I have seen many designs where the antenna/PA output is fed DC voltage through an inductor. Why not leave it out and feed the DC bias directly? It is L1a in the diagram below:
There are some values in the table below for some working frequencies. Is L1a also part of a filter/matching network? What is the general method of arriving at a certain value for it?
Source: MAX4146x EV KIT
In the light of the answers below I shall attempt to answer the last question myself.
An LC pass-band filter centered around the operating frequency, and the other parameters pretty much made up:
IN: Number of LC pairs: 1 Cutoff frequency (Fc): 314.8 MHz Passband: 315 MHz Impedance (Zo): 50 Z Ripple: 3 Z OUT: L: ~ 50 nH C: ~ 5 pF IN: Number of LC pairs: 1 Cutoff frequency (Fc): 433.8 MHz Passband: 434 MHz Impedance (Zo): 50 Z Ripple: 1.5 Z OUT: L: ~ 24 nH C: ~ 6 pF
Values calculated at https://www.allaboutcircuits.com/tools/bandpass-filter-calculator/
Maybe the values from datasheet were calculated similarly? Or maybe I did not understand all that much.
Probably miscalculated there... anyway, a bit more on the issue:
Generally,is stated that the output impedance of the bias circuitry should be kept small, in order to increase the linearity of theoutput bias stage. However, the output impedance is typically designed to have alargeresistance in order to reduce the noise contribution from the bias circuitry, and to avoidsignificant loading on the RF input port. For example, to use an inductor in the bias circuit to form lowimpedancenear DC,and highimpedance near the RF signal band