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Distributed Integrated Circuits: Wideband Communications for the 21st Century by Ali Hajimiri lobal communications mitted per second (i.e., bit rate) G have rendered our world a determines the speed of a digital smaller, yet more interest- communications system. C.E. ing place, making it possi- Shannon, the founder of modern ble to exchange visions, ideas, information theory, proved that the goals, dreams—and PoKéMoN maximum achievable bit rate of a cards—across our small planet. digital communications system Modern communications systems, increases linearly with the available such as the internet and portable range of frequencies (i.e., channel wireless systems, have added new bandwidth) and logarithmically dimensions to an already complex with the signal-to-noise ratio. world. They make us aware of our Thus, three critical parameters, similarities and differences and give namely, bandwidth, signal power, us an opportunity to communicate and noise, are the most important with people we have never met parameters in determining the per- from places we have never been. formance of any given communica- The fusion of education with com- Ali Hajimiri tions system. munication is already bringing One of the more common about new levels of awareness, methods of increasing the band- accompanied by creative upheavals million) of reliable active (e.g., tran- width, and hence the bit rate, of in all aspects of modern life. sistors) and passive (e.g., intercon- any given system is to migrate to However, the ever-increasing nect) devices. Further, they are rela- higher operating frequencies. The demand for more connectivity tively inexpensive to incorporate maximum speed of operation in inevitably increases the complexity into mass-market products. electrical systems is determined by of such systems. Integrated systems The realization of revolution- the performance of both active and and circuits continue to play a cen- ary ideas in communications passive devices. While in modern tral role in the evolution of compo- depends heavily on the perform- integrated-circuit technologies the nent design. Silicon-based integrat- ance of the integrated electronic single-transitor maximum frequen- ed-circuit technologies (particularly circuits used to implement them. cy of operation can be quite high, complementary metal oxide semi- Let’s consider some well-estab- actual circuits rarely operate any- conductor, or CMOS) are the only lished theoretical background for a where near these frequencies.1 This technologies to date capable of pro- moment. The maximum number of provides further motivation to pur- viding a very large number (over a bits (1s and 0s) that can be trans- sue alternative approaches to allevi- 1 A transistor in a given process technology is usually characterized by its unity-gain frequency shown as fT . This is the frequency at which the cur- rent gain (the ratio of the output current to input current) of a transistor drops to unity. While the unity-gain frequency of a transistor provides an approximate measure to compare transistors in different technologies, the circuits built using these transistors scarcely operate close to the fT and usually operate at frequencies 4 to 100 times smaller depending on the complexity of their function. There are two main reasons for this behavior. First, many systems rely on closed-loop operation based on negative feedback to perform a given function independent of the parameter variations. An open-loop gain much higher than one is thus required for the negative feedback to be effective. This higher gain can be only achieved by operat- ing the transistors at a lower frequency than the fT to provide the desired gain. Second, integrated passive devices necessary in most of the high- speed analog circuits have their own frequency limitations due to parasitic components that can become design bottlenecks. The combination of these two effects significantly lowers the maximum frequency of reliable operation in most conventional circuit building blocks engenious winter 2003 progress reports ate bandwidth limitations, particu- traveling wave on the input line. larly in silicon-based systems Each transistor adds power in which, despite their reliability, suf- phase to the signal at each tap fer from low transistor speed, poor point on the output line. The for- passive performance, and high ward traveling wave on the gate noise compared with other tech- line and the backward (traveling to nologies. the left) wave on the drain line are The complex and strong inter- Figure 1. A distributed amplifier consisting of absorbed by terminations matched relations between constraints in two transmission lines and multiple transis- to the loaded characteristic imped- tors providing gain through multiple signal modern communications systems ance of the input line, R , and out- paths that amplify the forward traveling in have forced us to reinvestigate our wave. Each transistor adds power in phase to put line, Rout, respectively, to avoid approach to system design. “Divide the signal at each tap point on the output reflections. and conquer” has been the principle line. Each pathway provides some gain and In a distributed amplifier, one therefore the whole amplifier is capable of used to solve many scientific and providing a higher gain-bandwidth product tries to avoid a “weakest-link” situ- engineering problems. Over many than a conventional amplifier. ation by providing multiple, equally years, we have devised systematic strong (or equally weak) parallel ways to divide a design objective paths for the signal. In the absence into a collection of smaller projects gle signal path, distributed inte- of passive loss, additional gain can and tasks defined at multiple levels grated systems and circuits rely on be achieved without a significant of abstraction artificially created to multiple parallel paths operating in reduction in the bandwidth by render the problem more tractable. harmony to achieve an objective. addition of extra transistor seg- While this divide-and-conquer However, this multiple signal-path ments. This is the direct result of process has been rather successful feature often results in strong elec- multiple signal paths in the circuit. in streamlining innovation, it is a tromagnetic couplings between cir- The extended bandwidth of the double-edged sword, as some of the cuit components, which makes it distributed amplifier comes at the most interesting possibilities fall in necessary to perform the analysis price of a larger time delay between the boundary between different and the design of distributed cir- its input and output, as there is a disciplines and thus hide from the cuits across multiple levels, a task trade-off between the bandwidth narrow field of view available at not crucial when using the “divide and delay in an amplifier. each level. Thus, approaching the and conquer” approach. Alternatively, one can think of this problem across multiple levels of This concept can be best seen approach as a method of absorbing abstraction seems to be the most through the distributed amplifier the parasitic capacitances of the promising way to find solutions not (originally suggested by Percival transistors into the transmission easily seen when one confines the and first implemented by Ginzton) lines and making them a part of search space to one level. sketched in Figure 1. This amplifier the passive network. Distributed circuit and system consists of two transmission design is a multi-level approach lines on the input and the out- allowing more integral co-design of put, and multiple transistors t Caltech, one of our most the building blocks at the circuit providing gain through multi- A exciting breakthroughs and device levels. This approach ple signal paths. The forward has been in the area of can be used to greatly alleviate the (to the right in the figure) silicon-based distributed frequency, noise, and energy effi- wave on the input line is amplified circuits for communication systems; ciency limitations of conventional by each transistor. The incident we have achieved unprecedented circuits. Unlike conventional cir- wave on the output line travels for- performance for communication cuits, which often consist of a sin- ward in synchronization with the blocks and systems. 12 13 In particular, we have used the This concept has been successfully have been made in this direction, a concept of distributed systems to demonstrated in the distributed watt-level, truly fully integrated demonstrate an extremely high- voltage-controlled oscillator of CMOS power amplifier has not speed voltage-controlled oscillator Figure 2b where alternative signal been demonstrated using the tradi- using a low-performance CMOS paths have been introduced to tional power-amplifier design tech- technology with small cut-off fre- change the electrical length seen by niques. quencies for the active and passive the traveling wave. components (see Figure 2). This Another component we oscillator uses the delay introduced have devised is the distributed wo main obstacles in the by the distributed amplifier to sus- active transformer (DAT) T design of a fully integrated tain electrical oscillation by contin- power amplifier. The design of power amplifier are the uous amplification of the signal a fully integrated silicon-based low breakdown voltages of around a loop. The oscillation fre- power amplifier with high output transistors and the high losses of quency is determined by the round- power, efficiency, and gain has been passive components. The low trip time delay, i.e., the time it takes one of the unsolved major chal- breakdown voltage limits the volt- the wave to travel through the lenges in today’s pursuit of a single- age swing at the output node, transmission lines and get amplified chip integrated communication sys- which in turn lowers the produced by the transistors. tems. Although several advances output power. The high passive loss Tunability is an essential fea- reduces the amplifier’s power effi- ture for such distributed voltage- ciency by dissipating the generated controlled oscillators (DVCOs), and power in the signal path.

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