Capacitance is a fundamental electrical property, the ability to store electric charge. For the majority of standard capacitors encountered in electronics, this ability remains a fixed characteristic of the device, unaffected by changes in the applied voltage. However, certain specialized capacitor types are engineered to vary with voltage. This nuanced behavior is crucial for various advanced electronic applications.
Understanding Capacitance Fundamentals
A capacitor consists of two conductive plates separated by an insulating material, known as a dielectric. Physical attributes like plate area, distance, and dielectric material determine its capacitance. Once a capacitor is manufactured, these physical dimensions and material properties are set, establishing its inherent capacity to store charge. This process can be visualized like a water tank, where the tank’s physical dimensions define its maximum water storage capacity.
Capacitance in Typical Circuit Components
For most capacitors widely used in everyday electronic devices, such as ceramic, film, and electrolytic types, capacitance is considered a stable, predetermined value. The relationship between charge (Q), capacitance (C), and voltage (V) in a capacitor is expressed by the formula Q = CV. When the voltage across an ideal capacitor changes, the amount of charge it stores changes proportionally, while its capacitance itself remains constant. While exceeding a capacitor’s maximum voltage rating can lead to its failure, this does not alter its fundamental capacitance value; instead, it typically results in damage or destruction of the component.
Instances Where Capacitance Changes
Despite the general rule, there are specific capacitor designs where capacitance intentionally varies with the applied voltage. This voltage dependence is a key feature for their specialized functions. Two notable examples include varactor diodes and ferroelectric capacitors.
Varactor diodes
Varactor diodes, also known as varicaps, are semiconductor devices that exploit the voltage-dependent capacitance of a reverse-biased PN junction. By changing the reverse voltage across the diode, the width of a non-conductive region called the depletion region is altered. As the reverse voltage increases, the depletion region widens, decreasing capacitance. Conversely, reducing voltage narrows this region, thereby increasing the capacitance.
Ferroelectric capacitors
Ferroelectric capacitors use special dielectric materials whose permittivity changes significantly with the applied electric field and voltage. These materials exhibit a spontaneous electric polarization that can be reversed by an external electric field. This causes their capacitance to decrease as the applied DC voltage increases, due to the alignment of internal dipoles restricting further charge change. Such capacitors are distinct from traditional ones because their dielectric properties are inherently nonlinear with respect to voltage.
Practical Considerations and Applications
Understanding capacitance behavior with voltage is crucial for circuit design. Fixed-value capacitors, with their predictable and stable capacitance, are widely used in applications requiring consistent performance, such as filtering electrical noise, setting timing intervals, and storing energy in power supplies. Their unchanging nature ensures reliable operation in many common electronic devices.
Conversely, components exhibiting voltage-dependent capacitance enable unique functionalities. Varactor diodes are integral to tunable circuits, including radio tuners and voltage-controlled oscillators, where adjusting a voltage allows for precise frequency modulation or tuning. This capability is vital in communication systems like mobile phones and wireless routers. Ferroelectric capacitors, leveraging their distinct voltage response and memory characteristics, find applications in non-volatile memory (FeRAM) and advanced sensor technologies, such as medical ultrasound machines and high-quality infrared cameras. Their ability to change capacitance with voltage makes them suitable for specialized memory and sensing functions.