Low noise amplifier (LNA) using CE3520K3
I am designing an LNA at 24 GHz using the discrete RF FET CE3520K3. I am implementing this design on 10 mil Rogers 4350B substrate. Just by looking at the S parameters (attached), I can see that the FET itself is inherently unstable at frequencies below 11GHz. So, I have been trying to make the LNA stable by various methods, but have been unsuccessful. The only way I could get unconditional stability at all frequencies is by adding a shunt resistor at the gate of the FET. I know that this increases the noise figure and the resistor has lot of parasitics at 24GHz, which might kill the design. I have looked for information on stabilizing very high frequency LNA design using discrete FETs in many textbooks and all over the internet, but could not find any information. Any suggestions are very welcome.
24 GHz microstrip bandpass filter on output is mentioned here:
I took a fast look at that FET but could not stabilize it in the ten minutes I spent, without messing up the minimum noise figure. I hope to mess with it later, I don;t like it when instability wins.
Does anyone have any experience with balanced LNAs? A friend of mine claims that almost automatically stabilizes the circuit but I am skeptical.
In my previous job, we had the same problem with a SiGe transistor. We copied the application circuit at the datasheet, but it was really bad regarding wideband stability. A low-pass type network including biasing circuit worked well. But it degraded noise performance a little. If you are not a big customer, usually vendors do not reply your e-mails.
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