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 fT and fmax and the 3-dB cutoff frequency fc of 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 fmax / fT ratios, an InP technology with fmax / fT of 120 GHz/190 GHz, and a GaAs technology with fmax / fT of 170 GHz/36 GHz. The higher influence of /max (compared to ft) on fc is quantitatively demonstrated. TWAs in both technologies were realized and measured, and good agreement between measurement and theory is obtained.
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