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Direct coupled oscillator

  1. Jun 4, 2005 #1
    Could some one please help me understand the current flow in this circuit (electron flow theory)... specifically, how does the capacitor charge, and how do the two transistors open/close? So far this is what I think...

    The current leaves the negative terminal, splits up into both branches... in the left path, the current goes into the emitter of the transistor (NPN) to the base, allowing current to flow from the emtter to the collector... much of the current returns to the positive terminal after leaving the collector, but some current takes the path through the resistor and the capacitor, charging it. In the right path, the current canot go past the transistor until it is opened by current flowing through the base to the emitter (PNP), but by this time, the capacitor is already charged and the current flowing through the circuit is practically nothing... this is as far as I get. I don't know how the current flows when the capacitor discharges...to discharge fully, energy has to be used up by the speaker.

    So if anyone could explain if what I have is right, and what happens after the capacitor charges, i'd appreciate it. Thanks.

  2. jcsd
  3. Jun 4, 2005 #2


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    I am more than rusty on this stuff, but I'll give it a shot. It appers to me to be a somewhat strange way to connect the speaker, but if this is what you were given, so be it. The capacitor will block the dc from flowing through it. If the voltage source is E then you should be able to figure out the dc voltage at the base of the PNP (also the voltage at the collector of the NPN), and at the base of the NPN, by considering base-emitter junction voltages. Since no dc flows through C, the two resistors will act as a voltage divider of the drop from the NPN collector to its base. So the dc voltage on the left side of C will depend on the ratio of the resistors. The base current of the PNP will depend on the size of those resistors (and how large E is), and the collector current of the PNP will be a large multiple of the PNP base current. The voltage at the collector of the PNP will be determined by the dc resistance of the speaker, because all of the PNP collector current has to flow there (not really a good idea to run dc through a speaker). When the capcitor is charged, the dc voltages will stabilize. Any small change in the base current of the PNP will produce a much larger change in the collector current and voltage, which will push ac through the capacitor, changing the base current of the NPN, changing the collector current. etc. Seems to be a rather unstable configuration. I guess that's why they call in an oscillator.

    I suggest you start by finding the dc voltages at the various points in the circuit, and then think about the effect an ac component will have to alter those voltages.
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