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Posts posted by walid

  1. The circuit in the following figure (from: http://www.hobby-circuits.com/circuits/power-supply/dc-power-supply/891/variable-dc-power-supply) represents a variable power supply
    Transistor Q1 is npn type
    This was corrected in the following figure
    I have thought carefully about this circuit, but I could not get to the mechanism of how it work!
    I think that when the transistor Q1 operates, it must turn off Q2.
    I try to simulate it using multisim10, but it didn't work, that is when I change the value of any of P1 or P2 the output is fixed at near the retectified voltage.
    Any help in clarifying the way this circuit work would be welcome, thanks

  2. Thanks audioguru
    I am very happy with your presence and I hope to be in good health and happy in your life.

    I have read in a book "The art of electroncs" page 66, the following

    I take it most likely the first stage is a source of signal with internal resistance (Zout1), and the second stage with input resistance of (Zin2),,,, what would you say?

    Note: I miss you here too long and I had to register on many forums and I put the same question to ensure the receipt of a convincing answer from one of them.

  3. Hi,
    If there are two BJT small signal amplifiers (common Emitter) cascade connected using a suitable decoupling cap
    The first preamp has an o/p imeadance Zo, and the second has an i/p impedance Zin
    Now we have Zo and Zin and a cap betwwen them
    My question is: Is the connection series or parallel? That, is the o/p voltage divided or the o/p current?

  4. This circuit was intended as 27MHz colpitts oscillator:

    and whem simulated using multisim10 sofware I get:

    I have some questions:
    Any osci. Needs a FB to sustain oscillations, I think that this FB from collector to base path through the parallel two caps; the internal transistor capacitance b/w collector and Emitter, and the C4(=4.7p) cap.
    From the Fairchild datasheets of the 2n3904 transistor I get:

    [img width=680 height=70]
    [img width=680 height=91]
    Also, there is a curve related the capacitance with REVERSE BIAS VOLTAGE (V) 
    My questions:
    (1) What is the approxi. value of the internal transistor capacitance b/w collector and Emitter at 27MHz?
    (2) How the phase shifted from 180 degree at the collector to 0 or 360 degree at the base? Please give me a step by step illustration.
    (3) The freq of the osci. was 25.4MHz while my calculaed value of the resonant freq was 27MHz? what factors affect this?

  5. china-bulb-torch.jpg

    I've built this emergency lighting circuit
    When the main voltage 230 V is present, LEDs turn off
    And when does not LEDs turn on
    Transistor and 1n4148 diode are responsible for making the LEDs turn off
    Question: How do the 1n4148 diode makes the transistor on and off?

    note: the burnt R = 5.6k
    When I implemented the circuit it worked very well

  6. Another reason for using many capacitors in parallel is it gives lower impedance to higher frequencies if they're spread across the PCB traces.

    I understand from your words that if I had two caps, one of them is equal to 5p and the second = 10p.
    Better to use the two in parallel to get 15p and not one 15p to make an easy path for the high-frequencies toward ground???
  7. Ok I get it,

    "AM is produced because the transistor's gain alters as the base current is varied and FM because the capacitance between the PN junctions also alters as the voltage varies.

    BUT: FM is alters as the voltage varies, you mean the voltage of the voice signal, not the voltage of the battery?

    in the circuit above, if we omit the voice section with its preamplifier, will remain only the oscillator section
    will it change its frequency with change in battery voltage?
    thank you

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