The nominal noise in an electronic circuit can be generally considered to be a general term for all signals other than the destination signal.

At first people called the electronic signals that caused the noise of such sound equipment as radios. However, not all the effects of non-purpose electronic signals on electronic circuits are related to sound, so the concept of noise has since been gradually expanded.

For example, the electrical signals that cause the white stripes on a video screen are also called noise. It may be said that any signal in a circuit other than the destination signal, whether or not it affects the circuit, is called noise.

For example, ripples or self-excited oscillations in the supply voltage can have an adverse effect on the circuit, causing the hum of the audio device or causing the circuit to operate erratically, but sometimes not. This ripple or oscillation should be referred to as a noise of the circuit. There is a radio signal of a certain frequency, which is a normal target signal to one receiver, and a non-target signal to another receiver, i.e. noise.

The term interference is often used in electronics and is sometimes confused with the concept of noise. In fact, there is a difference. Noise is an electronic signal, and interference is a kind of effect, which is an adverse reaction to the circuit caused by noise. While there is noise in the circuit, there is not necessarily interference. In digital circuits. It is often observed with oscilloscopes that a small spike in the normal pulse signal is not expected, but rather a noise. However, due to the characteristics of the circuit, these small spikes do not cause the logic of the digital circuit to be affected and confused, so it can be considered as no interference.

When a noise voltage is large enough to disturb the circuit, it is called the interference voltage. And a circuit or a device, when it can still maintain the normal operation of the maximum noise voltage added, called the circuit or device anti-interference tolerance or immunity. Generally speaking, noise is difficult to eliminate, but it can be tried to reduce the intensity of noise or improve the immunity of the circuit, so that the noise does not cause interference.

This stuff is mainly produced by the digital circuit and the power supply part of the circuit. In digital circuits, high frequency digital levels are common, and these levels can produce two kinds of noise:

1. Electromagnetic radiation, just like the antenna of a TV, interferes with the circuit next to it by emitting electromagnetic waves, which is what you call noise.
2. Coupling noise. There is a certain coupling between the exponential circuit and the circuit next to it, and the noise can directly affect other circuits on the electrical appliances.

Noise on the power supply: if it is a linear power supply, first of all the low frequency of 50Hz is a serious source of interference. Because the primary AC itself is not pure, and is the sine wave of the wave, easy to generate electromagnetic interference next to the circuit, that is, electromagnetic noise. If it is a switching power supply, the noise is more serious. Switching power supply operates at high frequency, and there is a dirty harmonic voltage in the output part, which can produce a lot of noise to the whole circuit.

Preventive methods: reasonable grounding, using differential structure to transmit analog signals, adding decoupling capacitors at the output end of the power supply of the circuit, using electromagnetic shielding technology, analog and digital separation, signal lines on both sides of the bottom line, grounding isolation, and so on. What I’ve told you is just the tip of the iceberg in terms of noise reduction. Even if you’ve been in electronics for 30 years, you won’t know all of these things, because it takes a strong technical foundation and a lot of experience to understand these things, but what I’ve told you is generally enough.

Background noise is caused by the circuit itself. Due to the impurity of the power supply, the improper phase margin and gain margin of the circuit and other reasons of the circuit itself and the device. This part needs to be improved in the circuit design.

Other noise is due to improper circuit layout and wiring, electromagnetic compatibility, interference between conductors, and so on.

Noise cancellation in analog circuits depends more on experience than on scientific evidence. Designers often encounter situations where the analog hardware part of the circuit is designed, only to find that the noise in the circuit is so great that they have to redesign and rewire the circuit. This “try and see” approach to design can be successful after a few twists and turns. However, a better way to avoid noise problems is to follow some basic design principles and apply basic knowledge related to noise when making decisions early in the design process.

Design method of low noise preamplifier circuit

Preamplifier plays an important role in audio system. This article begins by explaining how engineers should select the right components when designing a preamplifier for a home audio system or PDA. Then, the source of noise is analyzed in detail, and guidelines for designing low noise preamplifiers are provided. Finally, taking the preamplifier of PDA microphone as an example, the design steps and relevant matters needing attention are listed.

A preamplifier is a circuit or electronic device placed between the source and the amplifier stage, such as an audio preamplifier placed between a CD player and the power amplifier of an advanced sound system. Preamplifiers are designed to receive weak voltage signals from confidence sources. The received signal is amplified with a small gain, and sometimes is adjusted or corrected before it is even transmitted to the power amplifier stage. For example, audio preamplifiers equalize and tonal signals. When designing a preamplifier for a home sound system or a PDA, one of the most difficult problems is which components should be used properly.

Element selection principle

Because of the compact size and excellent performance of op amplifier integrated circuits, op amplifier chips are used in many preamplifiers at present. When we design the preamplifier circuit for the sound system, we must know clearly how to select the appropriate technical specifications for the operational amplifier. During the design process, system design engineers often face the following problems:

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 op amplifier, which is not acceptable to the op amplifier. If the input signal or common-mode voltage is too weak, the designer should use a high-precision operational amplifier with very low compensating voltage (VOS) and very high common-mode rejection ratio (CMRR). Whether to use a high-precision op amplifier depends on how many times the amplifier gain is required by the system design. The larger the gain, the more accurate the op amplifier is required.

2. What supply voltage does the operational amplifier need?
This problem depends on the dynamic voltage range of the input signal, the overall supply voltage of the system, and the output requirements, but the accuracy of the op amplifier can be affected by the different power supply PSRR (power supply ratio), of which battery-powered systems are the most affected. In addition, the power consumption is also directly related to the static current and supply voltage of the internal circuit.

3. Does the output voltage need full swing?
Low supply voltage designs usually require a full swing output to take full advantage of the entire dynamic voltage range to expand the output signal swing. As for the full swing input problem, the operational amplifier circuit configuration will have its own solution. Since preamplifiers are typically configured with inverting or non-inverting amplifiers, the input does not need to swing full because the common-mode voltage (VCM) is always less than the output range or equal to zero (with rare exceptions, such as single-supply voltage operational amplifiers with floating ground).

4. Is the gain bandwidth issue more of a concern?
Yes, especially for audio preamplifiers, this is a big concern. Since the human ear can only detect sounds in a frequency range of about 20Hz to 20kHz, some engineers design audio systems that ignore or underestimate this “narrow” bandwidth. In fact, the important technical parameters that reflect the performance of audio devices, such as low total harmonic distortion (THD), fast conversion rate (SLEWRATE) and low noise, are all necessary conditions for high gain bandwidth amplifiers.

Deep understanding of noise

Before designing a low-noise preamplifier, the engineer must carefully examine the noise from the amplifier. Generally speaking, the noise of an operational amplifier comes from four main sources:

1. Thermal noise (Johnson) : Thermal noise with wide band characteristics generated by irregular fluctuations in electronic energy of the current in the conductive body. The square of its voltage root-mean-square value is directly related to the bandwidth, electrical conductor resistance and absolute temperature. For resistors and transistors (such as bipolar and field effect transistors), this noise effect cannot be ignored because the resistance value is not zero.

2, scintillation noise (low frequency) : due to the continuous production or integration of the crystal surface of the carrier generated by the noise. At low frequencies, the scintillation appears as low frequency noise, but at high frequencies, the noise becomes “white noise”. Most of the scintillation noise is concentrated in the low frequency range, which will cause interference to the resistor and semiconductor, and the interference of the bipolar chip is greater than that of the field effect transistor.

3. Shooting noise (Schottky) : Shottky noise is generated by the current carriers with particle characteristics in the semiconductor. The square root mean square value of the current is directly related to the average bias current and the bandwidth of the chip. The noise has a broad-band character.

4. Popcorn frequency noise: the surface of the semiconductor will produce this noise if it is polluted, and the impact is as long as a few milliseconds to a few seconds. The cause of the noise is still unknown, and under normal circumstances, there is no certain mode. Cleaner processes used in the production of semiconductors can help to reduce such noise.

In addition, because the input stages of different operational amplifiers use different structures, the differences in transistor structures make the noise levels of different amplifiers also vary greatly. Here are two specific examples.

The noise of the bipolar input operational amplifier: the noise voltage is mainly caused by the thermal noise of the resistor and the firing noise of the high frequency area of the input base current. The low frequency noise level depends on the low frequency noise generated by the input transistor base current 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 resistance.

The noise of CMOS input operational amplifier: the noise voltage is mainly caused by the thermal noise of the channel resistance in the high frequency region and the low frequency noise in the low frequency region. The cornerfrequency of the CMOS amplifier is higher than that of the bipolar amplifier, and the broadband noise is also much higher than that of the bipolar amplifier. The noise current is mainly generated by the shooting noise of the input gate leakage. The noise current of the CMOS amplifier is much lower than that of the bipolar amplifier, but the noise current of the CMOS amplifier increases by about 40% for every 10(C) temperature increase.

The engineer has to know a lot about the noise problem and do a lot of calculations before he can accurately express the noise into numbers. To avoid complications, only the most critical parameters of the audio technical specification are selected here.

Here we discuss how to design a microphone preamplifier suitable for PDA use. As mentioned above, we must understand that the source is the input preamplifier signal. First, we must know the following information:
Type of microphone planned
Microphone output signal level
Microphone impedance and the frequency of the specified impedance
Gain specification, 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 ceramic microphone are as follows:
Impedance: 2.2K (operating at 1kHz)
Output signal: 200(VPP)
Audio input frequency range: 100Hz to 4KHz
Thermal noise: 2NV /(Hz preamplifier gain index: 500(non-inverting), up to 5 times gain on the first stage, up to 100 times gain on the second stage.