VCO: common emitter vs. common collector

[el_coronel]

Dear colleagues,

Recently I design in AWR MWO two types of X-band VCO using BFR840 transistor: with common emitter and common collector.
I always used to thought that CE circuit provides a higher value of output power than other ones. But I see the reverse situation: while CC circuit gives about 7 dBm of output power, CE scheme gives something around -10 and less dBm.
The CE is more preferable due to the transistor topology.

Would someone explain, please, what causes such a big power decrement while changing CC to CE?
Not being a big specialist in active devices simulation, may be I'm doing something wrong?

You could find the archived project file attached.

 

[pragash]

colpitts VCO requires CE configuration. hartley VCO requires CC configuration. the characteristic of the VCO depends upon type of VCO you design..

if you device not reccommended for CC configuration by the manufacturer, you better select another VCO which suits CC if you need 7dBm output power..

 

[el_coronel]

There are two "pro" arguments for using CE circuit:
1) Manufacturer recommended use CE due to the transistor pin configuration. This is actually not very strong argument.
2) CC that I designed seems to be very unstable, it could works fine on a determined frequency, but there appear a lot of "parasitic" 2pi phase shifts on arbitrary frequencies while frequency tuning. This is not strong argument too, but having a stable CE circuit, I want to use it and have a good output power (more than 0 dBm).
Although output power value is not very important, it is something like an educational project. I just think, that -10 dBm is very low value. Or it is not?

 

[pragash]

typical value of 0dBm will be adequate. -10dbm is low. if you have to use CE, you can add a buffer at the output but it comes with additional cost! since its educational project, i would recommend you to experiment another BJT with different characteristic and have stable intrinsic elements at CC configuration...