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Low Device Impedance - RF Cafe Forums

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

Amateur Radio | Antennas | Circuits & Components | Systems | Test & Measurement

Post subject: Low device impedance
Unread postPosted: Tue Apr 05, 2005 10:19 pm

Hi All :

I'm dealing with low-impedance FET for power amplifier design.
I realized that at some gate voltage biasing point, the IM3 balance performance (between IM3 high and IM3 low) is worse, while the absolute IM3 value is better.

when I tried lower gate voltage IM3 balance is improved, same as when I tried higher gate voltage. So means, at only this gate voltage, the phenomenon shown.

I guess it's probably canceling out phenomenon of the device?
I am not sure about this.
and also I observe this low-impedance device is very sensitive to gate voltage, instead of current.

I mean even though the drain current different by 15mA to 20 mA, the IM3 differs a lot, at this gate voltage.

Could you enlighten me in this issues?

will appreciate your advice :wink:

Thank you.

Jean :-)


Post subject:
Unread postPosted: Tue Apr 05, 2005 11:52 pm

Hello Jean,

I found out a similar phenomenon in my power amplifier which is based on LDMOS technology (Enhancement MOSFET), this phenomenon is called" sweet point" meaning the gate voltage level that gives you the best IM3 performance. If you go around this point you will se that IM3 degrades, so this is a singular level. RF power amplifiers manufacturers use to measure that level at various temperatures and design the bias circuit to give that voltage and closing a loop over it to maintain the bias level over temperature.

This issue has of course to do with the intrinsic structure of the device.



Post subject:
Unread postPosted: Wed Apr 06, 2005 1:02 am

Thank you for your helpful advice.

By the way, I'm not sure whether I could say LDMOS as low-impedance device? I'm not familiar with this LDMOS device as currently I'm using GaAs MESFET.

I'll appreciate if you could give me some information / white papers on this LDMOS sweet point issue. :wink:

Thank you so much.



Post subject:
Unread postPosted: Wed Apr 06, 2005 1:39 am

Hello again,

You can find useful information on LDMOS on Google Search. I am sorry, but I never invesitigated this issue thorughly.
The prominent manufacturer of LDMOS transistors is Freescale.
LDMOS are widely used for Cellular/UMTS/PCS bands, i.e. 800-2100MHz, however there are lower frequecies models down to 200MHz.

What is your frequency band?

By the way, as higher the output power the device is capable to as lower its output impedance will be.

Hope this helps.


Post subject:
Unread postPosted: Wed Apr 06, 2005 9:58 pm

Hi there,

Thanks again for your advice.

My amplifier frequency band is 1.8GHz.
I am wondering whether this phenomenon that appears in MESFET, is mostly happened at low-impedance MESFET?

Because I did not experience this at higher impedance MESFET. I am not sure about this.

Could anybody enlighten me? :wink:

Thank you so much.

Best regards,
Jean :D


Post subject:
Unread postPosted: Wed Apr 06, 2005 11:47 pm


It happens in higher impedance devices as well. It is a common phenomenon in these devices. The best way to observe this phenomenon is to move up and down with the gate voltage around this point, do that and you should see how IM3 products goes up and then up again around a singular minimum extremum point.

Can you give more details about your application? like output power?
Do you use some sort of linearizer?

Anyway, I have done comparisons of efficiency and linearity between LDMOS and GaAs FET at your band too. The LDMOS technonlogy leaves the GaAs FET way behind both at linearity and efficiency.

You can write to me too: itre27@012.net.il

Hope this helps

Posted  11/12/2012

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