In today’s world, there are several digital devices in use. Digital devices range from digital cell phones, digital TVs, digital cameras, etc. Experts predicted the end of analog devices in the early 1980s. Digital signal processing algorithms became more powerful while developments in IC technology offered efficient incorporation of these algorithms in silicon.
With the increasing benefits of digital devices, why are engineers still interested in analog devices? After all, analog designs are old and out of date. So, what is the interest all about? Well, these questions have been asked by those who didn’t have an idea of what analog design is about. In this article, we will understand why analog design has remained relevant in today’s world.
Why Analog?
In spite of the advances in digital circuits and the increasing demands for digital devices, analog circuits are relevant in today’s high-performance systems. There are some systems where it is difficult to use digital functions. Therefore, it is difficult to replace analog functions in these systems.
- Microprocessors and memories
The memories and microprocessors built today are a result of analog design expertise. Some problems associated with the timing and distribution of clocks and data across chips ensure that high-speed signals can also be viewed as analog waveforms.
- Wireless receivers
The antenna of an RF receiver picks a signal. This signal has an amplitude of a center frequency of 1 GHz and a few microvolts. Also, large interference accompanies this signal. Therefore, the receiver must ensure it amplifies the low level signal with low noise and functions at a high frequency. It is important to know that these requirements are crucial regardless of the type of signal it is.
The two principal functions of most electronic systems is to receive a signal and extract information from it. for instance, your cell phone gets an RF signal and processes it, after which it offers data or voice information. The same rule works for your digital camera. Your camera receives light intensity from different parts of an object and then processes this information to get an image.
Typically, the processing task is preferably performed by digital circuits. Also, the integration of analog circuits isn’t restricted to analog signals. You will need an analog designer if a digital signal is small for a digital gate to interpret. Despite the development in semiconductor technology, analog circuits still face challenges. Therefore, these circuits call for innovations.
Why Razavi Analog CMOS Integrated Circuit?
In the early 1930s, J.E Lilienfield initiated patented the idea of metal-oxide silicon field-effect transistors (MOSFETs). This happened before the bipolar transistor invention. However, MOS technologies become more relevant much later. Complementary MOS (CMOS) devices were officially introduced in the mid-1960s. Therefore, this triggered a revolution in the semiconductor industry.
As time went by, CMOS technologies made name in the digital market. Also, the CMOS gates only dissipated power during switching and needed very few devices. These two characteristics were different from that of their bipolar counterparts.
The next step was the application of CMOS to analog design. The feasibility of installing both digital and analog circuits on the same chip to enhance performance made CMOS technology a perfect choice. However, bipolar transistors were a better option since MOSFETS were noisier and slower.
The CMOS technology dominated the analog market because it improved MOSFETs speed. Furthermore, the speed of MOS transistors increased by orders of magnitude. Another benefit of MOS devices is that they can function with lower supply voltages. Also, CMOS circuits function form supplies around 1 V while bipolar circuit run from around 2 V.
Razavi Analog Design Challenges
In his book, Razavi identified some challenges associated with analog designs. Analog designers need to deal with these challenges. Here, we will be discussing some of these challenges and how to handle them.
Power consumption
The semiconductor industry is working towards achieving low-power design. Also, this effort is applicable to portable devices as it helps to enhance battery lifetime. It helps to minimize heat removal cost for larger systems. The scaling of MOS device has helped to reduce the consumption of power in digital circuits. However, it has more complicated effect on analog circuits.
Transistor imperfections
Scaling has made MOS transistors faster at the expense of their “analog” features. For instance, the highest voltage provided by a transistor diminishes with every new generation of CMOS. Also, the surroundings of a transistor may determine its properties.
Circuit complexity
The analog circuits developed in today’s world may comprise tens of thousands of transistors. This demands tedious and long simulations.
Reducing supply voltages
CMOS circuits’ supply voltage has reduced from 12V to about 0.9V due to device scaling. Also, several circuit configurations have not been able to survive this reduction in supply. Therefore, designers continue to look for new topologies that function at low voltages.
PVT variations
In terms of supply voltage and fabrication process, several circuit parameters and devices vary. The effects caused by design circuits and PVT make their performance ideal for a particular range of PVT variations.
Razavi Analog CMOS Processing Technology
Modern CMOS technologies require over 200 processing steps. However, we can look into the combination of some operations. Also, these operations include wafer processing, photolithography, oxidation, and etching.
Wafer processing
The wafer used in CMOS technology must have very high quality. Also, this means that the wafer must develop as a silicon body with a few defects like unwanted impurities and dislocations in crystal. In addition, the wafer must comprise the appropriate level and type of doping. This will help achieve the necessary resistivity.
Photolithography
This involves the transfer of the circuit layout data to the wafer.
Oxidation
One distinct property of silicon is that it creates a uniform oxide layer on the surface. Also, this enables the production of gate-oxide layers. Furthermore, silicon dioxide can function as a protective layer in several steps of fabrication.
Ion implantation
Ion implantation is a common method of adding dopants in which the doping atom hits the surface of the wafer and reaches the exposed areas.
Conclusion
Razavi analog explains the methodologies and theories behind the design of analog circuits. Also, analog circuits have remained relevant in today’s devices.
