calculate width mikrostrip

I have done a design for 12GHz microstrip LNA(Er=4.5 and 10.2). I used ADS2005A

I did my biasing network using microstrip and a drain resistor.
I used line calc tool to calculate the approximation for the inductor to microstrip and
while i just optimized the values of a radial microstrip stub as the replacement for the capacitor. I did this for the biasing network

My problem is:
1)how to actually calculate the inductance and capacitance required for the biasing network?For my design i used 1nH and 1.5pF but this is just a approximate value i used.

1) what is the actual calculation for the approximation of the inductor and capacitor to microstrip length and width in the Vdd biasing network for the amplifier?

i read the tutorial about microstrip approximation in www.rfic.co.uk but i still dont understand

I hope someone can guide me in this aspect. thanks

I am not sure exacty what you are trying to ask, but for a bias network , you are basically desiging a bias T. You want DC in one port and RF in the other two ports.

Are you trying to use discretes to make the bias network? Is this a narrow band design (<10% BW). If so choose a capacitor that has SRF equal to your center freq. Tee it off your out put 1/4 wave length and it appears as a open circuit, in other words it is not there to the RF signal but can pass DC to the transistor. After that I like to place a series inductor with SRF = to Fcen and then place my larger cap that removes bias noise.

Broad band gets more complicated. i often spend as much time modeling broad band bias structures as I do designing the gain.

C

thanks for the reply. Can u pls explain what is srf and the formula by which to convert the inductance and capacitance to microstrip lengths.

check this link , some formulas will be helpfull
http://www.zen118213.zen.co.uk/RFMic...Microstrip.pdf
http://www.zen118213.zen.co.uk/RFMic...tor_Basics.pdf

khouly

A SRF is self resonant frequency, where a cap becomes a short and an inductor becomes an open. Often a manufacturer will publish this data.

For a Transmission Line, TL, the impedance, Z = (L/C)^0.5

for 50 OHms L/C=2500. That is our L to C ratio.

Now assume a length of TL, say 1 meter. Lets assume propagation is c. The for 1V input, you have 1V/50 = 0.02A. The total Q on the 1 meter is I*1m/c or 66.6e-11 Columb. C=Q/V = 66pF/meter. L = 166.6nH/meter.

These are per unit length, you should be able to get the velocity of propgation from your TL calculator.

C

How do we calculate the inductance(rf choke) required for a biasing network.
my design is 12GHz LNA with Zin=15.143+j9.6410 and ZL=17.75+j4.99. Vdd is 8.3V with Rd=680ohm.

Actually i used microstrip but i want to know the formula for Rf choke calculation for understanding.

thanks