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The large-signal common-emitter power gain, GPE, is the ratio of the output power, POE, to the input power, PIE, in a common-emitter power-gain circuit. Most specifications give the PIE range for a given POE and supply voltage. Power gain typically decreases as the temperature of the transistor increases at high frequencies. The reverse may be true using a high-frequency transistor at low frequency. It is also a function of the power-output level and the transistor’s dc parameters. A typical dependence of power gain on power output is shown in Fig. 3. The closer the power output design center is to the transistor’s saturated power output, the greater the fall-off in power gain with increased drive, and the greater the variation from transistor to transistor.

Fig. 3. Typical dependence of power gain on output power from microwave transistors.

Power gain is normally measured with the input and output simultaneously matched for maximum power gain. Occasionally, some mismatch is allowed to improve efficiency. Power gain is typically specified in a narrow-band tuned circuit. Broader bandwidth power gain would be somewhat lower.

Power gain theoretically decreases with increased frequency at 6 dB per octave. In practice, it decreases slightly faster than this. How power gain typically varies with frequency is shown in Fig. 4. Theoretically, the power gain is 1 (0 dB) at fmax (maximum frequency of oscillation). At this point, all of the output power would have to be fed back to the input of the device to sustain oscillation. In good present-day devices, fmax can be considerably higher than fT, and thus significant power gain can be obtained at a frequency above fT. This is because power gain is proportional to fT/rb. Base resistance is consequently a major factor in high-frequency performance.

Fig. 4. How power gain decreases with frequency—a theoretical curve.

Collector-circuit efficiency (η)

Collector-circuit efficiency is an important parameter because it involves the power consumption of the power amplifier. Collector deficiency is defined by:

Assuming that the amplifier stage has reasonably high-power gain, the collector-circuit efficiency is then very nearly the over-all efficiency which is defined:

For a given tuned circuit, η varies with power output as illustrated in Fig. 5. Collector efficiency can very much be a function of the circuit design, but it also relates to some transistor parameters.

Fig. 5. Collector-circuit efficiency as a function of output power for a typical microwave transistor (ITT3TE440) at 400 Mc. Vcc = 40 V.