PCB Components and Their Functions

Key Takeaways

• The core network analysis values: resistance, capacitance, and inductance.

• Linear passives and their circuit applications.

• How non-linear and active semiconductor devices differ in circuit usage.

Common PCB components and their functions define standard circuit operations

Learning network analysis is a bit like learning a language: just like how a base alphabet can build words, sentences, paragraphs, and so on, there are just a few discrete circuit elements needed to build a world of electronic features. While this is an incredibly invigorating discovery, it’s also arresting. How can a handful of PCB components and their functions encompass the whole of electronic design? It’s key to understand that electronic design is a broad topic with multiple sub-disciplines, and these basic components often have unique roles for each.

Major PCB Component Families 

Passive Linear PCB Components and Their Functions

Resistance, capacitance, and inductance at the lowest level of abstraction represent a component’s passive response to current and voltage in the system. Resistance is everpresent; in mechanical terms, it is akin to friction. Just as frictionless kinematics is a simplification of real-world surface interactions, zero electrical resistance is unachievable. A zero-ohm resistor – a readily purchasable component – would seem to contradict this, but these resistors are only meant to provide a resistance value that is negligibly (for most cases) close to zero. 

Capacitance and inductance are both present, although not necessarily at the same time. Both values represent the reactance of a component or the opposition to alternating current; resistance is the opposition to direct current, and the two measurements form the vector components of impedance. Unlike resistance, which is limited to negative values only in specific cases (non-static resistance and non-linear devices), reactance can readily be positive or negative, with the former representing inductance and the latter representing capacitance. Importantly, reactance is frequency-dependent, favoring capacitance and inductance at low and high frequencies, respectively. 

Every component contains some amount of resistance and reactance, the unintentional inclusion of either in a component that is not designed to provide that parameter is known as parasitics. With the understanding that idealized circuit elements don’t provide a complete picture of real-world circuit response, consider the three components that act as fundamental building blocks:

• Resistors - Resistors oppose the flow of direct current; for two resistors to achieve the same voltage drop across the terminals requires a proportionally larger current for the higher resistance value. One of the major roles in-circuit for resistors is to act as current limiters to sensitive components. However, the work performed in limiting current generates Ohmic heat according to its power dissipation; this heat must be thermally routed away from its point of generation and any thermally sensitive components to prevent malfunction, reduction of service life, or fire hazards. Resistors are also used to divide voltage, bias operating conditions, and terminate transmission lines.

• Capacitors - A capacitor stores energy in the form of an electric field by building a separation of charge between two plates. Capacitors charge and discharge equivalent to the product of their resistance and capacitance; smaller capacitors store less energy, but can respond more dynamically to evolving circuit conditions (and vice versa). Since capacitance grows at low frequencies and falls to zero at high frequencies (the exact values will depend on the individual values of the capacitor), they can be used as high-pass filters that block DC and pass sufficiently fast AC. Capacitors can also condition signals for less variance, tune antennae frequencies, and enable consistent, reliable power distribution across an electrical system. 

• Inductors - The inductor is, in many ways, a foil to the capacitor: it stores energy within a magnetic field, has its reactance increase at high frequencies, and filters AC signals while passing DC. Yet it also performs many of the same and even complementary functions of a capacitor like ripple filtering, tuning, and building switched-mode power supplies. An inductor is built by wrapping an insulated wire around and through a toroidal ferromagnetic core.

Non-Linearity and Semiconductor Devices

A circuit composed of only passive, linear elements can achieve a surprising amount of functionality, yet active semiconductor devices allow for even greater applications. Semiconductors exist in the space between conductors and insulators and their state is easily controlled following a doping process that introduces excess electrons, electron vacancies, and junctions between electron-rich (n-type) and electron-poor (p-type) regions. 

The most basic semiconductor device is the diode. It is a single p-n junction that exhibits polarization, conducting extremely well in one direction and poorly (ideally, not at all) in the other. In this sense, a diode functions similarly to a check valve in a fluid system, acting as a one-way passage for current while preventing backflow. Diodes serve a variety of roles in circuits including voltage regulation, tuning, oscillating, and luminescence.

Unlike the previous components discussed, a diode is a nonlinear element: its current and voltage do not scale proportionally but are instead related by an I-V curve. The lack of linearity is evident in the stepped regions of operation, like the threshold and avalanche voltage, that require a particular voltage minimum to be met in one of the junction directions before non-negligible conduction can occur. However, exponential growth in either direction of the I-V curve is unsustainable, leading to permanent damage in the breakdown region or Joule heating in the forward bias direction.

The other major semiconductor device family is the transistor. It also exhibits non-linearity like a diode, but it is an active element, able to convert other forms of energy to electrical energy. There are two major branches of transistors:

• Bipolar junction transistor (BJT) - A BJT is constructed as two diodes sharing a common semiconductor junction, i.e., a p-n-p or n-p-n design. These three doped semiconductor regions correspond to the three terminals of the BJT: the base, collector, and emitter. The base is made active with a biasing voltage that enables the conduction path between the collector and emitter. For optimal performance, the BJT is designed to conduct from collector to emitter, however, it can still conduct from emitter to collector with reduced gain. In the case of forward- or reverse-biasing at both base-emitter and base-collector junctions, the BJT can function as logically high or low, respectively.

• Field effect transistor (FET) - Like the BJT, the FET contains three terminals – the source, gate, and drain – which are analogous to the emitter, base, and collector. In contrast, FETs conduct with only one charge carrier (electron or electron-hole) instead of the mix of electrons and electron holes in BJTs. The FET holds a couple of advantages over BJTs in general power roles. First, it has a reduced draw since the gate terminal only consumes power when switching. This allows greater miniaturization of the package. Second, FETs are constructed symmetrically, making no distinction between a forward and reverse operation, gain, etc.

Cadence Software: Model Simulations for Circuit Solutions

PCB components and their functions are critical to forming a basis for network analysis. Designers will want to carefully understand the different roles these components play to build a more comprehensive schema for electronic design. With the fundamentals down, engineers and designers can leave the specifics of circuit simulation to powerful systems. With Cadence’s PCB Design and Analysis Software toolset, design teams have an all-encompassing electronic system design package that can handle the intricacies of circuit design just as well as the complexities. Combined with the power and efficiency of OrCAD PCB Designer for system layout, design teams can quickly move from prototype to production in minimal time.

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