# Gain and Bandwidth - RF Cafe Forums

The original RF Cafe Forums were shut down in late 2012 due to maintenance issues. Original posts:

Qacer
Post subject: Gain and bandwidth
Unread postPosted: Thu Apr 28, 2005 1:11 am
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Joined: Sat Apr 23, 2005 2:09 pm
Posts: 17
Location: Tampa, FL
Hi all,

I was just reading the Pozar book and on p.553 it mentions that "it is preferable to design for less than the maximum obtainable gain to improve bandwidth."

Is there a formal derivation to this statement? I'm curious why.

Thanks!

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Jeanalmira
Post subject:
Unread postPosted: Thu Apr 28, 2005 1:25 am
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Joined: Tue Mar 15, 2005 11:43 pm
Posts: 65
Location: Singapore
Hi,

I think it's related to this condition "Gain-Bandwidth is equal to constant", which derived from Bode-Fano Theorem.

Therefore, by designing for less than the maximum obtainable gain, wider bandwidth will be obtained.

Thanks and Regards,
Jean

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Guest
Post subject:
Unread postPosted: Mon May 02, 2005 1:30 pm

The last post is absolutly correct. Fanos limit shows that for wider bandwidth, the gain will be reduced.

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Guest
Post subject:
Unread postPosted: Tue May 03, 2005 8:49 am

Aint Bode-Fano's criterion showing the VSWR-bandwidth product that a matching network can have for a given quality factor mostly for passive devices shuch as antenas?

I always had the impession that remplacing VSWR by GAIN was an oversimplification. In my mind it only work as stated if the device in unilateral.

Second, what is this GAIN-Bandwitdh tradeoff? What gain? A flat gain or the Maximum available gain over frequency, whatever value or rolloff it shows?

Very often people will design matching network to autocompensate the gain rolloff and obtain a flat response, by sacrificing the VSWR.

I see Bode-Fano as a good starting point to sea if a matching is feasable.

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Guest
Post subject:
Unread postPosted: Tue May 03, 2005 11:43 am

You are correct , it is a criterion for passive networks. The matching networks are passive, therfore, the wider the bandwidth the higher the loss, hence lower gain.

Posted  11/12/2012

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