JFET buffer does not work

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neazoi

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Hello, I have made a jfet pierce crystal oscillator that outputs 3vpp sinewave on 1Mohm oscilloscope.

I would like to amplify this signal and lower a bit the impedance. I have used this buffer **broken link removed** with MPF102 fet and 1M gate resistor.
For the source resistor I have tried 1M, 100k and 100R. However I see no amplification and no signal output at all!
When I changed the fet to a j310 I see some signal output but it is attenuated!

What could cause this behaviour?
 

By nature, a common drain stage can't increase the signal voltage, the voltage gain is always < 1. The selected Rs values look arbitrary wrong, I would expect a resistor in the range 0.5 to 5K.
 

FvM said:
By nature, a common drain stage can't increase the signal voltage, the voltage gain is always < 1. The selected Rs values look arbitrary wrong, I would expect a resistor in the range 0.5 to 5K.
Click to expand...

Oh thanks, I was not aware of that.
What topology do you recommend as a buffer for this oscillator? **broken link removed**

would a single power fet like irf510 driven directly by the drain signal, work?
 

A buffer's reason is that it isolates the input and output. This is what you circuit does. If you use a 1M gate resistor and feed the output from the source, it will come from a 200 ohm source.
So if you are feeding something like a common emitter amplifier, then its low input impedance would would damp the oscillator circuit and perhaps stopping it from working. Using the buffer between the two stages would allow the oscillator to function and would supply more power to the next stage.

Frank
 

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