Fachzeitschrift für Hochfrequenz- und Mikrowellentechnik
RF & Wireless Figure 7:
RF & Wireless Figure 7: Current vs. Voltage 800A3A Figure 8: Power vs. Load Impedance 350AH1 voltage is at a maximum with zero current, and a short where the current is maximum with zero voltage. The graphs in figure 2 and 3 are based on the minimum rated output of the amplifier across its entire operating frequency range. There most likely will be spots within the frequency range where the output power will exceed the specified minimum rated output power. To avoid unexpected results, always request a copy of specific production test data before placing an amplifier in service. Example 2: High Power Solid-State amplifiers by necessity employ active VSWR protection. Take, for example the 1000W1000D. • 80 MHz – 1000 MHz bandwidth • 1000 watts minimum RF output delivered into a 50 Ω output impedance • Active protection kicks in to reduce the gain when reverse power is measured at 500 watts; this is a VSWR of 6:1 when using the amplifier at rated power. • This fold-back protection limits the reflected power to 500 watts maximum The 1000W1000D is an example of one of AR’s high power amplifiers that folds-back when reverse power reaches 50% of rated power. Even though the amplifier does foldback, a considerable amount of power is still being delivered to the load. In many cases, other manufacturers of high power amplifiers would not be able to handle such conditions and forward power would either be shut-down completely or reduced drastically. In power critical applications, an impedance matching transformer similar to the one used in the AR 800A3A could be used to match the amplifier to the load. However, since matching transformers tend to be narrow band, this approach may prove impractical if the 1000W1000D were to be operated over its entire frequency band. In this case, a series of narrow-band transformers could be switched in to the application as the frequency dictated or simpler yet, the user could opt for a higher power amplifier. The above graph demonstrates that even though fold-back occurs at a VSWR of approximately 6:1, significant output voltage and current are still delivered to the load. Example 3: Much has been said so far regarding the importance of impedance matching. The 800A3A is an example of a unique amplifier that provides the user with selectable output impedance to match a wide variety of applications. • 10 kHz – 3 MHz bandwidth • 800 Watts minimum output power rating • An internal user selectable impedance transformer provides 12.5, 25, 50, 100, 150, 200, or 400 Ω to facilitate a closer match to the load impedance • Active protection kicks in when VSWR exceeds 6:1 to reduce the gain • This fold-back protection limits the reflected power to 400 watts maximum The internal impedance transformer of the 800A3A allows this amplifier to have output impedance that matches that characterized by a variety of applications. External transformers are available to extend the usefulness of the 800A3A to include even more applications. Figure 7 clearly highlights the benefits of an amplifier with an internal impedance matching transformer that facilitates a better match with varying loads. The range of output voltage and current is considerably greater than what is provided by a standard 50 Ω amplifier. Example 4: The salient characteristics of high power, broadband, and very low output impedance (typically
