TO OBTAIN LOW-NOISE CHARACTERISTICS in a monolithic GaAs FET broadband amplifier covering the VHF and UHF bands, the negative feedback approach has been shown to offer the most effective ~i r c u i t '~*~~. However, this circuit dissipates a large current due to the wide gate width required for achieving a large open loop gain (a gm) through which a low NF and a high gain are obtainable. This paper will offer a GaAs monolithic broadband amplifier design (Figure 1) in which the following approach has been taken to resolve the problem. The use of a cascade connection of three GaAs FET's with high input/output impedances affords a decrease of the dissipation current under a fixed open loop gain. The open loop gain G of a single FET amplifier can be expressed approximately aswhere RL is the load resistance and R, -J is the output resistance of the FET (= l/gd). Therefore, the high open loop gain can be obtained by increasing RL without increasing g, . The inputloutput VSWR is reduced by connecting the drain of the third-stage FET and the gate of the first-stage FET through a shunt negative feedback resistor Rf. The increase of the input/ output impedances of each FET leads t o a decrease of the bandwidth, though the open loop gain is increased. The role of the feedback resistors R2 and R3 of the second-and third-stage FETs, respectively, is to suppress the input/output impedances of the respective FETs, increasing the bandwidth of the amplifier. Automatic gain control (AGC) is provided by using dualgate FETs in the first-and secondstages. SPICE 2 program. It was found that the gain and the input/ output VSWR are principally functions of a set of Rf, R2 and R3 and a set of Rf, gml and respectively. equivalent circuit'. It was found that the NF is determined predominantly by gml and Rf. Figure 2 shows the result of the NF value calculated as a function of gml for various Rf values. The NF decreases with increasing gml (determined by the gate width W 1 of FET 1) and Rf. On the other hand, the dissipation The performance of the amplifier was simulated based on the The NF value of the amplifier was calculated using a simple and Huber, J., "A GaAs Monolithic Low-Noise Broad-Band 'Archer, J.A., Weidlich, H.P., Pettempaul, E., Petz, F.A. Amplifier", IEEE J. Solid-state Circuits, p. 648-652; Dec., 1981.
Amplifier", ISSCC DIGEST O F T E C H N I C A L P A P E R S ,P. 194-'Estreich, D.B., "A Wideband Monolithic GaAs IC 195: Feb., 1982. 3Nishiurna, M., Nambu, S., Hagio, M. and Kano, G., "A GaAs Monolithic Low-Noise Wideband Amplifier", I n t . S y m p . GaAs and Related Compounds, Japan, p. 425-430; 1981. Chairman: Allen F. Podell A. F. Podell Associates Palo Alto. CA current and the input/output VSWRincrease with gml and Rf, respectively.The foregoing simulation results helped to design the broadband amplifier. Device parameters chosen are listed in Table 1.Essential parameters gml and Rf were determined so as t o satisfy the target specification that NF <2.5dB and input/ output VSWR G2.5. The feedback resistances R2 and R3 of F...