Analog Circuit Design

Today’s modern circuit design is a bit of a conglomerate thanks to the availability of sophisticated process technologies that have made bipolar and CMOS (Complementary Metal Oxide Semiconductor), power and signal, passive and active components available, all on the same die. From this point on, it is up to the circuit designer’s ingenuity to assemble these components into the analog and/or logic building blocks deemed necessary to develop a chip’s intended system.

New analog blocks are invented all the time, whereas the digitalization of traditional analog blocks continues. A few examples of new analog functions available are charge-pump voltage regulators and LED drivers. A present-day example of digital technology tunnelling deep into the analog core functions is the digitalization of the frequency compensation in switching regulators’ control loop. So whilst digital technology circuits and processes continue to advance in leaps and bounds, analog keeps reinventing itself and rebuilding around a central analog core of functions that are extremely hard to crack.

There are two types of standard transistors today, NPN and PNP, with different circuit symbols. The letters refer to the layers of semiconductor material used to make the transistor. Most transistors in use today are NPN because this is the easiest type to make from silicon. NPN is one of the two types of bipolar transistors, in which the letters “N” and “P” refer to the majority charge carriers inside the different regions of the transistor. A charge carrier denotes a free particle carrying an electric charge. Most bipolar transistors used today are NPN because electron mobility is higher than hole mobility in semiconductors, allowing greater currents and faster operation. The NPN transistor is the leader of the traditional bipolar analog integrated circuits world. NPN transistors consist of a P-doped semiconductor layer (the “base”) between two N-doped layers. A small current entering the base in common-emitter mode is amplified in the collector output. In other words, an NPN transistor is “on” when its base is pulled high relative to the emitter.

In fact, in the most basic and most cost-effective analog IC processes, the chip designer has at its disposal just that; a good NPN transistor. The rest, PNPs, resistors, and capacitors, are just by-products, not much better than fleas on a dog’s back.

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