a signal source matches the input impedance of the next stage, or the load, particularly in systems that process high-frequency or fast-switching signals.
To ensure maximum power transfer, the characteristic impedance of the transmission line (if any) should also match the source and load impedances, observing the Maximum Power Transfer Theorem.

Buffer Amplifiers and Impedance Matching

A buffer amplifier can be used when impedance matching is necessary but cannot be achieved directly.
Buffer amplifiers provide high input impedance to prevent loading the source and low output impedance to drive the load effectively.
Examples of buffer amplifiers include voltage followers (a type of unity gain amplifier) and emitter followers in transistor-based circuits.

Impedance in Audio and RF Systems

In audio and RF (radio frequency) systems, incorrect impedance matching can lead to issues like signal loss, distortion, or potential damage to the equipment.
Sound professionals typically use audio transformers to match output impedance of an audio source to the input impedance of the speaker or microphone, ensuring maximum and undistorted signal transfer.
For RF systems, devices such as antenna tuners or matching networks are used to match the impedance of the transmitting antenna to the characteristic impedance of the transmission line.

Measuring Impedance

Impedance measurements often require the use of specialized equipment such as impedance meters or network analysers, particularly at higher frequencies.
In some cases, techniques such as the voltage drop method can be used to measure impedance, where a known current is passed through the circuit and the voltage drop across it is measured.