colpitts osc not working

Posted on Feb 7, 2014

I`m afraid the circuit you have there is nowhere near correct. For a start, there is no DC path to the transistor base, so no base bias current is available. Q1 collector and emitter are connected directly to the battery with no intervening impedance, so the transistor cannot deliver any useful output. Hmm. ok. so how do I figure out what R1, R2,

colpitts osc not working
Click here to download the full size of the above Circuit.

Re, RL, and Ce should be I guess I don`t understand what resistor biasing does/means. C1 and C2 in that tutorial should both be the load capacitance (18pF in this case), so that makes sense, but nothing else really does *sadface* Once you have a reasonable collector current, which looks as if it should be a bit over 1mA, for the values you have, the next step is to give the circuit a nudge to try to start it. In the real world noise voltages should do this, but a simulation may need an actual kick. I suppose that you are doing a Transient analysis using something like PSpice. You could try putting a pure current source in parallel with the crystal, and applying a small pulse, say 1mA high for a pulse length of half the crystal period (33ns for a 15MHz crystal). Break the connection between C1 and Q1 base. Make sure Q1 base is still biased. Verify Q1 bias as shown above. Then, connect a signal generator to Q1 base, and measure the voltage at C1, where you broke the connection. If the voltage at C1 is a little higher than the signal generator voltage, then you should get oscillation. I am guessing it won`t be, and you`ll have to make C1 smaller. well the 2nd circuit I posted works fine, it`s the first one that I`m trying to create to understand what is going on. there is something I`m not understanding so let me break down the pieces that don`t make sense in the 15Mhz circuit (working) to try and make sense of the non-working...

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