Stabilizing a Transistor

The scattering and noise parameters of a BJT at 4GHz are:

S11 = 0.65∠−95° S12 = 0.035∠40° S21 = 5∠115° S22 = 0.8∠−35°

Show how resistive loading can stabilize the transistor.

Ref. Gonzalez, Microwave Transistor Amplifiers: Analysis and Design 2nd ed., Prentice Hall, example 3.3.2, p.226


s2p Model



Simulation Results

With R1=Infinity, the transistor is potentially unstable:  K=0.547, |Δ|=0.504.
With R1=500Ω, the transistor becomes unconditionally stable:  K=1.02, |Δ|=0.413.
Smaller R1 values result in more stable amplifiers at the cost of reduced power gains.

Re-simulation with