From Wideband Amplifier Design

The f t of a transistor is given by the following equation,

(A.1)

where C jc is the collector base junction capacitance, C je is the emitter base junction capacitance, R c is the collector bulk resistance, ? f is the forward transit time for the transistor, and g m is the transconductance of the device equal to the collector current divided by V t. In turn, where k is Boltzman's constant, T is the temperature in kelvin, and q is the charge on an electron. V t is approximately 26 mV at room temperature.

Most of these parameters are highly nonlinear with respect to operating point. In fact, many of the SPICE parameters are used for the purpose of giving a value for C jc, C je, and ? f as a function of operating point (both voltage and current).

However, for a given operating point, it is possible to set the f t of the device by picking any one or any combination of these parameters. For example, suppose you did not want to bother with ? f and instead wanted to absorb all of the effects of ? f into C je for a particular operating point. Then for this one point only, you can use the following equation for f t,

(A.2)

where C je 1 equals

(A.3)

Substituting equation (A.3) into (A.2)...

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