Clapp Oscillator Problem

[ANTALIFE]

Hello

I have been trying to make a Clapp Oscillator from a JFET and some L/C/R's (see pic)

With these values the circuit should be oscillating at ~40kHz but I am not getting anything? I have triple checked all connections, soldered it on veroboard, tried biasing Gate, and tried using a 2N5484 JFET but still nothing...

Is there something else i'm missing?

[danadak]

C3 should be eliminated, so that inductor serves as bias connection
as well as part of tank.

Or bias with additional DC path gate to ground high valued R -




Regards, Dana.

[rhb]

It looks to me as if you are missing a 1N194 from the gate to ground.  The attached is from "Solid State Design for the Radio Amateur" by Hayward and DeMaw, which *really* should be reprinted by ARRL.  I know of know equal.

My understanding is that the Clapp is just a Colpitts with a capacitor in series with the inductor for tuning.

[Cyberdragon]

You don't need a diode for it to operate, it's just there to adjust the gate bias. The lack of any bias at all is what's causing the oscillator not to start.

[rhb]

I see two issues, R1 is rather large and V2 is rather small.

But my main purpose was simply to post the circuit.  I don't recall ever building this, but I built  several of the more basic circuits in the book. Sadly too busy finishing degrees and earning a living to spend more time on them.  I'm working on my penance and reformation ;-)

[CJay]

It looks to me as if you are missing a 1N194 from the gate to ground.  The attached is from "Solid State Design for the Radio Amateur" by Hayward and DeMaw, which *really* should be reprinted by ARRL.  I know of know equal.

My understanding is that the Clapp is just a Colpitts with a capacitor in series with the inductor for tuning.

I believe there's a PDF copy on the CD included with the recent reprint of EMRFD from the ARRL?

[T3sl4co1l]

The tank impedance is rather low for the amount of bias current (and therefore transconductance (gain)) the JFET has.

Try a smaller resistor.

The gate bias comments are also spot on.

Tim