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The mysterious connection of capacitors: Do you know why adjacent conductors form invisible capacitors?
As electronic technology advances day by day, as the frequency of circuits continues to increase, the problem of parasitic capacitance has attracted more and more attention. Parasitic capacitance, also known as stray capacitance, is the unavoidable and often undesirable capacitance caused by the close proximity of electronic components or circuit components. The process of capacitance formation is closely related to the electric field effect between conductors. In this article, we will delve into the definition of parasitic capacitance, its formation mechanism, and its impact on high-frequency circuits.
The formation of parasitic capacitance
When two conductors at different voltages are brought close to each other, they are affected by each other's electric fields and store opposite charges, forming a capacitor-like structure. This means that by simply changing the voltage between the conductors, we need a corresponding current in and out of the conductors to charge or discharge:
i = C dV/dt
Where, C represents the capacitance between conductors. In many cases, such as an inductor, it often exhibits the effect of containing parallel capacitance because its close windings create parasitic capacitances. When a voltage difference exists across the windings of an inductor, adjacent wires charge at different voltages, causing them to behave capacitively.
The influence of parasitic capacitance
In low-frequency circuits, parasitic capacitance is usually negligible, but in high-frequency circuits, it can become a major problem. Especially in amplifier circuits with extended frequency response, parasitic capacitance between the output and input may act as a feedback path, causing the circuit to generate unnecessary oscillations at high frequencies. This phenomenon is called parasitic oscillation.
In high-frequency amplifiers, parasitic capacitance combines with stray inductances such as component leads to form a resonant circuit, further causing parasitic oscillations.
In all inductors, the parasitic capacitance will resonate with the inductor at a certain high frequency point, causing the inductor to exhibit self-resonant characteristics, which is called the self-resonant frequency. When the frequency exceeds this range, the response of the inductor becomes capacitive.
Design challenges and solutions
High frequency circuits require special design techniques to reduce the impact of parasitic capacitance. These include careful separation of components and wires, guard rings, ground planes, power planes, and shielding between inputs and outputs. In addition, the use of line terminations and strip lines can also effectively reduce its impact.
In closely spaced cables and computer buses, parasitic capacitive coupling can cause crosstalk, allowing signals from one circuit to flow into another, causing interference and unstable operation.
With the advancement of technology, electronic design automation software can now calculate the parasitic capacitance of components and circuit board traces and include these effects in simulations of circuit operation, a process called parasitic extraction.
The influence of Miller capacitance
In an ideal inverting amplifier, accumulated parasitic capacitance creates a feedback impedance between the input and output. Even a small parasitic capacitance will be very important due to the presence of gain due to the Miller effect. This causes the amplifier's frequency response to be limited and behave like a low-pass filter.
The voltage gain of modern transistors can often reach ten to more than a hundred times, so the limitation of Miller capacitance on high-frequency performance cannot be ignored.
As early as 1920, Miller had proposed the concept of parasitic capacitance of vacuum tubes. In today's bipolar junction transistors, the parasitic capacitance between the base, collector, and emitter also shows dependence on voltage changes, which further highlights the importance of parasitic capacitance in frequency response.
Summary and reflection
Although parasitic capacitance is an unavoidable presence, through proper circuit design and system thinking, we can minimize its impact on circuit performance. As technology advances, so does the understanding of and countermeasures for parasitic effects. Are you ready to take on the challenge and minimize the impact of parasitic capacitance?
