Low-Noise Microwave Amplifier Design

Once I had to design a fast transistor amplifier, in common base. I did some tricks to bias it but it wasn't stable. As it hapens, Bob Pease was in town for a seminar (he is , also, a guru!). At the end I asked him for an advice - a 50 to 75 ohm resistor in the base. That removed any oscillation.

For s parameters, go with the data sheet (Agilent are good) or rent a device to measure the individual transistor s parameters.

Yep! that's always been the standard way to stop those spurious oscillations, the small value resistor is just enough to stop any resonance of the base capacitance with the previous stage inductance of track etc...

Look at any high frequency linear amplifier and you will see loads of 68 ohm resistors in the base lead!!

John.

Difficult to answer your question with the limited information provided.

In general you have to stabilise the transistor, either using feedback, resistive loading as you have suggested, or a combination of the techniques. The stabilization technique would likely be dictated by the amount of gain that you can afford to sacrifice. The new s-parameters can be calculated by matrix techniques using the s-parameters of each of the building blocks used. (It would likely be easiest to transform to T-parameters for cascaded elements and Y-parameters for parallel elements.) I would suggest the use of simulation software tools, that would make life a lot easier.