Double balanced mixer - Size tail device L?

[ee484]

Hi, everyone.

I have a simple question about Gillbert Cell Mixer (Double balanced).

RF signals are excited at tail device and LOs are at upper devices as conventional ones.

The tail devices (In analog simple differential pair, I called this device tail device because it acts as tail current source) are also connected to the current mirror.

Is there any reason that the tail devices should have longer channel length than switching devices (upper ones). In analog, since this device should act as tail current source, typically L should be larger than the input devices and Vdsat should be larger for better matching.

How about in Mixer? L should be the same as switching devices? or should be bigger? since they are also acting as a tail current source?

Thanks..

BJ

 

[hr_rezaee]

hi
i know that tail acts such as tails in opamps.
and M_tail sinks current.
but i have no idea about L size.
search google for "mixer tutorial" or "Gilber cell tutorial"
you can find many usefull text.

 

[hrkhari]

Hai:

Increasing L would conribute to an increase of flicker noise contribution, hence degrading the NF of the mixer.

Rgds