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Transistors in LNA work in satuation or linear?

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ethan

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why should transistors work in saturation in lna

Someone has posted related question before.

But my question here is for the upper transistor (stacked transistor) in one-stage cascode Source-degeneration LNA. Which region should it work? Saturation or Linear region?

Thanks a lot.
 

lna transistor

I think it is better in saturation region to make sure your LNA more stable!
Eventhough linear region provide smaller current and act like an active resistor, it's better to have a stable transistor...especially on LNA!

:D
 

Hi guamak_menanak,

The transistor you mentioned is the top one, which I mentioned?

I have seen one one-stage cascode source-degeneration LNA with frequency 2.4GHz. It had pretty good performance(with very good simulation results for every aspect), I found that the bottom transistor was in saturation, while the top one was in linear region. I was confused. That's why I asked this question.

So, do you think this LNA design was not that stable?

guamak_menanak said:
I think it is better in saturation region to make sure your LNA more stable!
Eventhough linear region provide smaller current and act like an active resistor, it's better to have a stable transistor...especially on LNA!

:D
Click to expand...
 

Hi guamak_menanak,

The transistor you mentioned is the top one, which I mentioned?

I have seen one one-stage cascode source-degeneration LNA with frequency 2.4GHz. It had pretty good performance(with very good simulation results for every aspect), I found that the bottom transistor was in saturation, while the top one was in linear region. I was confused. That's why I asked this question.

So, do you think this LNA design was not that stable?

guamak_menanak said:
I think it is better in saturation region to make sure your LNA more stable!
Eventhough linear region provide smaller current and act like an active resistor, it's better to have a stable transistor...especially on LNA!

:D
Click to expand...
 

hi,
to me in the cascode LNA design.
the bottom one is in saturation,
while the top one is also in saturation as well.
am i rite ?

cheers
:D
 

why worked in saturation region, in microwace freq they seems work in linear region
 

maxwellequ said:
Can you post a schematic ?
Click to expand...

Here it is.
____ VDD
|
|
[] L2
|
|
--| |---Vb
| M2
|
|
--| |---[]---Vin
| M1 Lg
|
[] Ls
|
|
------ GND

Right now I am talking about transistor M2, or we can talk about both M1 and M2. Which should work in Saturation? Or only M1 needs, not M2? or both?

Can Randyding give more specific comments? Like which in frequency range, the transistors of LNA should work in saturation or linear?
 

It is better to have all the transistor in the saturation region, to make sure that your design is stable if there are some voltage drop.

I'm not very sure on why sometimes we used linear transistor, if i'm not mistaken transistor in linear region act like a resistor. Furthermore, someone told me that linear transistor can save the power consumption.

Etahn = as for me, I'm using all the transistor in the saturation region. It makes my design simpler and easy to calculate :D
 

hi to all
both transistors MUST be in saturation only....M2 in saturation behaves like a common-gate stage and its input impedance is approximately 1/gm. This low load value helps reduce the miller effect thru cgd of M1 thereby making it stable. this is why M2 must be in saturation. as only then will it act as a very good current translator while giving low input impedance to M1. I have never heard of M2 being in saturation...as there does not seem to be advantages!!!
 

I think if transistor work in saturation the out put is not exactly related to input so information get lost!so for LNA i think it should work in linear.isn't it true?
 

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