What is the hardest circuit to design

Integrated analog circuit technology

1. Introduction

Analog integrated circuits have been around for more than 37 years. Despite all prophecies of doom that analog technology will very soon be superseded by digital technology, we can say today that the demand for analog integrated circuits is increasing to a considerable extent. The proportion of purely analog integrated circuits currently accounts for more than 10% of all integrated circuits. Of course, the analog circuits have been replaced by corresponding digital circuits in many applications (the best example of this is "digital audio"). But good analog developers are needed elsewhere today. If we want to digitize naturally occurring signals, we always need analog-to-digital converters and digital-to-analog converters together with anti-alias filters or reconstruction filters. Furthermore, we find that new applications are appearing whose demand for speed and low energy consumption makes the use of high-frequency analog input stages necessary. We find such applications in the field of wireless and wired digital communication. We are seeing the size of integrated circuits increase as more and more attempts are made to integrate entire systems. This increases the likelihood that at least some of modern integrated circuits are dependent on analog circuit elements that enable a connection to the real outside world. These analog circuit parts, which usually only make up a small part of the chip area, often determine the limits of the performance of the overall system and are nevertheless the parts of the integrated circuit that are most difficult to design.

2. Mode of operation and structure of the components

The design of analog integrated circuits is largely determined by the physical understanding of its components. The purpose of this chapter is to convey this physical understanding.

3. Manufacturing process

Most integrated circuits are made on the basis of silicon. The semiconductor material silicon has the great advantage that its oxide is very well suited as a masking material for the various technological steps. Silicon dioxide is temperature-resistant, can be structured easily and is robust against many chemicals. This allows inexpensive manufacture.

4. Design tools

At the beginning of the manufacturing process of an integrated circuit there is the draft and the design. Once the requirements for a newly designed circuit have been defined, we will look for suitable circuit variants. When we have found something suitable, it is important to roughly dimension our circuit. Many circuit designers these days still prefer paper, pencil, and calculator for this. We are now also familiar with computer-aided tools that help us with dimensioning. This is discussed in more detail in the “Symbolic Analysis” section. After the rough dimensioning, the review of our design must take place. This is done with the help of numerical analysis. These include SPICE and all SPICE-like derivatives. These powerful tools allow us to realistically simulate the circuit, taking all issues into account. The simulation will give us the information about whether our circuit is working as intended. We will find clues to help us in case we need to correct the component parameters. Once we have sufficiently verified our circuit design, we can convert it into a layout. We usually use complete program packages such as Cadence or Mentor Graphics to design integrated circuits. The basis of all these packages is a CAD program for drawing the circuit diagram and layout. There are also programs for the synthesis and analysis of circuits. I will look at these tools by way of example in this chapter. Although the use of synthesis tools in analog circuit technology does not yet have the same status as in digital circuit technology, we want to study the basic procedure here.

5. Basic elements

The essential signal processing elements in analog integrated circuit technology are differential amplifiers and multipliers. In addition to these main elements, we will need a number of auxiliary circuits. With a differential amplifier, we use a current source in the emitter circuit in order to achieve a high level of common-mode rejection. Current sources are also used as active load elements and for level shifting. In analog integrated circuit technology, it is usually important that the designed circuit is independent of fluctuations in operating voltage or temperature. We then derive the operating points of a circuit from a reference voltage source.

6. More complex functions

An ideal operational amplifier is a differential input, single output amplifier. It has an infinitely high gain Av, an infinitely high input resistance Rin and an output resistance Rout of zero.

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