Bandwidth Potential of Cascode HBT-Based TWAs as a Function of Transistor ${ f} _{\max} /{ f} _{ T}$ Ratio

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 f
_c
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 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 /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.