How to cascade an LNA?

I have a problem. I need a LNA with gain 40 dB. But until now I don't get it. The gain of LNA that I have is just 27 dB with noise figure 2 dB. I try to cascade them. Is it possible? If it is possible, how about gain, noise figure and impedance matching? Thanks a lot for your answer.

if u have a good matching , the gain will increase , the noise figure will decrease a little bit , coz the noise figure is mainly depend on the first stage NF , so u need to design a good matching circuit between the stages , and in the output to get max power transfer

khouly

By the way thank you for your answer. I have the next questions. Is the gain increase linearly? Or any particular calculation to calculate the increasing of the gain? If the input and output of LNA have 50 ohm, should I make impedance matching?

what u should do , is to match the input of the second amp to get MAX gain , this may increase the NF of the second stage , but the total gain will be Maxminzed as well

the gain in ideal case will be the two gain in dB added to each other

khouly

I would cascade the existing 27 dB low noise stage with a 13 dB stage with a worse noise figure but a better output intercept point. Since there is so much gain in the first stage, the noise figure of the second stage is not important. But the compression point might be of the cascaded pair.

I have tried to cascade my LNA. But the gain is not increase instead attenuation. What is the problem? Is their impedance didn't match? I read from the datasheet of LNA written that the input and output impedance is 50 ohm. Should I check the impedance again? Should I make impedance matching for this?

what is the part number do u use , and what is the max signal input to the LNA

khouly

The part number is ZRL-700-SMA and the max signal input is +10 dBm.

Hi!
I think you are using a too high input power and/or some oscillation has been generated.
I suggest you to insert an attenuator between the 2 amp ... you have 16 db of margin on the required gain so a 10 db attenuator should be fine: it also contributes to improve matching between the 2 amplifiers.

You may want to try Microwave System Calculator because problems like the one you are facing can be easily debugged with mwsc. [don't forget to visit the howto section]

http://digilander.libero.it/mwsc

I've prepared the input file for you: attached! :D

Regards,
Max

Yes, I agree. Make sure the simulation software is not "simulating" your amplifier in compression!

The other possibility is that the LNA stage is oscillating or conditionally stable, and has an apparent gain that is very high until you change the load and the gain changes dramatically. Try a 3 dB pad between the two stages.

It seems that a lot of people are having troubles on this forum with neophyte problems of simulation. Today's simulation programs often allow the input power and DC operating levels to be simulated. If you overdrive the input with too much power, then you are going to get strange results. Play around with the drive level to see for yourself what is going on. If some of your transistors are biased incorrectly, then they are going to get saturated and not work properly. Use the program's analysis tools to make sure that the input and output of every device has a non-clipped waveform centered at the correct DC quiescent operating point, and you will avoid much of this.