Hi,
I am supposed to design a 2Ghz colpits oscillator using any effective non-linear element..ie MESFET,BJT,JFETs, HBT etc
Can anyone guide me on the biasing circuit that one would need for the Colpits to run at 2Ghz. Most of the circuits i used earlier involved JFET or BJT's. But none of them work properly beyond a few Mhzs.
Any kind of help will be deeply appreciated.
Thanking you,
Pratik
Try using one of those ceramic transmission lines that are shorted on one end.
...Jim Thompson
-- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et |
Remember, your oscillator only needs a modest amount of gain (ideally 1.0, but in practice slightly more) at your intended operating frequency - and I can tell you from personal experience that it's very 'possible' to use a BJT for a 2 GHz oscillator. The "common base" colpitts is my favorite circuit - use a network tuned to 2 GHz to form an AC ground on the base, and use a capacitive divider to couple some of the output (collector) back to the input (emitter). You can start with a DC bias that sets I(C) and V(CE) to whatever the datasheet uses for the "typical" operating point, then vary from there as required. I'd recommend a transistor with Ft maybe 1.5 ~ 3x your intended operating frequency; i.e. use a 3 to 6 GHz Ft transistor. The higher the Ft, the more harmonics you'll have to contend with.
You will need to pay attention to the loading on the output though, the CB configuration ends up with a high output Z - a buffer stage is advisable to avoid "pulling", sometimes you can get away with very light coupling to the output.
I think I used an MRF901 'back in the day' to make 2.54GHz oscillators for field disturbance perimeter alarm sensors.
Good luck!
TJL
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