Recently I met an interesting question for RF amplifier design. As we know, by adding a series inductor at Emitter, we can increase the stability of a BJT Common-Emitter (CE) amplifier. When it comes to a Common-Base (CB) or a Common-Collector (CC) amplifier, what should we do to increase its stability?

My objective is at 2.14GHz: (1) the CB amplifier works as a “current follower” (voltage amplification); (2) the CC amplifier works as a “voltage follower” (current amplification).

What I have tried for a CB amplifier is:
(1) Adding a series resistor at output, but the S parameters becomes really bad, typically S11 and S22. Also, the voltage gain decreases much.
(2) Using the ADS optimization tool to improve stability and S-parameters performance, but the results are not good. Typically the CB amplifier cannot work as a “current follower”. Also, the voltage gain decreases much.

The attachments are ADS simulations files and some simulations results. I hope they would be useful for your analysis. Thanks!

for common drain ( or common collector) amplifier put a series RC circuit in parallel with the the gate of amplifier .

jianke,

If you study some of the older VCO architectures you can get an insight into what to avoid.

Common base amplifier Colpits oscilator comes to mind.  

Zbe is base emittor parasitic capacitor 1/(j w Cpi).
Ze  is parasitic capacitance to ground for example 1/(j w Cpara).

Impedance looking into the base  Zin= Zbe + Ze + Zbe*Ze*gm

Notice that Zbe*Ze*gm will be negative and real.  This is why the colpits works, but not what you want .  Positive resistance in series with the base, if greater than the negative should negate this oscillation mechanism.

http://rfcooltools.com