The relevance of fT and fmax for the speed of a bipolar CE amplifier stage

Expressions relating the bandwidth of a common-emitter (CE) amplifier stage and the small-signal CML gate delay time to directly measurable transistor parameters, such as f_T, f_max, and input bandwidth f_υ, are presented. They are valid for an arbitrary division of the base resistance and base-collector depletion capacitance into internal and external components. No resistance measurements are needed. It is shown that the transistor input bandwidth f_υ is an important figure of merit for the speed of a CE stage. Under a given bias condition, f_υ is determined by the base resistance and the cut-off frequency. In most cases the value of the maximum oscillation frequency f_max is only of minor importance. It would therefore be more meaningful to present besides f_T also f_υ instead of f_max as a figure of merit for transistors for high-speed, low-power analog and digital circuits