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A novel design of a single-stage microwave distributed amplifier is presented in this article. The design is based on the use of a Chebyshev polynomial to express the transducer power gain of the distributed amplifier. The obtained amplifier provides a considerable increase in the bandwidth as compared to both the conventional single-stage distributed amplifier and multi-stage distributed amplifier.

Conventional MESFET distributed amplifiers (CDA) are formed by two artificial transmission lines, called gate and drain lines, actively coupled by a number of MESFET transistors.¹ The gate and drain lines are made out of series inductances L_g and L_d and capacities C_g and C_d, respectively, of the MESFET transistors. In this circuit configuration, the idle ports of the gate and drain lines are terminated by the particular impedance Z₀, commonly equal to 50 Ω.

The conventional single-stage distributed amplifier (CSSDA) consists of one section^2-4 in which the gate and the drain line idle ports are open-circuits terminated, as shown in Figure 1. The structure is obtained from the dual-fed distributed amplifier⁵ in which the transducer power gain of this configuration can be improved up to 12 dB. This improvement is due to the fact that the voltage is doubled along the gate line and the recovered power will be absorbed by the loads at ports 2 and 3 within a conventional distributed amplifier structure. In this circuit, the gate line is not matched since it is open-circuited; this engenders a variable signal with frequency at the input of the transistor. The signal at the level of the load undergoes the same problem as the input signal. Unlike the CDA, the transducer power gain will oscillate within the bandwidth.

For the CSSDA amplifier, the 50 Ω matching condition of the gate and drain lines do not have meaning since both lines are open-circuited at one of their extremities. Thus, the matching constraint is raised, leading to more freedom on the choice or the determination of the gate and drain lines’ characteristic impedances, according to the wanted objectives. These characteristic impedances highly influence the circuit bandwidth. The goal that has been fixed is to look for the values of these characteristic impedances that lead to the enhancement of the bandwidth as compared to the bandwidth of the usual cases (CDA and CSSDA amplifiers) where 50 Ω is attributed to these impedances. Consequently, these characteristic impedances will serve the role of design parameters. The objective of the present work is to obtain a bandwidth at least twice as large as that obtained by CSSDA and greater than the bandwidth of the conventional distributed amplifier CDA. The amplifier with these performances has the abbreviated name of SSDA. To accomplish this, the frequency response (transducer gain) of the SSDA amplifier is approximated by a Chebyshev polynomial.⁵