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Design >> RF Design >> CMOS PA parallel power control?
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Message started by Visjnoe on Jul 1^st, 2009, 8:55am

Title: CMOS PA parallel power control?
Post by Visjnoe on Jul 1^st, 2009, 8:55am

Hello,

I'm looking into some possibilities for designing a CMOS PA @1GHz
(no prior experience). I have read some papers where the power is controlled by combining the output power from different parallel
stages, but they always use a transmisson line and/or matching network
before the power is combined.

I was wondering if it's not possible to connect different open-drain (cascaded) output stages directly together and enable/disable them
for power control. So all parallel stages would share 1 common output node which is connected to the output matching network.

Regards

Peter

Title: Re: CMOS PA parallel power control?
Post by pancho_hideboo on Jul 1^st, 2009, 9:26am

Visjnoe wrote on Jul 1^st, 2009, 8:55am:

I was wondering if it's not possible to connect different open-drain (cascaded) output stages directly together and enable/disable them
for power control.
So all parallel stages would share 1 common output node which is connected to the output matching network.

Possible.
See Fig 5.26(Fully digital amplitude control through multiple-switch transistors) of the following.
http://www.amazon.com/All-Digital-Frequency-Synthesizer-Deep-Submicron-CMOS/dp/0471772550/ref=sr_1_25?ie=UTF8&s=books&qid=1246466119&sr=1-25

Or see http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?isnumber=29863&arnumber=1362836&count=47&index=19

But there are two issues at least.

(1) Output impedance will be changed by power control since active transistor numbers are changed.
(2) Input capacitance is fairly large even for small power mode.

And there are many methods to realize on/off transistor.

Title: Re: CMOS PA parallel power control?
Post by Visjnoe on Jul 2^nd, 2009, 12:00am

Dear Pancho,

thank you very much for your reference papers, it was exactly the kind of material I was looking for.

FYI, in the meantime I found another publication (by Rofougaran and Abidi) where a similar approach is used for discrete power control of a PA:

http://ieeexplore.ieee.org/search/srchabstract.jsp?arnumber=586252&isnumber=12729&punumber=4485&k2dockey=586252@ieeecnfs&query=((rofougaran)%3Cin%3Emetadata+)&pos=4&access=yes

Kind Regards

Peter

Title: Re: CMOS PA parallel power control?
Post by waseda-rfic on Jul 2^nd, 2009, 1:34am

Visjnoe wrote on Jul 1^st, 2009, 8:55am:

Hello,
I have read some papers where the power is controlled by combining the output power from different parallel
stages, but they always use a transmisson line and/or matching network

in this way, it has more output power than a single PA. transmisson line or transformer is for power combining and match.

Title: Re: CMOS PA parallel power control?
Post by Visjnoe on Jul 2^nd, 2009, 4:48am

Dear Waseda,

I think I'm missing something here: what's the difference between power combining the output power of different PA/stages and controlling (increasing) the output power of 1 PA using different parallel stages?

I mean, I understand what the difference is, but for what reason would one prefer one method over the other as a means of power control?

Regards

Peter

Title: Re: CMOS PA parallel power control?
Post by waseda-rfic on Jul 2^nd, 2009, 5:54am

Visjnoe wrote on Jul 2^nd, 2009, 4:48am:

I mean, I understand what the difference is, but for what reason would one prefer one method over the other as a means of power control?

If output power is not so large, you can use one PA with output power control. but for large output power, one PA is not enough, you have to use power combining technique. And you can control each PA, and also you can make controlling within each PA if needed.