Double gate FET mixer biasing

Thread Starter

zachnikolic

Joined Feb 7, 2018
8
Hello,

I am trying to design a mixer using a double gate FET (BF908) to mix an audio signal with a local oscillator at 5 MHz. I am using a board provided to me by my school so I have a schematic provided to me attached below. Should I bias the gates differently? I am also required to drive a 50 ohm load, so does that mean I should make the impedance of the secondary side of the transformer equal to 50 ohms at 5 MHz?
Thanks
 

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Thread Starter

zachnikolic

Joined Feb 7, 2018
8
hi z,
Please post your LTS 'asc' file, add the resistor values and we can check your work.
E
Hello Eric,

attached are the asy file and sub file for the bf908 as well as the asc file. I am getting an error saying definition not found for "C" so I am unable to run a simulation.
Thanks
 

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MrAl

Joined Jun 17, 2014
8,868
Ok sorry last post,

Fixed the error by removing "L=" and "C=" from the sub file. Also figured out the biasing for the most part, now getting a clean mixed signal.
Hi,

The bias is probably equal voltages, but unequal currents are allowed (even though they may be low anyway). It's been a while since i worked with a dual gate MOSFET. I think back in the 1980's some time. With all the parts i have collected over the years, i have but one dual gate MOSFET package i got sometime in the late 1970's or early 1980's.
 
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