Coaxial 2-port S-parameter system for transistor characterisation in three configurations from 1-500 MHz

A description is given of a coaxial 2-port s-parameter measuring system for the characterisation of transistors in common base, common emitter and common collector configurations from 1 to 500 MHz. The system is inherently wideband, because no directional couplers or hybrids are used. The incident and reflected waves are derived from measurements of the total voltage across a coaxial line by direct contact with the inner conductor using a commercial vector voltmeter. A description is also given of a precision transistor jig that accepts transistors with 2.54 mm and 5.08 mm p.c.d. lead spacing, in any configuration, with negligible common-ground impedance. The apparatus is calibrated in terms of precision air-dielectric coaxial lines, but the overall system accuracy for active-device measurements is assessed by transforming the twelve 2-port parameters, in the three configurations, to the equivalent 3-port scattering matrix when rows and columns should sum to unity. Other checks are performed using a unique relationship between the six s11 and s22 2-port parameters which, according to theory, should also equal unity. Detailed results are given of measurements on a BF115 transistor at 500 MHz both in the passive and active states. It is concluded that overall system accuracy can be many times worse for active devices than one would expect from the accuracy of calibration and subsequent measurements on passive networks using the above check procedures. A typical 3×3 matrix of an active BF115 transistor is adjusted to sum to unity by an iterative process. The matrix is then retransformed back to the three 2-port matrices, and is compared with the initial measured parameters.