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Design >> RF Design >> transformer optimization for power amplifiers
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Message started by aaron_do on Jun 17^th, 2011, 12:17am

Title: transformer optimization for power amplifiers
Post by aaron_do on Jun 17^th, 2011, 12:17am

Hi all,

I have often seen papers where a transformer's performance is measured by its maximum available gain (MAG). However, in a power amplifier, we are not trying to achieve matched conditions at the output. The conditions which lead to MAG of the transformer may not lead to optimum output power or efficiency.

My questions are, what is a good way to determine transformer insertion loss, and do we know if the insertion loss is optimized?

thanks,
Aaron

Title: Re: transformer optimization for power amplifiers
Post by loose-electron on Jun 17^th, 2011, 3:07pm

empirical for an existing element or theoretical for a design in progress?

Sonnet or AWR has tools for the EM models of a design in progress.

Empirical testing? Have to design a test platform with appropriate balanced paths and calibration for a testing method. Somewhat similar to a test setup for a filter inserted into an RF path.

Title: Re: transformer optimization for power amplifiers
Post by aaron_do on Jun 17^th, 2011, 4:40pm

Hi,

I think my question wasn't phrased very well. I know how to use an EM simulator to extract an S-parameter model of a transformer. My question is on the theoretical side for a design in progress.

I am using the transformer in a power amplifier, and I have found that the insertion loss I am getting is quite a bit poorer than the maximum available gain of the transformer. But I know that I shouldn't be getting the MAG of the transformer anyway, because I am not trying to match the output to the load. So how do I know my transformer is being used optimally? I am thinking I need to do a load pull simulation and look at my required output power contour to see the optimum load impedance, and then use this info to modify my transformer, or possibly add matching of some kind.

thanks,
Aaron

Title: Re: transformer optimization for power amplifiers
Post by inductioner on Jul 26^th, 2011, 4:32am

Transformer's insertion loss can be considered as an independent parameter during initial PA design phase. It only hurts your PAE, but does not affect impedance transformation behavior. One simple way is to set IL=0dB in the first place and go back to run with EM model.

Title: Re: transformer optimization for power amplifiers
Post by aaron_do on Jul 26^th, 2011, 6:26pm

Hi inductioner,

thanks for the reply. Actually that is how I am designing now. But the insertion loss I achieve is poorer than the MAG of the transformer. So I'm wondering if there's a way to optimize the PA around the transformer to get the best possible insertion loss (i.e. close to the MAG), or vice versa. Perhaps I am not understanding something properly...

thanks,
Aaron

Title: Re: transformer optimization for power amplifiers
Post by aaron_do on Aug 1^st, 2011, 6:38pm

Hi,

after thinking on it more, I don't think MAG is very useful for transformer design because if I understand it correctly, it is not possible to achieve MAG for a lossy two-port passive device. You could argue that it is just a FOM of course...

cheers,
Aaron

Title: Re: transformer optimization for power amplifiers
Post by rfcooltools.com on Aug 2^nd, 2011, 1:24pm

aaron_do,

If the xfmr is on chip then you can play around with the turns ratio, to get max available gain from your xfmer (provided your bandwidth is small enough) resonate either the primary or secondary or both to achieve the most optimum match. Also consider a narrow band impedance transformer instead of a broadband transformer this should be the lowest loss, again provided that you bandwidth is small enough.

http://rfcooltools.com

Title: Re: transformer optimization for power amplifiers
Post by aaron_do on Aug 2^nd, 2011, 6:56pm

Hi rfcooltools.com,

when you say "max available gain", are you referring to the definition of Gmax with a simultaneous conjugate match at both ports? I don't know for sure, but I think this is not possible to achieve Gmax in a passive device when loss is taken into account. Take the simple case where the DUT is a series resistor. I cannot see any way to match this device at both ports using passive components. That's why I think using Gmax as an FOM for a XFMR is a bit arbitrary.

cheers,
Aaron

Title: Re: transformer optimization for power amplifiers
Post by rfcooltools.com on Aug 5^th, 2011, 9:40am

Aaron_do,

For a broadband match I agree a simultaneous match cannot be achieved, but as I pointed out over a band limited range of frequencies it is possible to match both simultaneously. Also I agree the FOM can be less than meaningful for all conditions. Is your PA a narrow band or broadband?

http://rfcooltools.com

Title: Re: transformer optimization for power amplifiers
Post by inductioner on Aug 7^th, 2011, 5:44am

Hi Aaron,

I think Gmax is usually used for transformer characterization, or as one of passive device merits of figure. When using XFMR for design, T or Pi model can be more useful to gain more circuit insight.

I designed a few CMOS PAs with integrated or off-chip XFMR. I was pretty much following the old-time PA design methodology : Use load-pull technique to do all the followings : 1. Power match to maximize PA output power 2. PAE match to target at power-aided efficiency 3. For linear systems (especially OFDM ones) linear match is also one of the constraints to check AM-AM and AM-PM.

In so doing, I worked on load-pull analysis (after a few some basic checks on P1dB, AM-AM and AM-PM, stability, etc.) for PA core. excluding XFMR, which I consider as part of PA output matching netowork in the configuration I am figuring. The partition is arbitrary. I had included on-chip XFMR as part of PA core, the result was the same, as it should be. But the parasitics of PA core can play a major role in matching optimization especially when dealing with > 5GHz operating frequency.

So, back to you question : I normally don't worry about transformer characteristic precisely from initial design phase; only the first order model parameters such as turn ratio, magnetically coupling coefficient k, rough L/C parasitic values and so on you will deal with. In the mean time, transformer can be designed to meet your electric and magnetic needs with the best achievable insertion loss (determined mainly by substrate rho, resistive loss, low magnetically coupling coefficient, etc.)

In final stage of design, surely we need to put everything together to see if the result is good.

Hope the above is useful to you.

cheers,
inductioner

Title: Re: transformer optimization for power amplifiers
Post by aaron_do on Aug 7^th, 2011, 6:35pm

Hi,

thanks a lot for both of your replies.

rfcooltools.com,
actually what I was saying is that for a passive lossy device, it is not possible to get a perfect match at even a single frequency (i.e. narrowband. No need to consider broadband). This to me is important because it tells me that Gmax doesn't have physical meaning for a transformer. As I said, for an example, try to do a two port conjugate match on a series resistor. I have discussed this with a colleague, and he has concluded that for a passive lossy device, a perfect match can only be achieved for an odd number of ports.

inductioner,
thanks for the great advice. I'm quite new to PA design, so I hadn't really thought of "linear match" to optimize AM-AM and AM-PM. I assume you mean to do a load-pull simulation to find the load which optimizes AM-AM and AM-PM. Apart from that, I think I'm using a pretty similar design methodology.

thanks,
Aaron

Title: Re: transformer optimization for power amplifiers
Post by Gloria_evans on Sep 27^th, 2011, 4:25am

Here is the method used by the rhombus industries must be useful for you.
Follow this link "http://www.rhombus-ind.com/app-note/fr-il.pdf"

Title: Re: transformer optimization for power amplifiers
Post by aaron_do on Sep 27^th, 2011, 6:20pm

Hi Gloria,

thanks for the reply. However, I don't see how that schematic could work. The load side is shorted to ground, so how can there be a voltage to measure?

thanks,
Aaron

Title: Re: transformer optimization for power amplifiers
Post by weber8722 on Nov 24^th, 2011, 8:04am

aaron_do wrote on Aug 2^nd, 2011, 6:56pm:

Hi, for the series resistor Gmax would be just 0dB, but the impedances for load and generator would become infinite (not so special in a Smith chart :)). So Gmax is just one FOM, but not the most important one, in your application Gmax may not matter much, because you are limited to a certain range of impedances. Therefore available gain matters more.

Bye Stephan