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Wideband Mosfet matching - RF Cafe Forums

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Extreme
Post subject: Wideband Mosfet matching Posted: Wed Oct 31, 2007 7:07 am

Captain


Joined: Sun Feb 04, 2007 2:58 pm
Posts: 9
Hi people,

I would like to learn how to match this mosfet to 50 ohm in wideband 88-108 MHz.
http://www.mitsubishichips.com/Global/c ... 06hvf1.pdf

I would like to know how to start,and what tools can be used.
Not asking for a fully explanation,but maybe some steps how to deal with this.

Thanks


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IR
Post subject: Posted: Wed Oct 31, 2007 2:18 pm

Site Admin


Joined: Mon Jun 27, 2005 2:02 pm
Posts: 373
Location: Germany
Hello,

I had problems to view the data sheet you provided.

Anyway, general rules for matching of RF power transistor:

1. Detemine the DC operating point of the device.
2. Measure the S-Paramaters at this operating point, or use the
S-parmaters provided by the manufacturer if applicable.

Note: Some manufacturers provide a more comprehensive simulation model (Like Spice model), which enables to perform also Harmonic-Balance simulation.

3. Design the matching networks based on the the provided/measured
S-paramaters for the required bandwidth of operation.

4. Ensure that the following conditions are met:

- Stability for the entire operating band and also out of it as much as possible.
- Achieving the required gain.

Tools which are useful to perform such design are: ADS, Microwave Office.

Hope this helps!


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Feng
Post subject: Posted: Wed Oct 31, 2007 2:35 pm

Captain


Joined: Sun Mar 19, 2006 5:25 pm
Posts: 21
Location: Burnaby
IR is right. The only S-parameter you can use is on the situation of Vdd=12.5 and Id=500mA. There are smart components in ADS for wideband matching, and you can find them at Impedance Matching. After you figure out the initial parameters of matching circuit, you may simulate it in SP or HB by using the s2p file created from the provide S-paramters.

As IR suggested, you may measure the S-paramters in order to do sweep simuation and observe the operating states.


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Extreme
Post subject: Posted: Wed Oct 31, 2007 2:35 pm

Captain


Joined: Sun Feb 04, 2007 2:58 pm
Posts: 9
I realize now,i ask an impossible question.

Let me ask you guys some partial question.

But i rediscovered RF-sim99 i have on my system.
It has an 2 port parameter function.
I copyd the S-parameters from the datasheet and made a S2P file of it.

The curve was on the smith chart and @100MHz i can read some data:
This is just output data.
Imp. 24.52R-j38.02R
Series equivalent 24.52R/41.87pF
Parallel equivalent 83.45/29.56pF

With an automatch i get a sharp match with 100nH and 32pF.

First question:
Is it correct to say:
@100MHz the output of the fet can be compared to real 24.52R with a 41.87pF cap in series.
Or a real 83.45R with a 29.56pF cap in parallel?

I think i'll wait for an answer before asking more questions.


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IR
Post subject: Posted: Wed Oct 31, 2007 2:55 pm

Site Admin


Joined: Mon Jun 27, 2005 2:02 pm
Posts: 373
Location: Germany
Hello Extreme,

Yes what you wrote is true.

Only one correction: For parallel equivalent the imaginary part is inductive, as the lower part of the Smith Chart for admittances is inductive.


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Feng
Post subject: Posted: Wed Oct 31, 2007 2:57 pm

Captain


Joined: Sun Mar 19, 2006 5:25 pm
Posts: 21
Location: Burnaby
I thing you can say that, but them vary with the frequency. Therefore, it is not a good news for a wideband matching. By the way, what is the bandwidth of the sharp match you got?


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Extreme
Post subject: Posted: Wed Oct 31, 2007 3:32 pm

Captain


Joined: Sun Feb 04, 2007 2:58 pm
Posts: 9
I have to say i am a absolute beginner in calculation/simulation with these kind of things.
Hobby'ing with RF for only 2 years now.
Mostly building designs or combine schematics.
http://fets.arecool.net
But i like to know why i build it this way,and learn designing myself.

But Feng,you bring me to another question.
You asked how wide my matching is.
I have a strong dip in the reflection line @100MHz
I think the wider the match, the reflection is getting more
So there has to be a compromise between them.
What would be an acceptable reflection coefficient for 88-108.
25dB or higher?


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Feng
Post subject: Posted: Wed Oct 31, 2007 4:34 pm

Captain


Joined: Sun Mar 19, 2006 5:25 pm
Posts: 21
Location: Burnaby
I visited your website and saw many amazing works although I don't know the language on it.

The practical return loss depends on the isolation between the transmitter and the antenna as well as how many reflection power can be endured by the transmitter, although the reflection power should be as less as possible. I really don't what is better for your transmitter, but for a basestation of mobile communication, the VSWR should be less than 1.3, the corresponding return loss is 17.69dB.


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Extreme
Post subject: Posted: Wed Oct 31, 2007 5:12 pm

Captain


Joined: Sun Feb 04, 2007 2:58 pm
Posts: 9
Ok now i can answer.
@18dB reflection and simulation @100MHz i get a bandwitdh from 94 to 106 MHz.

Just thought of someting:

If i would create a matching from coils and C's from Fet to 25Ohm @92MHz.
and followed by a matching from 25ohm to 50ohm @102MHz.

Could this be a way to increase bandwidth?

Thanks guys for your fast answers for so far.


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Feng
Post subject: Posted: Wed Oct 31, 2007 5:30 pm

Captain


Joined: Sun Mar 19, 2006 5:25 pm
Posts: 21
Location: Burnaby
It needs to calculate the serial connection matching networks first, and then you will find out whether this method increase the bandwidth or not. You may use simulation software to figure it out, or transform to Z matrixes to get their product.




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