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AM circuit design please help

Discussion in 'Radio and Communications' started by Walid, Jul 31, 2010.

  1. Walid

    Walid New Member

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    How should I select the values of the resistors for the following circuit to work as AM modulator ? where should i set the bias point ?
    AM Modulator

    random selection of resistors does not work , so what is the basis of determining the resistors' values? please help me. thank you
     
  2. Bloodninja

    Bloodninja New Member

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    this looks like a bjt amplifer, if you want to adjust this circuit then you have to put in mind the frequency reponse of the BC548 which then depends on the values of capacitors and resistors in the circuit
     
  3. RadioRon

    RadioRon Well-Known Member

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    To start with, simplify the bias arrangement to ignore the AC circuit. R5 and R4 are a voltage divider that sets a voltage at the base of the transistor. That voltage, less 0.7V, appears at the emitter. Since the emitter voltage is known and the emitter resistance R2 is also known, you can calculate the emitter current. The emitter current is almost identical to the collector current, so to simplify we can say they are equal. This collector current passes through R1 which then lets you calculate the voltage drop across R1 and you now know the collector voltage. Now you know the bias currents and the voltages at the transistor terminals. To design, choose a collector bias point that is half of your Vcc and work backwards to get the emitter and base resistances. The simplified approach assumes that the base current is a small part of the current through R4. If you lower the value of R2 a lot, then this assumption breaks down and you may have to reduce the base bias resistance to suit.

    The AC circuit simply provides an AC short across the emitter resistor and VG1, boosting the AC gain of the stage from the point of view of VG2. This circuit is a simple common emitter ac amplifier from the point of view of the carrier from VG2 so the bias design follows basic common emitter practice. There is nothing special about the bias resistors to make this an AM modulator. The only special thing to make this a modulator is the addition of VG1 in the emitter circuit.
     

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