mini brick mosfet driver

[ant17]

hi guys i have currently built a a colpits oscillator running at 10mhz. it is driving a IXFN140N30P mini brick mosfet directly through a 10 ohm resistor but the output wave is only 50khz the waveform is at the drain a square wave from the front but from the trailing edge it slopes down to just above ground almost like a sawtooth wave i. am using a 2uh inductor as a load current limiting resistor my point i am trying to get to is. what would cause it to run at a lower frequency than the one at the gate and what would cause such a weird waveform if you could help i would be greatful

 

[crutschow]

Post your schematic.

 

[ant17]

Post your schematic.

hi this is the basic circuit but mosfet modeled is not the one i have for my circuit i don't have a orcad model for it

 

[davoud]

If you refer to datasheet you can see your switch parameter :Rds & Ciss ,Coss,Crss that can couple with your inductor and make RLC filter. I think you should use a better switch for this frequency.

 

[FvM]

Whatever causes the generates the said 50 kHz signal, may be some kind of relaxation oscillation, the circuit can't work as intended. The oscillator is effectively shorted by the gate impedance.

If you don't know how the design RF circuits from the scratch, you either need to learn the basics or study known working circuits, preferably both.

 

[ant17]

hi guys i suspect the colpits oscillator does not have enough drive voltage to drive the mosfet at 10mhz 400mv P-P so i have been trying to amplify the voltage to 9v using a common emitter amplifier which use a 2n222a transistor which the setup is in the attached schematic but i can't get any amplification no gain at all from the amplifier but have tried it at 8khz and had amplification which sort of tells me the transistor is not fast enough but it has a transition frequency of 500mhz which i thought would be enough so i am really confused why no amplification at 10mhz could some one please help me

 

[davoud]

Your Mosfet is not fast

 

[FvM]

The amplifier stage has considerable gain at 10 MHz, but a low imput impedance that effectively shorts the oscillator. The oscillator will be a better transmitter without the additional amplifier stage.

I don't think that picking up arbitrary circuits and changing the component values at will will bring you success.