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1-I have designed an LNA amplifier at 5.5 GHz and a Dielectric Resonator at 5GHz.I was hoping to down convert the LNA output using a dual gate mixer (5.5GHz at RF and 5GHz at LO) but none of the dual gate FETs I came across operate at such high frequencies e.g BF998 2-While designing the FET mixer,can I use the S parameters for calculations?Aren't they specified for linear models while mixer is clearly non-linear.

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  • \$\begingroup\$ If you can't find a DG FET at the frequencies you need, consider the similarity between a DG FET and a cascode configuration of 2 normal FETs. \$\endgroup\$ – Brian Drummond Jul 31 '15 at 13:48
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Maybe for such high frequencies you would have better luck using an integrated mixer like the LTC5510 instead of rolling your own.

Excerpt from the datasheet:

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EDIT (prompted from a comment)

I doubt you can use a BF909 or a 2N25139 up to 5GHz. Look at their datasheets. The BF909 is intended for operation up to 1GHz:

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While the 2N25139 is specified up to 4GHz (even the non-linear model is valid up to 4GHz):

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As an alternative you could use HEMT devices (JFETs using exotic semiconductor mixes different from plain silicon), but they are darned expensive: see this search result page on digikey!

Here is an excerpt from the datasheet of the first of that list:

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  • \$\begingroup\$ I actually want to design my own circuit.I can use NE25139 or BF998 but have a tough time deciding whether they will operate at such high frequencies or not \$\endgroup\$ – muhammad muheeb Jul 31 '15 at 14:30

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