Distributed Amplifier

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Distributed amplifiers are a very resourceful example of distributed circuit design that incorporate transmission line theory into traditional amplifier design in order to arrive at an amplifier with a larger gain-bandwidth product than is realizable by conventional circuits.

N Stage Traveling Wave Amplifier

1 History
2 Current technology
3 Theory of Operation
4 References
5 External links

[edit] History

The design of the distributed amplifiers was first formulated by William S. Percival in 1936 [1]. In that year Percival proposed a design by which the transconductances of individual vacuum tubes could be added linearly, thus arriving at a circuit that achieved a gain-bandwidth product greater than that of an individual tube. Percival's design did not gain widespread awareness however, until a publication on the subject was authored by Ginzton, Hewlett, Jasberg, and Noe in 1948 [2]. It is to this later paper that the term distributed amplifier can actually be traced. Traditionally, DA design architectures were realized using valve technology.

[edit] Current technology

More recently, III-V semiconductor technologies, such as GaAs [3]-[5] and InP [6],[7] have been used. These have superior performance resulting from higher bandgaps (higher electron mobility), higher saturated electron velocity, higher breakdown voltages and higher-resistivity substrates. The latter contributes much to the availability of higher quality-factor (Q-factor or simply Q) integrated passive devices in the III-V semiconductor technologies.

To meet the marketplace demands on cost, size, and power consumption of monolithic microwave integrated circuits (MMICs), research continues in the development of mainstream digital bulk-CMOS processes for such purposes. The continuous scaling of feature sizes in current IC technologies has enabled microwave and mm-wave CMOS circuits to directly benefit from the resulting increased unity-gain frequencies of the scaled technology. This device scaling, along with the advanced process control available in today's technologies, has recently made it possible to reach an fT of 170 GHz and a maximum oscillation frequency (fmax) of 240 GHz in a 90nm CMOS process [8].

[edit] Theory of Operation

The operation of the DA can perhaps be most easily understood when explained in terms of the traveling wave amplifier (TWA). The DA consists of a pair of transmission lines with characteristic impedances of Z₀ independently connecting the inputs and outputs of several active devices. An RF signal is thus supplied to the section of transmission line connected to the input of the first device. As the input signal propagates down the input line, the individual devices respond to the forward traveling input step by inducing an amplified complementary forward traveling wave on the output line. This assumes the delays of the input and output lines are made equal through selection of propagation constants and lengths of the two lines and as such the output signals from each individual device sum in phase. Terminating resistors Z_g and Z_d are placed to minimize destructive reflections.

The transconductive gain of each device is g_m and the output impedance seen by each transistor is half the characteristic impedance of the transmission line. So that the overall voltage gain of the DA is:

$Av = n*g_m*\frac{Z_0}{2}$ ,

where n is the number of stages.

Neglecting losses, the gain demonstrates a linear dependence on the number of devices (stages). Unlike the multiplicative nature of a cascade of conventional amplifiers, the DA demonstrates an additive quality. It is this synergistic property of the DA architecture that makes it possible for it to provide gain at frequencies beyond that of the unity-gain frequency of the individual stages. In practice, the number of stages is limited by the diminishing input signal resulting from attenuation on the input line. Means of determining the optimal number of stages are discussed below. Bandwidth is typically limited by impedance mismatches brought about by frequency dependent device parasitics.

The DA architecture introduces delay in order to achieve its broadband gain characteristics. This delay is a desired feature in the design of another distributive system called the distributed oscillator.

[edit] References

[1] W. S. Percival, “Thermionic Valve Circuits,” British Patent Specification no. 460,562, filed 24 July 1936, granted January 1937.
[2] E. L. Ginzton, W. R. Hewlett, J. H. Jasberg, and J. D. Noe, “Distributed Amplification,” Proc. IRE, pp. 956-69, August 1948.
[3] E. W. Strid and K. R. Gleason, “A DC-12 GHz Monolithic GaAsFET Distributed Amplifier,” IEEE Trans. Microwave Theory and Techniques, vol. MTT-30, no. 7, pp. 969-975, July 1982.
[4] Y. Ayasli, R. L. Mozzi, J. L. Vorhaus, L. D. Reynolds, and R. A. Pucel, “A Monolithic GaAs 1-13-GHz Traveling-Wave Amplifier,” IEEE Trans. Microwave Theory and Techniques, vol. MTT-30, no. 7, pp. 976-981, July 1982.
[5] K. B. Niclas, W. T. Wilser, T. R. Kritzer, and R. R. Pereira, “On Theory and Performance of Solid-State Microwave Distributed Amplifiers,” IEEE Trans. Microwave Theory and Techniques, vol. MTT-31, no. 6, pp. 447-456, June 1983.
[6] R. Majidi-Ahy, C. K. Nishimoto, M. Riaziat, M. Glenn, S. Silverman, S.-L. Weng, Y.-C. Pao, G. A. Zdasiuk, S. G. Bandy, and Z. C. H. Tan, “5-100 GHz InP Coplanar Waveguide MMIC Distributed Amplifier,” IEEE Trans. Microwave Theory and Techniques, vol. MTT-38, no. 12, December 1990.
[7] S. Kimura, Y. Imai, Y. Umeda, and T. Enoki, “Loss-compensated Distributed Baseband Amplifier for Optical Transmission Systems,” IEEE Trans. Microwave Theory and Techniques, vol. MTT-44, no. 10, pp. 1688-1693, October 1996.
[8] D. Linten, S. Thijs, W. Jeamsaksiri, J. Ramos, A. Mercha, M. I. Natarajan, P. Wambacq, A. J. Scholten, and S. Decoutere, “An Integrated 5 GHz Low-Noise Amplifier with 5.5 kV HBM ESD protection in 90 nm RF CMOS,” 2005 Symp. on VLSI Circuits Digest of Technical Papers, pp. 86-89, July 16-18 2005.