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Posts posted by walid

  1. Thank you Hero999, you helped me

    To build an FM transmitter which doesn't produce much AM, you need to use a varactor diode as part of the tuned circuit, rather than altering the bias point of the base.

    Like this circuit?

    Another question:
    Why the transmitter frequency change when the battery goes down, despite that the calculation of LC tank frequency does not doesn't take the value of the battery into account?
    thank you alot

  2. The hartely oscillator has many drawbacks as do ther other simple oscillators. It could have to do with the resonant frequency of the LC and the ability to get high enough gain.

    Please i didn't understand.
    It may also have to do with any change in load conditions that could prevent or stop oscillation.

    what if we use a buffer circuit between?

    thank you KevinIV
  3. Hello
    When one design  preamplifier using BJT voltage-divider unbupassed Emitter config, one want max symetrical signal swing at the collector. To do this, VCE = 0.5 VCC &
    I (saturation) = 2 IC = VCC/(RC+RE)
    If I want a voltage gain = A, then

    A= RC/(RE+re), where re = 0.026/IE, so

    RC = A(RE+re) --------------(eq.1)

    as said before, for max symetrical signal swing at the collector,

    2 IC = VCC/(RC+RE)  ==>

    RC = (VCC/2IC) - RE  ------(eq.2)

    From (eq.1) & (eq.2) we get:

    A(RE+re)= (VCC/2IC) - RE, solvig for RE yields:

    RE = {VCC/2(1+A)IC} - (A/A+1)re

    question #1: Is there any error in either equations or concepts
    If yes, corrected me, please

  4. The 4.7nF capacitor creates a lowpass filter with the unknown value of the source resistance.
    The 10k resistor and 6.8nF capacitor create another lowpass filter.

    If the cutoff frequency of both filters is the same then their total is -6dB because each filter is -3dB.

    If the cutoff frequency of both filters is  not the same? is it sitll -6dB

  5. A transistor needs to have a high collector current (and a high base current which reduces its input impedance) and a low resistance collector resistor when it is in class-A and it needs to drive a low resistance load.

    Are you mean like emitter follower?
    What about a voltage divider configuration preamp. is there a situation here that we want a high IC current?
    I stay, think long time about it, choosing small current (IC) give us a high Zin a long batt life and a transistor works in light duty, why not choose a very small current like 0.1 mA or even 10 uA, i know that big resistors will be used but this is no broblem!
    thanks alot.
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