Transistor base current in series feedback oscillator

Hi,

I'm designing a dielectric resonator oscillator in series feedback configuration. I use SiGe HBT transistors which have Ib(max)=3 mA and Ic(max)=30 mA.

When designing without regard for Ib I get good results - required output power, required phase noise and the Ic is not exceeded. But the Ib is 10 mA or more.

I measure that Ib by placing an ammeter on the schematic (Agilent ADS) between base pin and the microstrip line that is coupled to the resonator.

When I try to maintain Ib below 3 mA I get 10 dB less output power and 10 dB worse phase noise. Do I measure that Ib correctly?

The transistor has to accept current from the DRO and reflect amplified signal through the same pin - so I think this high Ib is correct behaviour.

Harmonic Balance simulation says, that this current appears only at the resonant frequency (not at DC), so I think that current is not caused by bias circuitry.

Maybe I just can't get anything better with that transistor? (Infineon BFP740 @ 10 GHz)

Piotr

I doubt that you're "measuring" Ib correctly. Without additional indices, the Ib rating refers to the average (DC) current, not an AC magnitude.

Harmonic Balance splits Ib into DC and AC components. I have for example:
- 0.211 mA at DC
- 12 mA at resonant frequency
- 0.056 mA at 2nd harmonic
- 0.004 mA at 3rd harmonic
etc.

P.