Bandwidth Potential of HBT-Based TWAs as a Function of Transistor f_max/f_T Ratio

Published in:
IEEE Trans. Microwave Theory Tech., vol. 56, no. 6, pp. 1331-1337 (2008).
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Abstract:
The bandwidth potential of cascode HBT-based broadband amplifiers following the traveling-wave amplifier (TWA) concept is studied. An approximate expression for the gain of the circuit is derived, which is based on the transistor small-signal model and the artificial transmission-line parameters. In this way, a relation between the HBT cutoff frequencies f_T and f_max and the 3-dB cutoff frequency of f_c the amplifier is obtained. This is very useful for assessing the gain-bandwidth potential of a given HBT technology for cascode-based TWAs.
Applying these results, we study the potential of two technologies with different f_max/f_T ratios, an InP technology with f_max/f_T of 120 GHz/190 GHz, and a GaAs technology with f_max/f_T of 170 GHz/36 GHz. The higher influence of f_max (compared to f_t) on f_c is quantitatively demonstrated. TWAs in both technologies were realized and measured, and good agreement between measurement and theory is obtained.

Index Terms:
Distributed amplifiers, equivalent circuits, f_max, f_T, GaAs, HBT, InP, integrated circuit modeling, semiconductor device modeling..

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