help : transmitter circuit explanation

Discussion in 'The Projects Forum' started by t-bone, Apr 4, 2008.

  1. t-bone

    Thread Starter New Member

    Apr 4, 2008
    could anybody explain me the transmitter circuit. i have a small doubt. i am a beginner and please correct me if i am wrong.

    my doubt is ,the oscillator is in common emitter configuration. the feedback is given from the collector. the output at the collector of CE CONFIGURATION is INVERTED,right? so the feedback given from the collector is out of phase with the input. that becomes negative feedback. then it is not an oscillator. but the circuit is working my idea is wrong. so please explain me.
  2. Audioguru

    New Member

    Dec 20, 2007
    The RF oscillator transistor Q2 operates as common-base not common emitter. The base is shorted to ground at RF frequencies by C4.

    The feedback is through C6 from collector to emitter which both have the same phase so the feedback is positive resulting in oscillation.

    The audio transistor Q1 operates as common-emitter. It has R1 providing negative feedback.

    Doesn't the very simple circuit change its frequency when something gets near its antenna or moves away? And when its battery voltage runs down?

    Doesn't it sound muffled on an FM radio without any treble sounds?
  3. mik3ca

    Active Member

    Feb 11, 2007
    I got a question. Why is 470pF chosen as the grounding capacitor?

    I find that using a smaller value capacitor (like 150pF) works alot better than
    large value capacitors (like 1nF+)
  4. Audioguru

    New Member

    Dec 20, 2007
    At 100mHz, a 470pF capacitor has a reactance of only 3.4 ohms so it is like a dead short. It does not affect audio frequencies in this circuit.
    A 150pF capacitor has a reactance of 10.2 ohms which is also a dead short and makes no difference if they are both high frequency ceramic disc types with very short leads.

    But a 1nf capacitor reduces the audio high frequency response a little.