Design and analysis of novel high-gain and broad-band GaAs pHEMT MMIC distributed amplifiers with traveling-wave gain stages

Using the concept of traveling-wave gain stages, novel GaAs pseudomorphic high electron-mobility transistor monolithic-microwave integrated-circuit (MMIC) distributed amplifiers (DAs) are demonstrated to achieve high gain and over several octaves of bandwidth performance simultaneously for microwave and millimeter-wave frequency applications. The cascaded single-stage distributed amplifier (CSSDA) is used as traveling-wave gain stages to improve the gain performance of the conventional distributed amplifier (CDA). By adopting the low-pass filter topology between the CDA and CSSDA and tuning the gain shape of CDA and CSSDA separately, a broad-band and high-gain DA, called CDA-CSSDA-2, was accomplished. The detailed design equations are derived for the broad-band matching design of this CDA-CSSDA-2. Two other MMICs, namely, a two-stage CSSDA called 2-CSSDA, and another two-stage design called CDA-CSSDA-1, are also included in this paper. This CDA-CSSDA-2 achieves 22/spl plusmn/1.5-dB small-signal gain from 0.1 to 40 GHz with a chip size of 1.5/spl times/2 mm/sup 2/. It also produces a gain-bandwidth product of 503 GHz, which is the highest among all reported GaAs-based DAs. The flat group delay also demonstrates the feasibility of this design for future digital optical communications and broad-band pulse applications.

I. Introduction

The principles of the broad-band capability of distributed amplifiers (DAs) are well known [1] [2] [3]. The advantages of uniform gain, flat group delay, and low voltage standing-wave ratio (VSWR) over wide frequency ranges in DAs have also made it possible to implement a broad-band millimeter-wave receiver for digital optical communications [4]–[8] and other pulse applications. In addition to the conventional common-source topology to implement DAs, there were many other reported circuit topologies to implement high-performance DAs for digital optical communications, such as cascode [3], [4], [8], [10], dual-gate [11], matrix [12], differential [5], [13], attenuation compensation [14], twin-cascode [5], and cascade [9], [15]–[17] using GaAs, InP, SiGe, GaN, and CMOS foundry processes. However, the performances of the conventional distributed amplifier (CDA) are gain-bandwidth limited due to its optimum number of stages [2]. The cascode configuration (a common-source field-effect transistor (FET) connected with a common-gate FET) DA, known for its high maximum available gain, wide bandwidth, improved input–output isolation, and variable gain control capability, has been utilized in many applications such as distributed mixers [18]–[20], and DAs [3], [4], [8], [10]. To make a very compact monolithic microwave integrated circuit (MMIC) DA design possible, the cascode FET gain cell are sometimes realized as a dual-gate structure. However, the dc power consumption of the cascode DA is higher since the dc voltage across the cascode cell is doubled as in the CDA and also the dc current flows through the drain and gate termination resistors. The differential and twin-cascode DAs are promising topologies to obtain better gain performance and are less noisy than that of the CDA, but the chip size and dc power consumption are of concern. The attenuation compensation technique used in the DA design could reduce the gate- and drain-line transmission losses and enhanced the gain performance in the high-frequency band, but the stability in the high frequency and chip size will be issues. The cascaded single-stage distributed amplifier (CSSDA), unlike the CDA, does not need to equalize the phase velocity on the gate and drain lines, but still needs to match the characteristic impedance of inter-stage transmission lines. The CSSDA shows excellent performance with high gain, good gain flatness, lower input and output VSWRs, flat group delays, and a low noise figure.