What type of AC coupling capacitor is in the input of LNA ?

[jecyhale]

Hi, everyone

Usually, there is an AC coupling capacitor in the input of LNA as a DC block.
So is it internal capacitor or external capacitor?


Thanks in advance.

 

[Bhanumurthy]

Re: LNA matching

Hi,
The AC coupling capacitor to the input of an LNA should be an external one.
bye
Bhanumurthy.

 

[RF-OM]

Re: LNA matching

To jecyhale,

I think it depends on frequency and design requirements. If you design 5 GHz 3.3V LNA and need 2pF cap, definitely it can be internal. But when you do 5 MHz LNA that should be installed distantly (for example on high antenna mast) and therefore have high voltage range it may be impossible to have in inside the chip.

Best regards,
RF-OM

 

[nassim1]

Re: LNA matching

Dear colleagues,

That is right it depends with the deigned cicruit at operating frequency...

In high frequencies (microwave band) instead of DCblock you will find DC blocking coupled lines....

Mouloud

 

[jecyhale]

Re: LNA matching

Thanks RF-OM,
the frequency is about 1.5GHz, and the cap value should be 5~10pF.

Added after 3 minutes:

To jecyhale,

I think it depends on frequency and design requirements. If you design 5 GHz 3.3V LNA and need 2pF cap, definitely it can be internal. But when you do 5 MHz LNA that should be installed distantly (for example on high antenna mast) and therefore have high voltage range it may be impossible to have in inside the chip.

Best regards,
RF-OM

I want to know, whether the parasitic capacitor of external cap is less than that of internal mim cap or not, for example 10pF.
I have done the extraction of internal mim cap.
The parasitic cap of 10pF mim cap is about 200fF.
So it is not easy to match it because the total parasitic cap will be as large as 400fF with ESD and pad.
Thanks.

 

[RF-OM]

Re: LNA matching

Dear jecyhale,

External capacitor definitely will have smaller parasitic capacitance than your numbers. The exact numbers depends on substrate and ground plane under the pads. If you remove ground plane under the pads with about 10% spare margin you may get very low parasitic capacitance. At your frequencies you definitely will get better results with external cap. Do not forget to choose right value by minimum impedance point (if your matching needs do not require another solution). If you give substrate data I can tell you exact number for parasitic capacitance.

Best regards,
RF-OM

 

[jecyhale]

Re: LNA matching

Dear RF-OM

Thank you very much.
What is the substrate data you want?

BR
Jecyhale

 

[RF-OM]

Re: LNA matching

For calculating parasitic capacitance I need substrate dielectric constant and thickness as well as the size of your cap. For example 0201 or 0402. If you will tell me your frequency, bandwidth and required cap voltage I may find the best cap from DC blocking point of view (not necessary the best for matching if you need it).

BR,
RF-OM

 

[Ruritania]

Re: LNA matching

jecyhale -- You need to calculate the input impedance of your LNA core. If the input impedance is low and already matches to 50ohm (CG topology, for instance), you have to use the off-chip cap, coz it would too bulky to integrate it on chip.

Hope it helps,
Ruri