How much output impedance has on Class C RF amp output without its matching network?

[Willen]

(RF amp for FM band and power approx 1 to 5 watt.)
Another question- I want to use fix capacitors instead of trimmer on the output impedance matching (Pi) network, are there any online calculator?

 

[ronsimpson]

Can you give us our schematic?

Typically the Pi network is adjustable to match the frequency and impedance. It is important to pass the frequency, reject harmonics and get the maximum power to the antenna.

 

[Sangoma]

Also there is a reason why trimmers are used, the value is not going to be exactly the same on each build, a slight difference in position or construction of inductors will change the value. It might be close, but not right for optimum performance.

 

[MikeMl]

Transistor classC amps usually have an output impedance which is low, like a few Ω. If you try to use a pi-net the Q is too low to provide sufficient harmonic suppression. Pi-nets are more likely to be seen in tube amplifiers which have an output impedance of hundreds of Ω.

You need adjustable trimmer capacitors to tune the final stage.

 

[ccurtis]

A handy, versatile matching network calculator that I use: https://home.sandiego.edu/~ekim/e194rfs01/jwmatcher/matcher2.html

 

[Willen]

Can you give us your schematic?

Amp like this , we discussed before.

 

[ccurtis]

Hi. This page does a pretty good job of explaining the output impedance of a transistor in a RF class C amp stage: https://hem.passagen.se/communication/pa313.html

There is a resistive (real) component to the transistor's output impedance that is not accounted for in the explanation, but it is generally only a few ohms and was neglected. The transistor's output capacitance is accounted for, however. You are not likely to find output impedance data on a transistor's data sheet, except for high power RF transistors specifically manufactured for that purpose -- such as Fig 5 in the attached, for example.