PCB Noise Reduction with Linear Devices in Electronics
When I first got into electronics and PCB design as a student, I had the idea that noise suppression required a lot of elaborate (and expensive) equipment. I had visions in my mind of building complicated mainframe-style systems for the sole purpose of coaxing the perfect signal from a device. Sure, you can go that route if you like. But like many aspects of engineering, the best solution is often the simplest.
Most PCB designers are considering the linear or nonlinear nature of electronic components without realizing it. Many design rules can be justified by understanding the difference between linear and nonlinear components. Understanding how these devices differ will help engineers and designers create interesting new devices or improvements on old components.
Noise is especially important in nonlinear devices. A nonlinear amplifier can output a signal with significant noise amplification if noise is present on the input. Thankfully, there are a number of strategies that can be used to reduce noise in PCBs using simple linear devices.
Linear vs. Nonlinear Devices
The distinction between linear and nonlinear devices is very important in electronics. In short, a linear electronic device (also known as a linear circuit) relates an input signal to an output signal using a linear mathematical function. If the strength of the input voltage or current is doubled, the output signal is also doubled.
If this is too mathematical for you, then consider an example with a sinusoidal AC signal. If a sine wave signal is applied to a linear device, then the output will also be sinusoidal. Any circuit that is composed exclusively of ideal resistors, capacitors, and/or inductors is a linear circuit. Other examples of linear circuits are simple amplifiers, differentiators, integrators, some filters, and unsaturated operational amplifiers.
Contrast this with nonlinear circuits. A nonlinear circuit applies some nonlinear mathematical function to the input signal. As an example, consider a full-wave rectifier. This circuit applies the absolute value function to an AC signal.
Other examples of nonlinear devices include diodes, low noise amplifiers, and a number of other transistor-based devices. Transistors are unique in that they are approximately linear when the input voltage is low. When the input voltage is high, the output saturates, and the transistor behaves like a nonlinear circuit.
Vacuum tubes: the earliest nonlinear circuit element
Noise Reduction with Linear Devices
Power supply noise in sensitive systems has been a problem for low power and RF designers. In the past, a regulator would need to be built directly on a PCB board. Now regulators come packaged in ICs and have been available for decades. Despite the fact that these ICs have been around for so long, regulator ICs available today still generate the same level of noise as regulator ICs that were available in the 1970’s.
With low power design becoming more popular, systems require even lower noise figures than can be provided by regulator ICs. PCBs with wireless capabilities need ultra-low phase noise oscillators to be able to transmit and receive digital data with high throughput. Let’s not forget about ringing in audio devices, where noise becomes particularly evident (and annoying) once it reaches an amplifier and is sent to a speaker.
There are some unique linear circuits that can be used to lower the noise floor in your power supplies. An excellent circuit that can remove residual ripple voltage and other noise sources is a capacitance multiplier. A capacitance multiplier combines resistors, capacitors, and a transistor in a linear circuit. This circuit causes a small capacitor to behave as if it were a much larger capacitor. A transistor with a higher beta value creates a larger multiplication factor.
You might be asking, why shouldn’t I just use a single large capacitor? The first reason is the footprint of a typical capacitor. For a given insulator and voltage rating, capacitance scales with the area of the plates. The large capacitance values that can be attained with a multiplier circuit would require huge capacitors (both in size and in capacitance). The capacitance multiplier also has the advantage of better removal of harmonic content compared to a large capacitor.
The transistor plays an important role as the output current gain will determine the largest multiplication factor that can be achieved with a capacitance multiplier. The transistor must be operating in the linear region. If the transistor saturates, gain is reduced and the capacitance multiplier becomes less effective.
Capacitors of various sizes
Even though transistors are typically used in capacitance multiplier circuits, an operational amplifier can also be used instead of a transistor. The operational amplifier must be operating in the linear regime, i.e., the inputs must be unsaturated. The capacitance multiplier circuit can achieve much larger multiplication by forming it into a Darlington pair.
Higher order filtering might require nonlinear components (usually diodes). Another option is to combine the capacitance multiplier into a loop filter. This allows higher order filtering using only linear components.
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