tf and fT are two closely related parameters for a bipolar model. Usually the parameter tf is included in the model and fT can be calculated from it and other parameters. tf is used to model the effect of the excess charge stored in the transistor when it is biased in the forward active region. i.e. the base – emitter junction is forward biased and the base collector junction is at 0 Volts. It is needed to calculate the emitter diffusion capacitance. fT is the transistor’s unity gain bandwidth defined as the frequency where the common emitter, zero load, small signal current gain extrapolates to unity ( Ref:”Modeling the bipolar transistor”. Ian Getreu). CAD programs use many different ways to use tf to convert to fT. However, this blog simply provides a way to get the conversion done to estimate the fT from tf. This provides the engineer a quick way to see what he may be dealing with without a lot of calculator overhead. If he needs a very accurate number he can always use an expensive simulator. This simple conversion is given by: fT(max)= 1/(2*pi*tf). This simple conversion assumes a zero value for the transistor’s internal collector pad to collector pin resistance. This resistance is assumed to be very small so this expression is a good estimate. Another assumption is that this is the maximum or peak value of fT. A calculator based on this expression is available from the Signal Processing Group website for download free of charge. Please visit the SPG website for other items of interest in analog and RFMW design.
