What's the important of stability simulation for the LNA????

[ahmad_abdulghany]

Hello all,
I have another questions..
What's it ment by 'stabiltity test' or 'stabiltiy circules' in case of a design of discrete LNA using BJT..??

How to simulate stability on Agilent ADS??

Also, what's the DC bias circuits i should use in my design of the LNA??

Can it be simply one or two stages of common emmiter amplifier with noise matching at input and output for minimum noise figure?

Sorry for long question, but please try to add even any small comment, any help will be appreciated,

Thanks alot in advance,
Ahmad,

 

[teteamigo]

Re: What's the important of stability simulation for the LNA

Before say anything: I made at last semester an LNA, and simulate Noise, Matching Networks (Source and Load) and overall Stability.

For ADS:https://home.sandiego.edu/~ekim/e194rfs01/ads_stability.pdf

The active semiconductor device imposes limitations by stability.

With Source and Load Impedances with positive real part we 'think' that guarantees Stability, but this isn't the general situation what it's a GREAT MISTAKE.

Read this technotes:
**broken link removed**

 

[ahmad_abdulghany]

Re: What's the important of stability simulation for the LNA

teteamigo,
Thank you for your reply,
I read the articles you attached above, and i found no details about my other questions...
About circuit configuration of the LNA (CE, CC,....etc.), DC biasing topologies, ... and other questions,

Can you help concerning them,
Or anyone share knowledge,
Thanks,
Ahmad,

 

[khouly]

Re: What's the important of stability simulation for the LNA

due to high gain and reverse gain S12 of the transistor , at low frequencies the transistor become unstable this make the AMP OSCILLATE for some load and source terminattion

if the amp is unstable some times the input impednace become negative , and S11 become larger than one

check gonzalez book or pozar about stabilty analysis of the amplifiers

to check the stabilty there are many ways in @DS
u can add in the schematic a stabilty factor K measurment K > 1 the amp is uncondictionally stable , this mean that the amp will not oscillate at all for any source and load valuse
if K < 1 this mean that the amp in potenially unstable , and need matching for stabilty or add stabilzation network

khouly

 

[ahmad_abdulghany]

Re: What's the important of stability simulation for the LNA

Khouly, Jazakom allaah khayra
Thank you very much for help,
But can you give me some details about biasing circuits of the LNA?

Also, i need an answer for the question, Can the LNA be simply one or two stages of common emmiter amplifer?! Please answer.

Thanks in advance,
Ahmad,

 

[khouly]

Re: What's the important of stability simulation for the LNA

4 sure the LNA can be single stage with low noise and moderate gain
and u can add a second stage for gain enhancment , and this stage will not have a restrict noise figure

about the biasing circuits u can make both active or bassive bias circuit , it will depend on the how stable u want the bias point ,

the passive is more simple than active for sure , active bias circuit need an extra transistor

khouly

 

[ahmad_abdulghany]

Re: What's the important of stability simulation for the LNA

Khouly, thanks alot for your help...

I want to know how active biasing is done please..
Also, for my next stage afther the LNA (mixer), its senstivity is bad, so i should increase output of the LNA to fire the mixer. then, Can i simply duplicate my single stage LNA to be two stages and couple them by a capacitor?
I will try on simulator, but i need any theoritical considerations comments...

Thanks in advance,
Ahmad,

 

[teteamigo]

Re: What's the important of stability simulation for the LNA

Active bias guarantees constant quiescent point. With passive bias there is a open loop, and there isn't a way to collector current remain constant, then variation insensitive (like temperature that affects: junction BE voltage, current gain β; and so on).


See datasheet **broken link removed**

But there is other examples....