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Design >> RF Design >> Any advantage of using both N and P MOS in an LNA?
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Message started by Horror Vacui on Aug 23^rd, 2018, 6:19am

Title: Any advantage of using both N and P MOS in an LNA?
Post by Horror Vacui on Aug 23^rd, 2018, 6:19am

Hi RF designers,

In most of the publications the RF transistors of an LNA are almost NMOS transistors only. If we assume that PMOS devices are fast enough for the operating frequency, would it make sense to employ the PMOS as well?

Thinking in the frame of one stage designs, and assuming that a pmos is a similar transistor than the nmos, we have two devices connected in parallel. So we might have twice the gm but twice the gds as well for the same current, so neither the gain, nor the output noise change.
Here I have assumed that the real part of the output impedance is determined by the output impedance of the transistors. Is it true in general?

For me it is a strange result, that having a pmos as well does not improve the circuit performance. Could you confirm or debunk the above statement? Can you give examples where using both N and PMOS improves the performance?

Title: Re: Any advantage of using both N and P MOS in an LNA?
Post by Marios on Sep 20^th, 2022, 9:10am

I think that NF is usually independent of device gds. The reason is that both source noise and signal experience same gain. What we are after is to minimize noise added by device itself. This is done by increasing device gm.

In the same manner that adding another NMOS in parallel with the original NMOS increases gm by a factor of 2 (thus decreasing noise contribution by device), adding a PMOS on top of the NMOS inceases gm by a factor of two (assuming gm_p = gm_n). However it is usually achieved without consuming more power (assuming no extra headroom is required).

Marios

Title: Re: Any advantage of using both N and P MOS in an LNA?
Post by Marios on Jun 7^th, 2023, 6:59am

Adding to what I wrote above, a load is usually needed on top of NMOS. If that load does not contribute signal gain and is simpy a current source, then it will add noise. Hoever, if the load takes part in signal amplification, then it might still contribute the same noise but it also contributes to the signal amplification thus lowering NF.