Colpitts Oscillator not oscillating

Discussion in 'Homework Help' started by yoan03, Dec 24, 2013.

  1. yoan03

    Thread Starter New Member

    Dec 5, 2013
    I have been trying how to design a crystal-controlled oscillator, but in the book Im reading "Introductory Electronic Devices and Circuit" by Robert T. Paynter says to replace the inductor for a crystal. I tried to simulate the circuit(Attached in the bottom) on multisim and didnt oscillate, then I tried on my breadboard and same result. Previously I used an inductor with 1mH and the circuit oscillate at 2.5Khz on both: in multisim and on my breadboard.

    I dont know if Im making a mistake with the capacitor C1 and C2, but what do you think guys where the mistake?
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
    The capacitors C1, C2 and C4 are about an order of magnitude too high...
  3. yoan03

    Thread Starter New Member

    Dec 5, 2013
    So which value for c1,c2, and c4 is good for the circuit to operate as an oscillator?
  4. Efron

    Active Member

    Oct 10, 2010
    You should be using values for c1 and c2 so that the "reactance" of those capacitors 1/(2πfC) are high enough for the circuit to oscillate.

    In your example, at f=32kHz, the "reactance" of c1 and c2 are almost 0 seen by the crystal/inductor. In other words, the crystal sees short-cut to ground.

    The c4 role is different. The "reactance" of this capacitor should be as much near to 0 as possible.


    Additional to that, I guess the base resistors are quite low values for an common emitter amplifier. This will make the input resistor of this amplifier to be low (some KΩ only). Although the circuit could oscillate in these conditions, it will be harder for it to do it in comparison with a higher input resistor amplifier.

    Note that the your tank circuit is connected in parallel to the input of the amplifier. If the input resistor is too low, the circuit will not oscillate (well, it will not keep oscillating) even when using correct values of c1 and c2.

    You could/should use x10 values for the base resistors.
    Last edited: Jan 2, 2014