# How to design matching network of wide frequency lna?

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#### chb8795412

##### Newbie level 5
how to design matching network

HI.
i have used BFG540 (philips) to design a wide fre. lna for 45MHz~850MHz
After testing ,the input VSWR of lna is more than 2.5,but my design request is less than 1.5.
I want to ask how to design the input matching?
thanks!! 8O

#### BigBoss

##### Advanced Member level 5
lna matching network

Have you ever used " feedback" ? If you haven't , you never get a matching over a wide band . Additional to , you have to apply either series and parallel feedback to obtain flat S21 and acceptable S11 and S22. In this case Noise Figure will also degraded..

Parallel feedback will consist of R-L-C circuit , series feedack will be only one resistor ( maybe a parallel capacitor to it )

Otherwise , S21 will drop 6 dB per octave and S11 and S22 will be changed with the freuency.

amntt

### amntt

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#### flatulent

##### Advanced Member level 5
matching network for microwave lna

If you can stand the reduced gain and degraded noise figure, you can put a small value attenuator on the input.

The other method described above is the best way. AdLab has a two port program that will include such feedback networks when it calculates the stability and other parameters of single transistor amplifiers.

It is available from http://home.t-online.de/home/weberconnect/esoftw2.htm and requires a share ware fee of $45 US or so. This set of programs is well worth the$45.

#### chb8795412

##### Newbie level 5
design matching network

Thanks Bigboss and flatulent very much!
i still two questions :
1.In my lna design ,there is a л-type high-pass filer before zhe lna .Through changing the value of L and C,the vswr is stilll more than 2.5 on 100M~270M,Is there other types of netwok ?
2.On designing matching network,the wide and length of transmit line is important ,is there anything should pay attenion to?
thanks!!

#### flatulent

##### Advanced Member level 5
matching network for lna

If you know the input impedance vs frequency, you should be able to have a CAD program make the network. The more L and C elements you have, the closer match you can have over the frequency range.

I have used the veripole two port program I mentioned above to cut and try the feedback elements to get a good 50 input.

My favorite is genesys 6.5 which is simple to use and has an optimizer.

The old motorola mimc matching program allows you to cut and try element values. It is attached below.

#### godz

##### Member level 1
matching network at 2 frequencies

chb8795412 said:
Thanks Bigboss and flatulent very much!
i still two questions :
1.In my lna design ,there is a л-type high-pass filer before zhe lna .Through changing the value of L and C,the vswr is stilll more than 2.5 on 100M~270M,Is there other types of netwok ?
2.On designing matching network,the wide and length of transmit line is important ,is there anything should pay attenion to?
thanks!!
Hi chb,
Concerning the transmission line attached to the matching network, the optimum is to design the transmission line to have a characteristic impedance (Zc) equals to the impedance level you are matching to, for instacne, 50ohms. To do this task you need to know the transmission line type you r using(microstrip, coplanar, coplanar with ground plane), the PCB board information(Er, substrate thickness and conductor thickness) and finally the frequency of interest. The following link **broken link removed** has an appelet for designing a coplanar transmission line with ground plane.
My advice for you, At this frequency the width of the TL is expected to be wide, so try to shorten the length of the transmssion line as much as you can, then you don't have to design according to Zc, and make the width small (~0.5mm). But if the length is greater then lamda/10 then follow the TL design rules.
regards,[/url]

#### chb8795412

##### Newbie level 5
rlc matching network

Thanks very much to all.
The forum is the best .

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