Re: gEDA-user: OFF: capacitors for RF power amplifier

[Wojciech Kazubski <wk0@xxxxx>]

> I'm currently designing a power amplifier for the HF (3-30MHz) radio
> band.
> 
> I am selecting capacitors for the low pass harmonic filter bank at the output.
> My question is what kind of capacitors should I use? I apply not more then
> 100V of say 30MHz maximum.
> 
> My best bet is to use X7R capacitors with as much DC voltage rating as I can
> get. I don't know if there's any connection between the DC and AC losses.
> 
> 
> Thanks,
> Levente
> 
The biggest problem can be the current handling capacity. Very few capacitor makers specify this. Usually if a capacitor is rated for 100V this means 100V DC wthout any current. Only special purpose capacitors for RF power applications have HF current ratings, see Anerican Technical Ceramics for example:
http://www.atceramics.com

For RF do not use X5R/X7R (good only for supply bypassing), use NP0/C0G or similar low loss ones. Their maximum capacitance is limited to few nanofarads in 1210 case, so it may be necessary to connect few in parallel to get desired value. Also if you do not have capacitors with high current ratings, connect several smaller ones in parallell instead of one big.
Foil type capacitors are suitable for lowest frequencies (LW/MW) and have to be low loss. Polipropylene or teflon ones are low loss and usable, while poliester ones are too lossy. Parasitic inductance can also be a problem.
Check your PA circuits with circuit simulator for resonance effects. In this case currents and voltages can be much higher.

Wojciech Kazubski


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