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[raphael]

Good morning
I have seen in a schematic, a tank circuit connected to the output of an integrated transmitter. The given values of such circuit depends on the frequency of interest. My question is, according to you what is the goal of such circuit? To make a resonator at a specific frequency or to match the output impedance of the transmitter to the input impedance of the (assumed) antenna.
Thank you in advance for your reply

[Schubert]

I assume by tank circuit you mean a parallel inductor and capacitor.

Tank circuits are not used to make resonators or impedance matching. Tank circuits do present an infinite impedance at a specified frequency. Hence these circuits can be used to filter specific frequencies (i.e. notch filters). Far above and below resonant frequency the circuit is short (i.e. passes all energy). The low frequencies pass through the inductor and high frequency energy passes through the capacitor.

[raphael]

thank you Schubert
the tank circuit in question was a kind of PI network (+ 1 capacitor between the inductor and the 2nd capacitor). i have found a paper (RF amateur) that explains that you can split this Pi structure in 2 parts:

1/a tank circuit (first capacitor+1 part of the inductor), that helps to have a good load resistor value (#Vcc²/Pout) at the amplifier/transmitter output+ a good balance for the Q factor (gives good component inertia + prevents from strong harmonic level)

2/a matching network (second part of the inductor+ second capacitor) to match this load resistor to the impedance of the antenna

Do you agree? It is fuzzy to me because i have already seen what you have written concerning the L//C structure...

[Schubert]

If it is a pi network, it is not a tank circuit. Pi networks are primarily used for impedance matching or filtering. What is the circuit diagram?

[raphael]

Dear Schubert
what do you mean by diagram? Bode diagram or the schematic?
I don't have any bode diagram. The schematic is a PI structure with an additional cap in series (after) with the inductor.

According to you, is the (correct) load resistance that must be present at the ouput of an RF amplifier (with output transistor), equal to the output impedance of this transistor?

Thank you