What's the general design procedure for trying to do a wideband impedance match to a device (in this case an LNA) which has very frequency-dependent impedance?
As a personal project, I'm trying to design an LNA for the RTL-SDR receiver which operates on the full 24 MHz – 1800 MHz range (it's very wideband, I know).
As I'm a student I've got the full versions of Agilent ADS, Genesys, and EMPro. Designing the amplifier itself in ADS is pretty easy – plop down an RF transistor, pick a bias point that's stable or near stable, and add neutralization if necessary to bring the transistor into stability.
My question is this: how do I match it to 50 ohms in and 75 ohms out? I know that I should match to the optimum impedance for low noise which I can extract from ADS (or Genesys if I export noise data with my S-paramters).
If the impedance was flat over frequency I could just do something like a Chebyshev match, but the impedance for optimum noise figure is frequency dependent. How do I handle something like this?
Best Answer
If you want to match the impedances at the input and at the output you could use pieces of transmission line and open circuit stubs between the transistor and the "IN"/"OUT" you are talking about.I always prefer open circuit stubs because I find short circuit difficult to create.You must be given the optimum reflection coefficient of your transistor.Use that as the source reflection coefficient (Since you want to minimize the noise figure) and proceed with TL and stub design.At the output you can maximize the gain.Output does not have much to do with noise.Just use conjugate matching.