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Design >> RF Design >> Need help - Impedance Tuning in Power Amplifier
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Message started by RFJunkies on Feb 12^th, 2012, 6:58am

Title: Need help - Impedance Tuning in Power Amplifier
Post by RFJunkies on Feb 12^th, 2012, 6:58am

Hi there,

Can anyone help me on the tuning of the PA circuit?I have difficulties on tuning the circuit because of little knowledge in RF.This is my mini project in my class. I try numerous attempt to get the individual value of the resistor,capacitor and inductor but seems cannot get my gain. Please help me with the value and teach me how to do them

Thanks,
newbie,
Kazu :-[

Title: Re: Need help - Impedance Tuning in Power Amplifier
Post by aaron_do on Feb 12^th, 2012, 5:11pm

Hi,

first of all, the resistors R_s and R_L should be fixed (50-ohm for example). R₁ and R₂ should be chosen for whatever bias point you want (Class A, B, C...). L₁C₁ form a resonator with ω₀²=1/L₁C₁. Same goes for L₂C₂. You can determine L₃ using the smith chart, or approximately by finding the total input gate capacitance, and then resonating it with L₃. Lastly L₅C₄ transform R_L to a lower impedance. You first need to determine what impedance that is by the amount of power you need (call it R_LL). Remember that the maximum power the circuit can deliver is approximately V_DD²/2R_LL. You can then use the smith chart. If you don't like the smith chart, L₅ can be calculated by Q=R_L/ωL₅ and R_LL=ωL₅/Q (those are approximations). Also, ω₀²=1/L₅C₄.
That should get you started, but you probably need to fine tune. Also, any node which you want to determine whether it is resonating or not, simple inject 1A AC current into it and check the AC voltage. Remember to disable other AC sources first though...

regards,
Aaron

Title: Re: Need help - Impedance Tuning in Power Amplifier
Post by RFJunkies on Feb 13^th, 2012, 6:20am

Hi Aaron,

First of all thanks for the explanations :) it gives me alot of help. I wil try and give u my feedback.

I need to implement the circuit using CMOS technology and simulate using mentorgraphic. Because of the problem in tuning to match it using input signal of 2.4GHz the circuit doesnt show any gain at output node. Can u advise further?

thanks

Title: Re: Need help - Impedance Tuning in Power Amplifier
Post by aaron_do on Feb 14^th, 2012, 12:38am

Hi,

I suggest you try and tune it first, and if you can't get it to work, you can try posting the actual schematic that you designed, as well as some simulation results.

regards,
Aaron