Why Inductor should be operated above SRF

[ah.vinod]

Hi,

I have very basic doubt. In RF designs, why should a Inductor should not be operated above SRF region even though just above SRF the impedance is still High. I am looking for RF choke application used in typical LNA MMIC. i am using 1 uH inductor whose SRF is 400MHz while my frequency of operation of LNA is 40MHz to 860MHz. Untill 860MHz the RF Choke or inductor is having Impedance in KOHms. so can i go ahead and use it.

Regards
Vinod A H

 

[keith1200rs]

Above resonance an inductor isn't an inductor anymore - it will be a capacitor (or at least predominantly capacitive). So, if you want the characteristics of an inductor it is no use. If you wanted the characteristics of a capacitor then you should use a capacitor.

Keith.

 

[ah.vinod]

Hi,

I understood the point, but instead of seeing the component as inductor or capacitor, i am more interested in seeing its impedance versus frequency plot which has high impedance value just above SRF. So can i not still use it for blocking those frequencies above SRF. i am only interested in few 100MHz after SRF frequency where still the impedance is high in a typical inductor.

Regards
Vinod A H

 

[keith1200rs]

But what function is is supposed to provide in your circuit? The phase will be totally different to if it was working as an inductor. "Ohms" are not the only consideration - if you were just interested in that you would use a resistor.

Keith.

 

[ah.vinod]

Hi,

Attachment shows my application. the Inductor Lc is used as RF choke. I cannot use a resistor because it will alter my dc bias point of LNA. the LNA output contains 40MHz to 860MHz while the RF Choke SRF is 400MHz and posses high impedance greater than 600 Ohms till 860MHz. So can i still use it.

Regards
Vinod A H

 

[keith1200rs]

My guess would be that there is a risk the LNA would be unstable. I may be wrong - check the LNA datasheet to see what it says about the load.

Keith.

 

[ah.vinod]

Hi,

the LNA used is RF3827 part from RFMD. attached is the datasheet. can u please confirm whether the Choke with SRF 400MHz can be used.

Also can u elaborate on the concept of dependency of load and choice of RF choke SRF and it effect on instability of LNA. I am very new to RF board design with LNA's.

Regards
Vinod A H

 

[keith1200rs]

You are probably better off posting on the RF forum:

Keith

 

[ah.vinod]

Thank u very much for guiding me to right place and also giving me better knowledge of LNA circuits

 

[keith1200rs]

I am not sure I helped that much - you have strayed outside my area of expertise. I see your problem - 1uH at 1GHz isn't normal (page 7 of the data sheet). My guess is that the extra compensation on the output has something to do with it, but it is only a guess. The guys on the RF forum know what they are doing though!

Keith.

 

[FvM]

The said parallel capacitance is about 0.15 pF, only slightly above "nothing" in a 50 ohm system