Gilbert cell biasing
Well guys thank you all so much for your help, I immensely appreciate it!!

Well while discussing the matter in the comments with jonk (thanks for the link) and Marcus Müller, I had the idea of simply treating the upper diff pair as a simple load that has 20mA through, and made a voltage divider biased current sink, and calculated the resistance values with the same method of biasing a base divider biased common emitter biased amplifier, and with simulation i got mixed signal.

So here's a recap:

i biased the upper pair with simple base resistance biasing method I made the following circuit:







simulate this circuit &ndash; Schematic created using CircuitLab

Well I wanted my Q point for each transistor to be 6V, 10mA, for that the value R1 and R2 is 600Ohms, and to make sure that the base runs with ic/hfe (with hfe around 200) R3 and R4 have 226kOhms.

Now thanks to the discussion I've had with the guys i thought about treating the whole block as a load itself that requires 20mA, therefore i added the following circuit instead of the constant current source:







simulate this circuit

And what i obtained from the simulation (Multisim) is the following:



(sorry didn't know how to resize it)
The expected results are shown, and it's the same situation for the Gilbert made out of this simple balanced mixer.

Only 2 issues bother me, the 226k resistance (very high value) I'm wondering if i can remove it or not.. and inputs need to be very small..

theory behind it as well as schematics can be found on:
http://michaelgellis.tripod.com/gilbert.html
and the link jonk proposed:
https://smallwonderqrp.blogspot.com/2016/02/home-brew-your-own-mixer-ics.html

So how catastrophic does this look?? And what do you guys think? Please be gentle haha