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When one compares the small-signal data so far plotted with large-signal measurements in an operating circuit, good correlation is observed. A setup for a 400-Mc large-signal input-impedance measurement, with a tuned and matched collector circuit load, is shown in Fig. 13. Test results are shown in Fig. 14. The slope of the rb curve is sensitive to the heat sink employed. If a smaller heat sink were used, rb would increase with PIE. The large-signal rb, is seen to be lower in Fig. 14 than for the small-signal condition shown in Fig.14 than for the small-signal condition shown in Fig. 8 as would be expected. The input reactance is less than the small-signal value due primarily to the effects of the feedback from the collector load. But the package Q is 3.4 – much higher than the small-signal value.

Fig. 13. Large-signal input-impedance test setup for 400-Mc measurement. Test results are shown in Fig. 14.

Fig. 14. Large-signal input-impedance curves. Note that the large-signal rb shown here is considerably lower than for the small-signal case shown in Fig. 8.

Power gain as a function of transistor parameters

A significant trade-off between power gain and the other required transistor parameters exists for each particular circuit operation. Power gain is a function of frequency, and at high frequencies approximates 6 dB/octave roll off. The lower the frequency where a transistor is used, the greater the sensitivity of power gain to hFE. At quite low frequencies, as expected, power gain is controlled more by the hFE of a transistor than by other basic parameters. A 400-Mc transistor, for instance, demonstrates this phenomenon as shown in Fig. 15, when operating at the lower frequency of 250 Mc. A large lot sampling of the devices at this frequency gives the distribution shown. Obviously, low-beta transistors have significantly lower gain than those with high beta. The range is roughly from 6 to 10 dB for the normal hFE range of a device family. Thus, there is a significant trade-off between the power gain that can be expected and the other required device parameters which correlate to hFE.

Fig. 15. Power gain vs dc beta. At lower microwave frequencies, the power gain is controlled more by hFE than by other parameters.

In addition to correlation of power gain to hFE, it would also correlate to VCE(SAT). This is especially true where high VCE(SAT) devices are caused by either non-uniform current distribution or (for resistor stabilized devices) higher value resistors in the emitter.

Saturated power output trade-offs

Saturated-power output capability is a significant and often overlooked characteristic of a power transistor. Among those presently marketed by many manufacturers, there is a wide range between the saturated-power-output capability and the specified power-output capability. Many designers tend to design around the saturated-power output if they can obtain this saturated power output without exceeding the dissipation limits of the transistor. In addition, the power a transistor dissipates under a high VSWR load condition is partially a function of saturated-power output capability. A significant trade-off exists between saturated power output capability and hFE. This is because of the current pinch-off effect under the emitter as previously described, where current peaks are more difficult to obtain with a high hFE.

Fig. 16. Gain saturation curves at 330 Mc. The lower dc beta transistors show significantly higher power gain.

Thus, device saturation would be expected at a lower level. Measurements of a device family, selected such that other parameters which affect saturated power output are equivalent in all devices at 330 Mc, are shown in Fig. 16. Here, a low hFE device has a significantly higher output level than one having high hFE. Also, note the differences in power gain as a function of hFE.

Another trade-off in the saturated-power output is with VCE(SAT). The higher the VSAT the lower the saturated-power output for a similar hFE. Table 1 shows roughly the trade-off between VCE(SAT) measured at dc and at saturated POE (the rf VSAT will be considerably higher but will also correlate). This is done on a high hFE unit to emphasize the correlation. A lower hFE device would not be quite as sensitive to the trade-off.

Table 1. Correlation of VCE(SAT) to POE(SAT) (For hFE = 60-65 units)

Unit

VCE(SAT) (V)

POE(SAT) (W)

1

0.70

13.5

2

0.70

13.5

3

0.72

13.4

4

0.75

13

5

0.75

12.8

6

0.75

12.5

7

0.85

12.5

8

1.0

10.5

Saturated-power output would also correlate to the BVCEO transistor breakdown voltage because a high BVCEO would indicate a high resistivity collector. This, in turn, would indicate a higher VSAT or a lower saturated-power output. However, as previously noted, a high BVCEO unit would have less COB and more power gain. The trade-off between all of these parameters becomes somewhat complex.