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Hello, i need a little help with inductor used in jfet

Discussion in 'General Electronics Discussion' started by michael1978, Jun 21, 2018.

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  1. michael1978

    michael1978

    388
    2
    Mar 17, 2012
    Hello can somebody help, what do the inductor L1 is to stop high frequency of wat, this is circuits
    LC OSCILLATOR.png
     
  2. duke37

    duke37

    5,364
    772
    Jan 9, 2011
    This is an oscillator using Q1 and Q2 as the feedback circuit. It is a type of multivibrator.
    The frequency is determined by the tank circuit consisting of L1 and capacitors to earth at each end. CV is variable to trim the frequency to the desired output frequency.

    The frequency can be roughly calculated from the equation F=1/(2*pi*srt(L*C)). Take C=38pF. The actual frequency will also depend on stray capacitances and inductances.
     
    michael1978 likes this.
  3. michael1978

    michael1978

    388
    2
    Mar 17, 2012
    Thank you very much…..Have a nice wekend. ;)
     
  4. michael1978

    michael1978

    388
    2
    Mar 17, 2012
    O sory, can you just tell me where discharege
    O Duke i have some question, i search on the internet, to find how the circuits works, but i did not find something like this………..is possible to describe how work the circuit please
     
  5. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
  6. michael1978

    michael1978

    388
    2
    Mar 17, 2012
    I take from that book, but he dont explain, only that gate drain is configuret Q1 and Q2 Q3 source follower, Q2 is coupled via 22p and back to stage Q1, 18Ω resistor is to stop parastic oscillacition, more not, i still dont understand, for that i ask help, just little bit how it works.
     
  7. duke37

    duke37

    5,364
    772
    Jan 9, 2011
    To make an oscillator, the loop gain must be more than one. Normally, as here, the gain is much more than one but it is reduced when the amplitude reaches a limit. This high gain can generate high frequency parasitic oscillations which are not wanted. The 18Ω resistor will damp the high frequency circuit to give a loop gain of less than one. Resistors or lossy ferrite beads are used in critical positions which will depend on the devices used and the actual layout.
     
  8. michael1978

    michael1978

    388
    2
    Mar 17, 2012
    Duke i will like just to know how charge and discharge L1 and C just that i want to know thnx
     
  9. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    This question makes absolutely no sense in this contect. The circuit is an oscullator and C1 L1 will be "charged" and "discharged" alternatingly with a frequency of approx. 9 MHz.
     
  10. michael1978

    michael1978

    388
    2
    Mar 17, 2012
    Thats is right LC charge and discharge i know, but i never see something like this circuit, but like Duke say, after is feedback Q1 thrugh Q2, because i never use fet transistor, for that its new for me. and Q3 get signal from Q2 right? of not…..and Q3 is output?:(
     
  11. michael1978

    michael1978

    388
    2
    Mar 17, 2012
    What is wrong, i dont understand, i dont know where is output...…….
     
  12. duke37

    duke37

    5,364
    772
    Jan 9, 2011
    In #6 you explain just how ir works. I do not see how to explain it further.
    Q3 is fed AC through the 120pF capacitor. Q3 is a source follower where the transistor will give the same AC voltage out but be capable of higher currents, sufficient to drive 50Ω.
     
  13. michael1978

    michael1978

    388
    2
    Mar 17, 2012
    Ah for that i ask , do i know of not., THANK YOU DUKE:), have a nice day….
     
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