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colpitts osc not working

Thread starter #1
Alright so I'm trying to make sense of a Colpitts Oscillator... and nothing is working :(

Here is what I have.



Now that MPS2222 transistor is runnin 200HFE, and that 25mhz crystal requires 18pF load capacitance...

Two questions:
1. Why does there need to be a transistor in oscillator circuits?
2. Why isn't this working!?

Thanks all for any suggestions!
 
#3
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...

The best thing for you to do would be to find an example of a properly designed oscillator, perhaps from a textbook or a web search.

When you have got that, there might also be some issues to deal with in getting an oscillator simulation to start, but first let's get the circuit right.
 
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Thread starter #4
No dice :( Q1 base still sittin tight at 4.69V.. and emitter isn't seeing any action at all

Thanks adjuster... I've done a circuit as such before based on a google images rampage... but now I'm trying to understand why and how it works.

I've done the following:


But now I need to understand what happens if I start changing parts around and such...
instead of a 15Mhz crystal what happens if I use one that requires a different load capacitance ect ect...resistors...ect..
 
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Thread starter #6
Hmm... ok... so how do I figure out what R1, R2, 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*
 
#7
There is something odd here: with no oscillation and the resistor values shown, the base should be about 2.2V, the emitter should be at about 1.6V.

Best to get the DCs right first.
 
#8
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).

Oops - nanoseconds, not milliseconds!
 
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#9
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.
 
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Thread starter #10
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 25Mhz circuit....

Here is a pic:


The orange question marks are pieces I do not understand the function/purpose of.
Those green arrows in parallel with the crystal, they are 100pF each, and when in parallel you add them together, so does that mean the crystal is receiving 100 load capacitance on each lead? Abnormally high if that's the case.

How do I calc the voltage going across the base and emitter? I understand there has to be a 0.6V+ greater than the emitter on the base in order to turn the transistor on.

Trying to break this down to the basics.... so confusing.

Thanks again guys for all the help... really appreciate the time you're taking.
 

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