Standard noise in electronic device circuits can generally be considered as a collective term for all signals other than the overall target signal.
Initially, the electronic signals that caused noise in microphones and other recording devices were called noise. However, the adverse effects of some unintentional electronic signals in electronic circuits are not all related to sound, so the definition of noise has gradually expanded.
How is noise generated in circuits? Noise is ubiquitous in circuits, but we can systematically understand its sources. Noise sources inherent in circuits themselves are called intrinsic noise. Intrinsic noise mainly includes thermal noise, shot noise, and contact noise.
First, thermal noise is generated by the thermal disturbance of electrons inside the resistor. This noise exists in all components containing resistors, and its voltage value is very little affected by temperature. Thermal noise is a type of resistor noise and one of the main sources of noise in circuits.
Secondly, shot noise is related to the current flowing through the potential barrier. It is caused by current fluctuations resulting from random electron (or hole) emission. This noise exists in vacuum tubes and semiconductors. In vacuum tubes, it originates from the random emission of cathode electrons; while in semiconductors, it is generated by the random diffusion of charge carriers and the random generation and recombination of hole-electron pairs.
Finally, contact noise is caused by changes in conductivity resulting from imperfect contact between two materials. This noise can occur at any contact point in a circuit, such as switches and relays. Contact noise also occurs in transistors and diodes, as well as in composite resistors formed by many particles molded together.
Besides the three types of inherent noise mentioned above, other types of noise, such as electromagnetic interference, may exist in a circuit. These noises can all affect the performance and stability of the circuit. Therefore, when designing a circuit, appropriate measures need to be taken to reduce the impact of noise in order to improve the circuit's performance and reliability.
To reduce thermal noise voltage, we can take measures such as reducing the system's resistance and bandwidth, lowering the circuit's operating temperature, or using parametric amplifiers. Shot noise has a constant power density with frequency and its amplitude follows a Gaussian distribution. Contact noise becomes very large at low frequencies and is often the most significant noise source in low-frequency circuits. Understanding the characteristics of these noises helps us to better design and optimize circuits.
For example, harmonic distortion or self-oscillation in the switching power supply voltage can potentially cause adverse effects on the circuit, resulting in microphones transmitting noise or causing circuit malfunctions. However, sometimes these adverse effects may not occur. This type of harmonic distortion or oscillation should be called circuit noise. There are also electromagnetic signals of a certain order. For one receiver that must receive this signal, it is a normal target signal; for another receiver, it is a non-target signal, i.e., noise.
The technical term "interference" is often used in microelectronics, and sometimes it's confused with the concept of noise. However, they are different. Noise is the signal from an electronic device, while interference refers to a certain effect, a side effect of noise on a circuit. A circuit may contain noise, but not necessarily interference. In data circuits, you can often observe small peaks mixed in with a digital oscilloscope within a normal pulse signal. These are not expected; they are actually noise. However, due to the characteristics of the circuit, these small peaks do not affect the logic of the data circuit and will not cause errors, so they can be considered as interference.
When the noise voltage is large enough to interfere with the circuit, this noise voltage is called the interference voltage. Alternatively, the larger noise voltage required for circuit components to function normally is called the interference immunity or immunity of the circuit or component. Generally, noise is difficult to eliminate, but we can try to reduce the noise's intensity or the circuit's immunity to make it less likely to cause interference.
Noise in switching power supplies: For linear regulated power supplies, the initial low frequency of 50Hz is a significant source of interference. Because the AC current exiting the primary stage is impure and has a sinusoidal waveform, it easily causes electromagnetic interference (EMI) to surrounding circuits, i.e., electromagnetic noise. In switching power supply circuits, the noise is even more severe. Switching power supply circuits operate at high frequencies, outputting a portion of the output current and voltage containing dirty harmonics, which will cause significant noise to the entire circuit.
How can noise be suppressed in electronic circuits?
This noise is primarily generated by the digital circuitry and power supply components. High-frequency digital levels are prevalent in digital circuits, and these levels can generate two types of noise:
1. Electromagnetic radiation, like a television antenna, interferes with nearby circuits by emitting electromagnetic waves, which is what you call noise.
2. Coupled noise refers to the coupling between digital circuits and adjacent circuits. The noise can directly affect other circuits in the appliance, and this type of noise is more severe.
Noise present in the power supply: For linear power supplies, the low-frequency 50Hz is a significant source of interference. Because the incoming AC current at the primary side is inherently impure and is a wavy sine wave, it easily generates electromagnetic interference, or electromagnetic noise, in adjacent circuits. The noise is even more severe in switching power supplies. Switching power supplies operate at high frequencies and have very dirty harmonic voltages at the output, all of which contribute significantly to the noise of the entire circuit.
Prevention methods include: proper grounding, using differential transmission for analog signals, adding decoupling capacitors at the circuit's power output, employing electromagnetic shielding, separating analog and digital grounds, routing signal lines with ground wires on both sides, and isolating ground lines, etc. In fact, what I've mentioned is just the tip of the iceberg when it comes to noise reduction. Even someone with 30 years of experience in electronics wouldn't fully grasp all these techniques, as understanding and mastering them requires a strong technical foundation and considerable experience. However, what I've told you is generally sufficient.
The noise floor is caused by the circuit itself, due to impurities in the power supply, inappropriate phase and gain margins, and other inherent circuit and component issues. This needs to be addressed during the circuit design phase.
Other noises are caused by factors such as unreasonable circuit layout and wiring, electromagnetic compatibility, and interference between wires.
Noise reduction in analog circuits relies more on experience than scientific evidence. Designers often find that after designing the analog hardware, the circuit is too noisy, forcing them to redesign and rewire. This "trial and error" approach can sometimes succeed after some trial and error. However, a better way to avoid noise problems is to follow basic design principles and apply fundamental noise-related principles when making decisions early in the design process.
Design method of low noise preamplifier circuit
Preamplifiers play a crucial role in audio systems. This article first explains how engineers should properly select components when designing preamplifiers for home audio systems or PDAs. It then analyzes noise sources in detail, providing guidelines for designing low-noise preamplifiers. Finally, using a PDA microphone preamplifier as an example, it outlines the design steps and related considerations.
A preamplifier is a circuit or electronic device placed between the signal source and the amplifier stage, such as an audio preamplifier placed between a CD player and the power amplifier in a high-end audio system. Preamplifiers are specifically designed to receive weak voltage signals from the signal source. The received signal is amplified with a low gain, and sometimes even adjusted or corrected before being sent to the power amplifier stage. For example, an audio preamplifier can equalize the signal and perform tone control. Whether designing a preamplifier for a home audio system or a PDA, a major challenge is determining the appropriate components to use.
Component selection principles
Operational amplifier integrated circuits (OPAs) are widely used in preamplifiers due to their small size and high performance. When designing preamplifier circuits for audio systems, it is crucial to understand how to select the appropriate specifications for the OPA. System design engineers frequently encounter the following issues during the design process:
1. Is it necessary to use a high-precision operational amplifier? The amplitude of the input signal level may exceed the error tolerance of the operational amplifier, which is unacceptable. If the input signal or common-mode voltage is too weak, the designer should use a high-precision operational amplifier with extremely low compensation voltage (Vos) and extremely high common-mode rejection ratio (CMRR). Whether to use a high-precision operational amplifier depends on the required amplification gain of the system design; the higher the gain, the more accurate the operational amplifier needs to be.
2. What supply voltage does an operational amplifier require? This depends on the dynamic voltage range of the input signal, the overall system supply voltage, and the output requirements. However, different power supply rejection ratios (PSRR) will affect the accuracy of the operational amplifier, with battery-powered systems being most affected. Furthermore, power consumption is directly related to the quiescent current of the internal circuitry and the supply voltage.
3. Does the output voltage need to be full swing? Low supply voltage designs typically require a full-swing output to fully utilize the entire dynamic voltage range and expand the output signal swing. Regarding the issue of full-swing input, operational amplifier circuit configurations offer their own solutions. Since preamplifiers generally use inverting or non-inverting amplifier configurations, a full-swing input is not required because the common-mode voltage (Vcm) is always less than or equal to zero in the output range (with very few exceptions, such as single-supply operational amplifiers with floating ground).
4. Is the gain bandwidth issue even more worrying? Yes, especially for audio preamplifiers, this is a very worrying problem. Since human hearing can only perceive sounds in a frequency range of approximately 20Hz to 20kHz, some engineers neglect or underestimate this "narrow" bandwidth when designing audio systems. In fact, important technical parameters reflecting the performance of audio devices, such as low total harmonic distortion (THD), fast slew rate, and low noise, are all essential conditions for high-gain bandwidth amplifiers.
In-depth understanding of noise
Before designing a low-noise preamplifier, engineers must carefully examine the noise originating from the amplifier. Generally, operational amplifier noise mainly comes from four sources:
1. Thermal noise (Johnson): Thermal noise with broadband characteristics is generated by irregular fluctuations in the electron energy of the current in an electrical conductor. The square of its root mean square voltage is directly related to the bandwidth, the resistance of the electrical conductor, and the absolute temperature. For resistors and transistors (such as bipolar and field-effect transistors), since their resistance is not zero, the influence of this type of noise cannot be ignored.
2. Flicker noise (low frequency): Noise generated by the continuous generation or integration of charge carriers on the crystal surface. In the low-frequency range, this flicker noise appears as low-frequency noise. Once it enters the high-frequency range, this noise becomes "white noise". Flicker noise is mostly concentrated in the low-frequency range and can interfere with resistors and semiconductors. The interference experienced by bipolar chips is greater than that experienced by field-effect transistors.
3. Shooting noise (Schottky): Schottky noise is generated by current carriers with particle-like characteristics within the semiconductor. The root mean square value of this current is directly related to the average bias current and bandwidth of the chip. This type of noise has broadband characteristics.
4. Popcorn frequency: This noise is generated when the surface of a semiconductor becomes contaminated. Its effects can last from milliseconds to seconds. The cause of this noise is still unknown, and it does not exhibit a fixed pattern under normal circumstances. Using cleaner processes during semiconductor manufacturing can help reduce this type of noise.
Furthermore, because different operational amplifiers employ different input stage structures, the differences in transistor structure lead to significant variations in the noise levels of different amplifiers. Below are two specific examples.
Noise in a bipolar input operational amplifier: The noise voltage is mainly caused by the thermal noise of the resistor and the high-frequency shooting noise of the input base current. The low-frequency noise level depends on the low-frequency noise generated by the base current of the input transistor flowing into the resistor. The noise current is mainly generated by the shooting noise of the input base current and the low-frequency noise of the resistor.
Noise in CMOS input operational amplifiers: Noise voltage is mainly caused by thermal noise from the channel resistor in the high-frequency region and low-frequency noise in the low-frequency region. The corner frequency of a CMOS amplifier is higher than that of a bipolar amplifier, and its broadband noise is also much higher. Noise current is mainly generated by the shooting noise of the input gate leakage. The noise current of a CMOS amplifier is much lower than that of a bipolar amplifier, but for every 10°C increase in temperature, its noise current will increase by about 40%.
Engineers must have a deep understanding of noise issues and perform extensive calculations to accurately represent this noise in numerical terms. To avoid complicating the issue, only a few of the most critical parameters from the audio specifications will be selected here.
In the above equation, S and N both represent power.
PDA microphone preamplifier circuit
Here we discuss how to design a microphone preamplifier suitable for PDAs. As mentioned above, we must understand that the signal source is the signal input to the preamplifier. First, we must know the following information: the type of microphone to be used.
Microphone output signal level
Microphone impedance and frequency of specified impedance
Gain specifications: The gain may be limited by the gain-bandwidth product of the operational amplifier.
Input signal frequency range
Noise regulations
For example, the technical specifications of a certain ceramic microphone are as follows: Impedance: 2.2kΩ (operating at a frequency of 1kHz)
Output signal: 200 (Vpp)
Audio input frequency range: 100Hz to 4kHz
Thermal noise: 2nV/(Hz) Preamplifier gain specification: 500 (non-inverting), first stage can reach 5 times gain, second stage can reach 100 times gain.
We use Formula 1:
Equal input noise (EIN) = Total input reference noise (%) × Input frequency range
Output noise = Equal input noise × Gain = 545.81nV × 5 = 2.73uV (applicable to level 1 gain) or 545.81nV × 100 = 54.58uV (applicable to level 2 gain).
Total output noise of the two amplification stages
The signal-to-noise ratio of a 1-volt output voltage is approximately 85.3 dB = 20 × log(1V ÷ 54.58uV).
The total output noise of the circuit is approximately the square root of the sum of the average root-mean-square values of each noise source. Furthermore, the majority of the output noise typically originates from the source with the highest noise level. Please note that this circuit is only suitable for single-supply designs. The input and output capacitors (C1 and C4) are optional; engineers can choose based on their specific needs. Applicability depends on the input and output connections of the user's system. If the microphone output has DC compensation, then input capacitor C1 is required to block the DC signal. The output capacitors can serve the same purpose.
Most microphones currently on the market are either high-impedance (around 2kΩ) or low-impedance (a few hundredkΩ). Both types of microphones can use the preamplifier design described above. High-impedance, high-output microphone preamplifiers are relatively simple and can be configured using either non-inverting or inverting amplifiers. Because their frequency response is relatively flat, no special equalization is needed, and the input level is relatively high, so the amplifier has very low noise requirements. However, high-impedance microphones are extremely sensitive to noise from unknown sources and magnetic fields. Low-impedance, low-output microphone preamplifiers can also use either non-inverting or inverting amplifiers to amplify the input signal. The requirements for frequency response and equalization are roughly the same as for high-impedance, high-output preamplifiers. If the microphone's output level is low, engineers must carefully select a low-noise operational amplifier. A good low-noise operational amplifier should produce low input reference voltage noise, and the noise should not exceed 10nV/(Hz).
Analysis and Measurement of Inherent Noise in Operational Amplifier Circuits
Noise can be defined as any unwanted signal in an electronic system. Noise can degrade audio signal quality and cause errors in accurate measurements. Board-level and system-level electronic design engineers want to determine the extent of noise in their designs under worst-case conditions, find ways to reduce noise, and develop measurement techniques to accurately verify the feasibility of their designs.
Noise includes inherent noise and external noise, both of which affect the performance of electronic circuits. External noise originates from external noise sources; typical examples include digital switching, 60Hz noise, and power switching. Inherent noise is generated by the circuit components themselves; the most common examples include broadband noise, thermal noise, and flicker noise. This series of articles will introduce how to predict the magnitude of inherent noise in a circuit through calculation, how to use SPICE simulation technology, and noise measurement techniques.
