Transistor Selection for oscillators

Notes from a cubical discussion

Bias current should be kept low to reduce junction heating, which increases noise. On the other hand, reducing bias current does reduce FT. The transistor should have an FT 15-20 times greater than the oscillation frequency. It seems that the ideal transistor should have enough FT with low bias current, but this isn't the complete story. To get high FT at low bias current, the base width and area is made very small, but this raises the current density and junction heating, which increases noise. So the best transistor should have high enough FT at low bias currents without increasing noise too much.

The transistor in an oscillator circuit should never be saturated and never be cutoff. Overdriving the transistor increases phase noise. In a Colpitts configuration, where the feedback path is tapped off the emitter path and LC tank elements are on the base path, add anti-parallel pairs of diodes to the emitter path to ground. This will prevent the transistor going into cutoff mode.