I. Principles of Analog Integrated Circuits
Analog ICs have long been primarily used in consumer electronics, and have maintained steady growth in recent years. According to a Databeans market research report, the global analog IC market experienced a compound annual growth rate of 12% from 2003 to 2009. This figure exceeds the growth rate of other products, indicating significant potential for the high-performance analog market in the future. Market statistics from the Semiconductor Industry Association (SIA) also show that the analog market will rapidly surpass the digital market in the next two to three years. Analog devices will become the mainstream of analog integrated circuit applications in the digital age of markets and products.
In information technology, digital integrated circuits are the main players, processing information carried by digital signals, which are discrete values in terms of time and quantity. However, signals in nature change continuously in terms of time and quantity, such as wind noise and water flow. Such signals are called analog signals, and correspondingly, circuits that process analog signals are called analog circuits, while integrated circuits used to process analog signals are called analog integrated circuits. Clearly, digital circuits cannot directly interact with nature. For the sake of convenience in processing or transmission, and to fully utilize the advantages of digital systems, analog signals are first converted to digital signals, then input into a modern digital system with high capacity, high speed, strong anti-interference capabilities, and good security for processing, before being converted back to analog signals for output.
The main component of integrated circuits is the transistor, and analog integrated circuits are no exception. However, analog integrated circuits utilize the amplification function of transistors, while digital integrated circuits utilize the switching function of crystals. Early analog integrated circuits mostly used bipolar transistors. Due to the maturity of CMOS technology, the slow speed of early CMOS circuits has been overcome, and it has the advantages of low power consumption and convenient process upgrades (CMOS can be scaled down proportionally). Nowadays, analog integrated circuits and mixed-signal integrated circuits (digital and analog circuits integrated together) are also commonly designed and implemented using CMOS.
II. Applications of Analog Integrated Circuits
The basic circuits of analog integrated circuits include current sources, single-stage amplifiers, filters, feedback circuits, and current mirror circuits. Higher-level basic circuits composed of these include operational amplifiers and comparators, and even higher-level circuits include switched-capacitor circuits, phase-locked loops (PLLs), and ADCs/DACs. Based on the response relationship between the output and input signals, analog integrated circuits can be divided into two main categories: linear integrated circuits and nonlinear integrated circuits. The former typically exhibits a linear relationship between the output and input signals; the shape of the output signal is similar to the input signal, only amplified by a fixed coefficient. Nonlinear integrated circuits, on the other hand, exhibit a nonlinear response to the input signal, such as a quadratic or logarithmic relationship, hence the name nonlinear circuits. Common nonlinear circuits include oscillators, timers, and PLL circuits. A typical application of analog integrated circuits is shown in the figure below. External natural signals collected by various sensors or antennas, such as those for temperature, humidity, optics, piezoelectricity, and acoustics, are pre-processed by analog circuits and converted into suitable digital signals for input into a digital system. The signals processed by the digital system are then post-processed by analog circuits and converted into analog signals such as sound, images, and radio waves for output.
Compared to the automated design methods of digital integrated circuits using EDA tools based on standard cell libraries, analog circuits retain the manual design approach. While there are numerous circuit drawing and simulation software tools and some mature circuit cells available, designing good analog integrated circuits relies more heavily on the designer's experience. This is because analog circuits need to consider more factors. In addition to the speed, power consumption, and area considerations of digital integrated circuits, performance indicators such as gain and accuracy must be taken into account, as well as the impact of noise, crosstalk, temperature, and device nonlinearity on performance.
