Variable capacitors are adjustable components that play a crucial role in many electronic circuits. There are different variable capacitor types and their adjustability makes them suitable for various applications including tuning circuits, oscillators, and filters. The working principle of a variable capacitor entails varying the distance between plates or the overlap of plates to change the capacitance. In this article, we discuss what variable capacitors are, how they work, and their common applications in electronic systems. See our article on types of capacitors to learn about other varieties.
What is a variable capacitor?
Some electronic applications require components that can adjust circuit properties in real-time. Variable capacitors fulfill this need by allowing users to fine-tune capacitance to achieve desired circuit behavior. They are essential in systems where precision tuning is critical, such as radio frequency (RF) circuits, oscillators, and filters. Figure 1 shows a variable capacitor

Variable capacitors operate by altering the physical configuration of their plates or dielectric material. Changing the distance between the plates or varying the overlap area adjusts the capacitance. For example, in air-variable capacitors, a rotating plate adjusts the overlap with a fixed plate, while in trimmer capacitors, a screw mechanism changes the plate spacing. This adjustability enables precise control over circuit parameters, which is critical in tasks like frequency tuning and impedance matching.
Variable capacitors and fixed capacitors differ primarily in their adjustability. While variable capacitors allow users to modify their capacitance values dynamically, fixed capacitors have a constant capacitance value. Furthermore, variable capacitors are typically used in low-power circuits, while fixed capacitors are widely used in both low- and high-power applications due to their robust construction and broader range of capacitance values.
Variable capacitor symbol
Different symbols are sometimes used to represent different variable capacitor types. These symbols may differ slightly based on regional standards or schematic conventions. Figure 1 shows some of the common variable capacitors including trimmer capacitor symbol (preset capacitor symbol) and dual (ganged) capacitor symbol.

How a variable capacitor works
Variable capacitors adjust their capacitance by changing the physical configuration of their internal components, such as the distance or overlap between plates. Air variable capacitors, one of the most common types, consist of a set of fixed plates (stator) and a set of movable plates (rotor). By rotating the rotor, the overlap between the plates changes, allowing users to adjust the capacitance.
Vacuum variable capacitors, on the other hand, enclose the plates in a vacuum-sealed housing. This design minimizes energy losses and allows the capacitor to operate at higher voltages and frequencies. They commonly find applications in high-power RF circuits and industrial applications.
Trimmer capacitors are small adjustable capacitors suitable for fine-tuning. Ceramic trimmer capacitors feature a ceramic dielectric material placed between two conductive plates. The adjustment mechanism typically involves a screw or slider that moves the plates relative to each other, changing the capacitance value.
Film trimmer capacitors use a thin plastic film as the dielectric material. The arrangement of their conductive plates allows adjustments by altering the overlap between them or their distance from the film. This construction ensures consistent performance and precise tuning capability.
Variable capacitor types and uses
Variable capacitors can be classified based on the principle they use to adjust capacitance. The two most common variable capacitor types are mechanically controlled variable capacitors and electronically controlled variable capacitors. To start with, mechanically controlled variable capacitors adjust capacitance by physically changing the position of the plates or dielectric material. This category includes air variable capacitors, vacuum variable capacitors, and trimmer capacitors
Electronically controlled variable capacitors can be categorized into two types: voltage-tuned capacitors, such as varactors, and digitally tuned capacitors, like MIM (Metal-Insulator-Metal) capacitors. Varactors change their capacitance based on reverse-bias voltage, while MIM capacitors use digital control to modulate capacitance. Both types provide precise tuning for modern electronic applications.
1. Mechanically controlled capacitors
a. Air variable capacitors
Air variable capacitors use air as the dielectric and feature a lightweight construction and high voltage handling capabilities. These capacitors mainly find uses in radio tuning circuits, where their adjustability allows precise frequency selection. Their simple design and high reliability make them suitable for applications requiring minimal dielectric loss.
b. Vacuum variable capacitor
Vacuum variable capacitors utilize a vacuum as the dielectric, offering excellent insulation and the ability to handle very high voltages. They typically find uses in high-power RF applications, such as transmitters and industrial heating systems. Their robust construction and low dielectric losses make them ideal for demanding environments where durability and precision are critical.
c. Trimmer capacitors
Trimmer capacitors are typically small and are suitable for fine-tuning circuits. Engineers commonly use ceramic trimmer capacitors in compact devices like oscillators and RF circuits due to their stability and reliability. Film trimmer capacitors, on the other hand, are used in applications requiring greater precision and durability, such as communication equipment and instrumentation.
2. Electronically controlled capacitors
a. Varactors
Varactors, also known as varicaps or voltage-controlled capacitors, are semiconductor devices that act as variable capacitors. Their capacitance varies in response to changes in the reverse bias voltage applied across them. As the reverse voltage increases, the capacitance decreases, and vice versa. Varactors find applications in frequency tuning, RF circuits, voltage-controlled oscillators (VCOs), and phase-locked loops (PLLs), where precise control over capacitance is needed. Unlike mechanical variable capacitors, varactors offer faster response times and compact size.
b. MIM capacitors
MIM capacitors are commonly used in integrated circuits (ICs), especially in RF (radio frequency) applications. The structure consists of two metal layers separated by an insulating dielectric material. These capacitors offer high capacitance density, small size, and stable performance across a wide range of frequencies. They find applications in analog and mixed-signal circuits, such as in filters, oscillators, and tuning circuits.
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