Skip to Main Content
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.
Microwave Theory and Techniques, IEEE Transactions on (Volume:56 , Issue: 6 )
Date of Publication: June 2008