October, 1967

Although dc parameters, as covered in Part 1 of this series, give a great deal of information about a transistor for use at microwave frequencies, rf parameters are important, too. The parameters to be considered are hfe, GPE, η, rf BVCEO, and VCE(SAT). Each will be defined and discussed in turn.

Hf forward-current transfer ratio (hfe)

The high-frequency common-emitter forward-current transfer ratio of a transistor is the ratio of the short-circuit output current divided by the input current. This is the high-frequency Beta of the device. The theoretical curve of current gain vs frequency is shown in Fig. 1 and is very closely approached in practice.

Fig. 1. Theoretical curve—hfe vs f.In practice, the high-frequency current transfer ratio varies with frequency in a manner closely approximating the theoretical curve shown.

The frequency at which hfe is 1 (or 0 dB) is usually considered as being fT, the current-gain times bandwidth of the transistor. Parameter fT is usually estimated by multiplying hfe by the frequency of measurement. In practice, however, measured hfe times frequency of measurement does not give fT – especially for high-power rf transistors. The external hfe is not necessarily that of the internal structure and thus is not always an exact measurement or estimate of fT. The difference is cause by the division of input current between the input resistance depends upon the emitter inductance and whether there is resistor stabilization in the emitter structure. Thus, the measured hfe may not be the actual internal gain of the structure. Although external current gain is important, in estimating power gain it is the internal current gain that should be used. This is especially so if emitter inductance is to be neglected or if any emitter resistance will otherwise be used in the power gain formula. All transistor manufacturers’ data sheets specify hfe as the external measurement. However, the internal (current) gain-bandwidth product can be somewhat higher depending upon device type and packaging.

Parameter hfe determines many circuit capabilities as well as some of the dc transistor parameters. It is not a measure of power gain, because base resistance (which can be established somewhat independently of hfe or fT) also enters into determination of power gain.