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.
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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.
Input and output admittances
To design adequately a high-frequency power-transistor circuit, the approximate impedances to which the input and output must be matched need to be known. In effect, it is the large-signal impedances that are important. These are difficult to determine with available measuring equipment, but accurate measurement techniques have been established. At present, mostly small signal parameters are specified. These place the circuit design in the right ball park for the large-signal impedance values, which are now being specified on some manufacturers’ data sheets.
Common-emitter admittances are normally given in the transistor specifications such as input admittance y11e (the input admittance with output short-circuited) and output admittance y22e. Typical forward-bias curves for these small-signal values are shown in Fig. 2. The large-signal input resistance will be less than the small-signal value. Thus, the circuit designer must compensate for this. The large-signal output admittance will be closer to the small-signal value if the latter is measured at less than the large-signal circuit operating voltage.
Fig. 2. Typical forward-bias curvesfor small-signal values of common-emitter admittance and large-signal input resistance. Curves were obtained on a type ITT-3TE440 transistor at 400 Mc.
Typically, the small-signal measurement of y22 at 10 V will agree closely with the large-signal value at around 28 V. Sometimes, the equivalent series circuit input resistance and reactance are given rather than the shunt values.