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A VARIABLE IMPEDANCE TRANSMISSION LINE OR INDUCTOR FOR MMIC APPLICATIONS

IP.com Disclosure Number: IPCOM000006457D
Original Publication Date: 1992-May-01
Included in the Prior Art Database: 2002-Jan-04
Document File: 3 page(s) / 143K

Publishing Venue

Motorola

Related People

John M. Golio: AUTHOR [+3]

Abstract

Variable tuning elements are diicult to achieve on GaAs Monolithic Microwave Integrated Circuits (MMIC). This article describes a method to realize a variable impedance transmission line and a variable impedance spiral planar inductor.

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MOTOROLA INC. Technical Developments Volume 15 May 1992

A VARIABLE IMPEDANCE TRANSMISSION LINE OR INDUCTOR FOR MMIC APPLICATIONS

by John M. Golio, Joseph Staudinger and Warren L. Seely

  Variable tuning elements are diicult to achieve on GaAs Monolithic Microwave Integrated Circuits (MMIC). This article describes a method to realize a variable impedance transmission line and a variable impedance spiral planar inductor.

  Transmission lines can be realized inseveral differ- ent forms using MMIC fabrication technology, A varia- ble impedance ,transmission line can be created on a GaAs substrate with any type of transmission line structure using the method discussed. The characteris- tic impedance (Zo) of a transmission lime is a linear com- bination of the characteristic impedance of each pair of conductors present within the structure. In the simplest lossless two conductor case, this characteristic imped- ance is the square root of the ratio of inductance (L) to capacitance (C) per unit length of the structure: Zo=JL/C. The problem reduces to being able to change this inductance-to-capacitance ratio in order to make the transmission line variable as desired.

  A top view of a viable microstrip approach 17 is shown in Figure la., a cross section in Figure lb, and a cross section at a fmed value of Ids in Figure lc. The RF signal will be applied across the center stripe of surface metal 10 to the back side ground plane 11, which form the required two conductor transmission lime. On each side of the center stripe, there are ohmic contacts 12 and 13 to N+ implants 14 and 15, forming repeatable low resistance contacts to the N- implant 16, which stretches just below the surface from one contact to the next. Cen- ter stripe 10 also must form a reverse biased schottky contact with channel 16. This construction is much like a normal metal-semiconductor field effect transistor (MESFET). The differences lie in the spacing between

center conductor 10 and the contacts 12, 13, which are quite large for the variable transmission lime, the drive method which is into and out of the center stripe, and the desire to maintain a sheet of current in "channel" 16. As the reverse bias to center stri...