Re: New ideas for the Audio TX/RX pop and PA output stage improvements on ubitx.net

Jerry Gaffke
 

I find that train of thought rather confusing.
Not sure where that 0.3amps average comes from, I'd think you could just find
the impedance from 1/(2*pi*Hz*Farads) and divide that into the 50 volts RMS to find current
rather than bother with the calculus.
Not at all clear how a 0.1% dissipation spec somehow means a max of 0.2 Amps at 30mhz.
A few more details would be appreciated.

I think most builders just try some np0's and if they get warm then replace with a bunch of smaller value caps in parallel.
I'm not aware of any Bitx40 LPF caps blowing out, and I doubt they are anything special.

Jerry


On Sat, Feb 17, 2018 at 02:11 pm, K9HZ wrote:

The current through a capacitor is given by I=C*dV/dt so the average current over time is I=C*int[dV/dt]/t… For your example: let’s use a 100pf capacitor in an 30 MHz circuit at 50 VRMS which is 140V P-P.  Iavg = 0.42 * 0.707 = 0.3 amps average.  Now the spec from that capacitor in your finding is 0.1% at 1 VRMS at 1MHZ which is 0.00028A * 0.707RMS *1000% = 0.2 Amps continuous at 30 MHz.  Yikes we are over by 50%!  Note that this DOES vary with capacitance… that’s why the larger the capacitor, the more current capacity is needed.

 

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