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Signal folding to the LO stage in a Folded Cascode

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designtech

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Hi,

I was designing a folded cascode mixer. To save voltage headroom, i used a tuned LC tank instead of a current source at the top, so that the signal sees a large impedance and folds to the LO stage. However, the Q-factor of inductor being small , about 5 for me, the impedance seen at resonance is pretty small, about 200 ohms. So not all the useful RF current is going to the LO stage, but some is going durectly to ground.
Could anyone suggest a remedy/ alternative for the folded scheme?
Thanks a lot!
 

e.Horus

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Re: Folded Cascode

Check:

E. Abou-Allam, J. J. Nisbet, and M. Maliepaard, “Low-voltage 1.9-GHz RF
receiver in 0.5-μm CMOS technology,” IEEE Journal of Solid-State Circuits,
pp. 1434-1443, October 2001.

You should expect less conversion gain than a gilbert mixer.
 

designtech

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Re: Folded Cascode

Hi,

Thanks for the reference. I have also used an LC tank the way its shown in the paper. The problem is that they have access to high Q inductor, but our inductor being on-chip has low Q. so the impedance seen at resonance by RF signal is small and gain drops drastically. Any suggestions/ comments would be beneficial
Thanks
 

orley

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Re: Folded Cascode

Q of inductor is a bottleneck of RF design. You may add a buffer(Source Follow) at output. Another solution means LC tank spilt with mixer, but it's not your mean.

Regard!

:D
 

e.Horus

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Re: Folded Cascode

If the frequency is low enough you may be able to use an external tank circuit.

If the frequency is high enough, such that you don't need a large inductor, you may be able to use the inductance of bond wire which is typically .8 nH/mm - 1 nH/mm.
 

designtech

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Re: Folded Cascode

My frequency is 900 MHz..any suggestions?

Thanks
 

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