Active vs. Passive Transducers: Key Differences Explained

This article compares active and passive transducers, highlighting their key differences and providing examples of each type.

Introduction

Transducers play a crucial role in converting physiological signals from the human body into measurable forms. These signals can originate internally (e.g., blood pressure), emanate from the body (e.g., infrared radiation), or be derived from tissue samples (e.g., blood). Physiological signals are typically categorized by:

• Biopotential
• Flow
• Pressure
• Dimensions
• Displacement (including velocity, force, and acceleration)
• Impedance
• Temperature
• Chemical composition

Biomedical instruments rely on various transducers to measure different human parameters, aiding in the diagnosis and treatment of diseases. Physiological signals, based on their type of energy, can be further classified as bioelectrical, biochemical, bioacoustic, biomagnetic, bioimpedance, or bio-optical.

What is a Transducer?

A transducer is a device that converts one form of variable or energy into another. Often, it’s necessary to convert physiological variables into electrical signals, as these are easily processed. The relationship between input and output variables can be linear, logarithmic, or follow a square law.

Transducers should be designed to extract minimal energy when interfaced with the body and ideally be non-invasive.

Transducers are broadly classified as either active or passive based on how they convert non-electrical variables into electrical signals.

Active Transducers

Active transducers directly convert the input variable into an electrical signal. They are popular due to the ease with which modern digital computers can process electrical signals.

Examples of Active Transducers:

• Moving Coil Generator:
  • Working Principle: The motion of a coil within a magnetic field induces an output voltage.
  • Application: Measures velocity and vibration.
• Photovoltaic Cell:
  • Working Principle: Voltage is generated in the semiconductor junction of a solar cell when stimulated by radiant light energy.
• Thermocouple:
  • Working Principle: An electromotive force (EMF) is generated across the junctions of two dissimilar metals when one junction is heated and the other is cooled.
  • Application: Measures temperature, radiation, and heat flow.
• Piezoelectric Transducer:
  • Working Principle: An EMF is generated when an external force is applied to a crystalline material like quartz.
  • Application: Measures sound, acceleration, vibration, and pressure variation.

Passive Transducers

Passive transducers modify either the excitation voltage or modulate carrier signals. Unlike active transducers, they require an external power source to operate. Passive transducers typically consist of passive circuit elements (resistors, capacitors, inductors) that change their value as a function of the physical variable or signal being measured.

Examples of Passive Transducers:

• Resistive Transducers: Potentiometers, strain gauges, photo-resistive cells, and photodiodes.
• Inductive Transducers: Inductance of a coil changes by varying its physical dimensions or the permeability of its magnetic core. Examples include induction displacement transducers, variable reluctance transducers, and LVDTs (Linear Variable Differential Transducers).
• Capacitive Transducers: Capacitance depends on the area of the parallel plates and the distance between them. An example is a passive capacitance transformer.

Tabular Difference Between Active and Passive Transducers