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LC Oscillator...Help

Discussion in 'General Electronics Discussion' started by afsci, Aug 18, 2011.

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  1. afsci


    Aug 18, 2011

    I am trying to design an LC Colpitts oscillator.


    This is the circuit. I came up with this based on the Colpitts oscillator shown on this website, except that I did not have a high frequency-blocking choke to connect the collector to the positive power supply, and used the 10 K ohm resistor instead. The potentiometer biases the transistor.
    What happens when I turn the circuit on is (1) It does not oscillate until I turn the power supply up to its maximum of 18 V. (2) The oscillation, at ~2 MHz, is very weak (millivolts). Why does the circuit require 18 V and how can I increase the signal amplitude? I tried connecting the collector to +V through the primary winding of a transformer (as a replacement for a choke) and got no oscillation. On another note, I tried using a the primary winding of a transformer as the inductor in the tank circuit, and that also didn't if it's possible to make that work in some way, that would be helpful to because I have more transformers than I do inductors.
    Last edited: Aug 18, 2011
  2. davenn

    davenn Moderator

    Sep 5, 2009
    your cct is not correct and isnt like the one in the link you gave
    you have the 10k fixed resistor wired incorrectly you have no output from the collector via a capacitor. you must use the RFC between the supply and the collector.
    Replace the 10k variable with a fixed resistor. its the capacitors C1 and C2 and the inductor L that set the freq, not a variable pot.
    The RFC, L2 is essential to isolate the power supply from RF oscillations, without it the oscillator would become unstable

    Try starting again and wiring your cct the same way as they did in the link
    and report back on how you got on.

    Electronics 101 hint ..... when you are given a circuit, dont change it and then wonder why it doesnt work .... Note the second line in my signature below ;) :)

  3. afsci


    Aug 18, 2011
    Thanks! I just got another RF choke today. I've been trying to teach myself from the web how to properly set up the biasing and gains for transistor amplifiers, which was actually why I put the pot there, not for setting frequency.

    I'll try that and see how that works.
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