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LNA and mixer question

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fanshuo

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in LNA and Gilbert mixer design, I did not find any one to use a PMOS current source as output resistance to boost the voltage gain.
What is the reason for this?
 

in the Mixer design the gain is not the main factor , the mixer has many spec's so very high gain is not always needed.

the LNA most use Inductor in the load or even a resistor

khouly
 

high is not always wanted
but why nobody is using it in any case?

I did know any necessarity to use a inductor or resistor as load, what is the reason?
 

i think it can be used to control the output impedance. so the matching will be easier

khouly
 

Two reasons:

1. Noise: PMOS is much more noisy than Ind/Res

2. Frequency: PMOS will degrade the high frequency gain.
 

1.PMOS does not necessarily generate more noise than resistor
2Whether it will degrade the high frequency gain in LNA depends on its size. Further more even it degrades the high frequency gain, why it is not used in mixer either
 

1. Yes, but very occasionally.

2. I only have experience on LNA. I can't help u on Mixer design

3. Normarlly LNA use inductor as loading. And u can make the gain of the LNA to be a bandpass response at the desired freq. By using the PMOS, u will have a lowpass response. And a very high gain is presented at low frequency. Maybe hundreds times higher than the gain at ur desired frequency. That is no good.

4. Another thing I forget to mention at the beginning is the linearity consideration.

5. Actrually when u realy start to design an LNA, u may find using PMOS as loading is very impractical. How to bias the PMOS is a big problem.
 

    F

    fanshuo

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about you third point, if you use a resistor as loading, it also has high gain at low frequency.

I think you are right at point 5. Bias is a problem it is easy to use inductor.

I think another reason is that the output resistance of a MOS transistor is not well defined. It's accurate value is not modeled very well.
 

Correct. That's why I never use res for LNA loading.
 

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