RFC at the collector leg of transistor

Hi,

When designing an amplifier or oscillator in common-emitter configuration,
at the collector leg, sometimes the resistor is replaced with inductor?

What is the purpose of replacing the resistor with inductor?

An inductor is used to increase the collector load impedance. In an oscillator, there is usually a parallel capacitor with the inductor, forming a high-Q resonant circuit.
In wideband RF amplifiers, an inductor feeds the DC current to the amplifier while the RF spectrum is coupled to its output via a capacitor.

Is there any reason when designing a Low Noise Amplifier that I would prefer a collector resitor over a collector inductor?

Since resistor also can feed the DC current to the amplifier, why do I need an inductor to feed the DC current to the amplifier?

hi
while you are biasing the amplifier you should take care of loading the output to stop gain decreasing.on routine is using inductor but you should consider the SRF.so
please see this web page:Mini-Circuits then go to amplifier then first part:Biasing of Constant Current MMIC Amplifiers (e.g. ERA SERIES)

many LNA MMICs are matched o input and output impedance of 50 Ohms under recommended conditions. One is using an inductive bias tee in collector circuit. If you replace it with a resistor, this resistor is connected in parallel with the load impedance of 50 Ohms and may cause a mismatch. Then the amplifier has a lower gain and may also increase its noise figure. I have tested various MMIC LNAs and other "gain blocks" with collector inductors as well as with resistors, and found not much difference. A resistor bias tee offers a lower-frequency extended response but it is not often needed. Otherwise I prefer a coil as I can use it to adjust amplifier frequency response over a desired bandwidth.

Yes, there is. The resistor might give you better stability for a transistor LNA.

Hi, I have checked out the document.
It shows the loss that is caused by the biasing resistors.
However, it didnt explain how exactly by adding a RFC can do the circuit better.

That fact that chokes do not have DC losses is usually a good enough reason to use them. It's just a waste of available power and voltage to use a resistor.