Thank you, MrAl and rumpfy.
MrAl,
This looks like an RF circuit.
Yes, this is a RF power amplifier.
In an RF circuit you adjust the center frequency once to the carrier frequency and leave it.
I think your "center frequency" is "resonant frequency of LC", right?
That works because most of these circuits have a much higher carrier frequency where the signal bandwidth is small compared to the carrier. So the circuit stays tuned for the carrier and works with the signal too because the Q is low enough to allow the low end and high end of the spectrum to pass. So the Q has to be low enough but that usually isnt hard to get.
Thanks, I am happy to know that. Here is a picture illustrated LC filter with various Q.
My question now: How about if the circuit used with two different center frequency fc1 and fc2?
Is there a way to adjust resonant frequency fo from fc1 to fc2?
Or we have to manually adjust fo?
rumpfy,
In the power stage of the amplifier, you have to supply the output transistor with DC power.
Thanks, got it.
This power is supplied through L1. The impedance of L1 is 'zero' at DC but increases as the output frequency rises. By using a suitable value of inductance, the impedance of L1 can be effectively 'open circuit' to the high frequencies being amplified by the power amplifier.
I understand what you said here but there is one thing that I am still confused. Why we need to block high frequencies (through using RF choke)?
Any high frequency energy can then be supplied to the load 'R', and not lost in the power supply.
Could you explain more? I don't know why these high frequencies relates to energy loss in power supply.
Aha, in small signal model, L1 is in parallel with R. At high frequencies, the current through L1 is zero and therefore all current is diverted to R. And no losses in RF choke, right?
The network LC comprises a tuned circuit and this circuit is designed to be resonant at the frequency required. At frequencies far different to the resonant frequency, there will be a signal loss in the tuned circuit. The resonant frequency of the tuned circuit is given by f = 1/ 2*PI * sqrt(LC). The formula shows L and C can be any value so long as their product gives the correct resonant frequency. However, the frequency range over which the resonant frequency of the network is resonant can be adjusted by the choice of L versus C. This controls the Q of the circuit.
It is important to have a tuned circuit LC in the output network so that the fundamental resonant frequency is transmitted and any harmonic frequencies are attenuated by the action of the tuned circuit. The transmitter MUST only transmit the resonant freqency.
I got it.
It really helps. Thanks.