Inductors are often used for RF amplifiers.

[Osawa_Odessa]

Hi, can anyone tell me why "Inductors are often used for RF amplifiers"?
What is the purpose of them?
Thanks.

 

[chuckey]

Inductors are one half of a tuned circuit, the other being capacitance. The capacitance might be an intrinsic part of a transistor, so adding a bit of inductance will reduce the effect of any stray capacitance over a limited frequency range. if an amplifier is required to amplify a limited range of frequencies, then adding an inductor and extra capacitance will do this, by presenting a high load impedance.
Frank

 

[sandeep_sggs]

i think the problem essentially is to realize a tuned circuit.Today maximum market is occupied by the CMOS. We know that MOS input side is essentially a capacitor, and to get a tuned circuit, inductive reactance(XL) should cancel capacitive reactance(XC). Hence inductor s used to get a tuned circuit going....Hope this helps. If you want to read further a book - "The Design of CMOS Radio-Frequency Integrated Circuits, Second Edition-by thomas lee" will be a good starting point!

 

[Osawa_Odessa]

Hi, thanks for helps.
I have read this:

RF Chokes
The impedance of an inductor increases in direct proportion to the AC frequency. It is near zero ohms at low frequencies and almost infinite at very high frequencies. This property allows inductors to pass DC while blocking higher frequency signals. Radio frequency (RF) "chokes" are components that squelch these high frequency signals and prevent them from interfering with other circuits.

Read more: https://www.ehow.com/about_6517825_explanation-inductor.html#ixzz2aWGIKwb6

Could you tell me why we need to block AC signal?

 

[spudboy488]

So you can "steer" voltage where they need to go. Think of an antenna system where there is a remote DC relay at the antenna end. You could send 2 cables (1 for the RF and 1 for the control of the relay) or you can send both on a single cable. Use an inductor to supply the DC voltage to the cable and use a capacitor to supply the RF to the cable. The inductor allows the DC to pass and blocks the RF. The capacitor allows the RF to pass and blocks the DC. At the other end, repeat to send the DC to the relay and the RF to the antenna. This is what a bias tee does (https://en.wikipedia.org/wiki/Bias_tee)

 

[Fabien]

Indutors are used as DC blockers. You bias the transistor with a DC voltage, but you don't want the AC going out of your circuit via the biasing. So the inductors block the AC signal.