What are the functions of Input Network in power amplifier?

[hanhan]

Hi,
I am reading about RF power amplifier. Here is the block diagram of a RF power amplifier.
Can anyone tell me the functions of "Input Network" here?
And also I have read that RF choke used to provide approximate current constant. Is this right?

 

[ronsimpson]

Input network can be: Output network might be:
1)Band pass filter, low pass filter, high pass filter.
2)Impedance matching circuit. example:75ohms in 200 ohms out.

approximate current constant= yes

 

[hanhan]

Thanks ronsimpson,

Input network can be: Output network might be:
1)Band pass filter, low pass filter, high pass filter.
2)Impedance matching circuit. example:75ohms in 200 ohms out.

That makes sense!

approximate current constant= yes

Is my understanding correct?
RF choke is used to block RF signal and pass DC or low frequency signal. Therefore, the current flowing through transistor is only DC component or low frequencies and it is a constant.

 

[ronsimpson]

A choke wants to took the current the same. (constant current) [not voltage]
A capacitor wants to keep voltage the same [not current]

The choke is big enough that current can not change much during one cycle of the RF.
It is true the the RF stays off the power supply in part because the choke blocks RF current from getting to the supply and there should be a capacitor from supply to ground that keeps the supply voltage constant.

The real purpose of the choke: If a resistor was used it will get hot. power loss The output voltage will be some where between ground and supply (max).
A choke wants to have 0 volts average across it. When the transistor is on, it makes half of the waveform from supply to ground. When the transistor is off it swings above supply by the same amount. So the output is from ground to 2x-supply. (max)