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Active Directivity is a measure of how much an amplifier's source match potentially affects its output impedance, or its load match affects its input impedance. High active directivity is a good thing.

What is the typical Active Directivity of LDMOS amplifier components measured in dB? Is it typically high, low, or does it vary widely between manufacturers and models?

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  • \$\begingroup\$ Active directivity is likely to be influenced far more strongly by the topology of the amplifier including how the transistor is connected to input/output, than the particular microfabrication techniques selected for the transistor. At least from a first pass analysis, an LDMOS based oscillator would not be any different than an oscillator with the same topology and comparable parasitics, but with a lateral MOSFET instead. \$\endgroup\$ – nanofarad Apr 15 at 0:32
  • \$\begingroup\$ @nanofarad, then why does it say in the link that "HBT amps have notoriously low active directivity, often just 2-3 dB"? (microwaves101.com/encyclopedias/…) Isn't HBT (Heterojunction bipolar transistor) a particular microfabrication that is notably different than LDMOS and more like a MOSFET than a BJT (like the HBT)? \$\endgroup\$ – KJ7LNW Apr 21 at 0:09
  • \$\begingroup\$ If anything, HBT doping yields to low base resistance more unlike LDMOS -- and in fact any feedthrough in a CS topology is going to be only capacitive in nature and quite small. In any case, asking about this without talking about topology is probably overlooking a much larger contribution; the TGA8061 from your link is three-stage CS, while there's nothing I can find on the CGB714's topology. In the end this is easier to discuss with specific parts that have good datasheets. \$\endgroup\$ – nanofarad Apr 21 at 1:13

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