[SI-LIST] Re: Decoupling Capacitor for LNA and PA

Jongbae Park,

For typical LNA and PA applications, the output signal is biased
with a DC voltage, and the DC voltage is isolated from the power planes
through an inductor (isolator) and resistor (damper).  If these two
elements are properly chosen, then the LNA and PA should not need
further decoupling in most applications.

However, if your power planes are extremely noisy, then decoupling
should be of benefit if properly designed and used.

I'll leave the "design" of the decoupling circuit up to the rest of this
list (there's been numerous discussions on this topic in the past), but
the use of the decoupling components with respect to LNAs and PAs
just to make sure we're not overlooking something basic.

As mentioned earlier, the DC supply usually is connected to the circuit's
output signal, because these circuits are usually open-drain/open-source
circuits, and the voltage biases the internal transistors.  The outputs
are then AC-coupled with capacitors on the output.

If decoupling capacitors were placed across the LNA's supply voltage
pin to ground, they would be in direct contact with the output signal,
and would effectively short the output signal to ground.  This is
could explain your simulation results. ???

Alternatively, the decoupling should be placed on the power-plane side
of the inductor/resistor, such that the decoupling circuitry has no
directly impact on the signal path but only improves the noise levels
on the power planes leading up to the inductor/resistor network.

If you're placing the decoupling circuit correctly, then the other
option to consider is that the decoupling circuit has created a
resonant circuit whereby the resonant frequency is close to the
carrier frequency.  If this is the case, you can use series resistors
to dampen the resonance, different capacitor values to shift the resonance,
or different parts' placement to change inductance and subsequently
shift the resonance.

Good luck,
Pat

 

> -----Original Message-----
> From: jbtera77@xxxxxxxxx [mailto:jbtera77@xxxxxxxxx] 
> Sent: Tuesday, May 30, 2006 2:06 AM
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Decoupling Capacitor for LNA and PA
> 
> Hi all,
>  I have used decoupling capacitors in digital circuits. And I 
> know that the
> decoupling capacitor is very helpful for the power/ground 
> impedance and SSN
> reduction.
> 
>  But I wonder if the principle of decoupling capacitor is 
> also acceptable
> for analog circuit such as LNA and PA.
> 
>  One of my colleagues has simulated the effect of the 
> decupling capacitor
> for the LNA he has designed by himself. By adding several decoupling
> capacitors on LNA chip (it has ideal power/ground source, ie, 
> no p/g noise),
> the output gain characteristics of LNA have changed. So I 
> thought that we
> better not apply the decoupling capacitors in the analog 
> chips such as LNA
> or PA, but I am not sure about that.
> 
> How could the decoupling capacitors affect LNA's signal 
> output? I don't
> think the decoupling capacitor causes the mismatch of the 
> signal input or
> output.
> 
>  Somebody has any idea about these phenomena?
> 
> Thanks in Advance,
> Jongbae Park.
> 
> 
> Jongbae Park
> Ph.D Candidate
> Terahertz Interconnection & Package Laboratory,Department of 
> Electrical
> Engineering,
> Korea Advanced Institute of Science and Technology (KAIST)
> E-mail) pjb77@xxxxxxxxxxxxxxxxxx
> URL) http://tera.kaist.ac.kr/
> 
> 
> ------------------------------------------------------------------
> To unsubscribe from si-list:
> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
> 
> or to administer your membership from a web page, go to:
> http://www.freelists.org/webpage/si-list
> 
> For help:
> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
> 
> List FAQ wiki page is located at:
>                 http://si-list.org/wiki/wiki.pl?Si-List_FAQ
> 
> List technical documents are available at:
>                 http://www.si-list.org
> 
> List archives are viewable at:     
>               http://www.freelists.org/archives/si-list
> or at our remote archives:
>               http://groups.yahoo.com/group/si-list/messages
> Old (prior to June 6, 2001) list archives are viewable at:
>               http://www.qsl.net/wb6tpu
>   
> 
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field

or to administer your membership from a web page, go to:
http://www.freelists.org/webpage/si-list

For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field

List FAQ wiki page is located at:
                http://si-list.org/wiki/wiki.pl?Si-List_FAQ

List technical documents are available at:
                http://www.si-list.org

List archives are viewable at:     
                http://www.freelists.org/archives/si-list
or at our remote archives:
                http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
                http://www.qsl.net/wb6tpu

Other related posts: