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audioguru said:I would use a 2N3904 transistor with 100k for R2. The transistor's and battery's current will be much lower and the transistor's input impedance will be much higher.
The oscillator doesn't have much power because the transistor is turned on too much and the emitter resistor wastes power at high current. You can't reduce the value of the emitter resistor much, or you will need to increase the value of the feedback capacitor so it isn't loaded down. Then the larger feedback capacitor would load down the tank and also lower the frequency of the oscillator.mstechca said:doesn't the 100K make the transmitting range lower?
I'm looking for higher transmitting range.
Is there an equation that indicates the capacitor value required based on the emitter resistor or vice versa?You can't reduce the value of the emitter resistor much, or you will need to increase the value of the feedback capacitor so it isn't loaded down.
Then its like I'm getting two transmitters ganged together. I don't want to deal with two controls to adjust the frequency.If you want more output power, then add a tuned-collector amplifier transistor without an emitter resistor. Then the antenna will be on the amplifier, reducing its frequency-changing effects on the oscillator.
I use batteries.Do you have a regulated supply for the oscillator?
The feedback capacitor plus the transistor's capacitance plus stray capacitance forms a voltage divider with the emitter resistor. In my FM transmitter's oscillator, if I reduce the emitter resistor to 47 ohms for a higher power output then the transistor would need a gain greater than 4 to oscillate.mstechca said:Is there an equation that indicates the feedback capacitor value required based on the emitter resistor or vice versa?
In my FM transmitter, the tuned circuit in the output amplifier is broad enough to cover the entire 20MHz band, because the antenna's impedance is so low it damps the tuned circuit.I don't want to deal with two controls to adjust the frequency.
Without a voltage regulator, doesn't the frequency of your oscillator change as the battery's voltage runs down?I use batteries.
Why would it require higher gain?audioguru said:The feedback capacitor plus the transistor's capacitance plus stray capacitance forms a voltage divider with the emitter resistor. In my FM transmitter's oscillator, if I reduce the emitter resistor to 47 ohms for a higher power output then the transistor would need a gain greater than 4 to oscillate.mstechca said:Is there an equation that indicates the feedback capacitor value required based on the emitter resistor or vice versa?
That to me is obvious.audioguru said:If its impedance was zero then the output would be zero and the circuit wouldn't oscillate.
This is the part I know and understand well.The tank at the collector is a high impedance parallel LC circuit, therefore affects frequency...
I don't understand why, when the tank, the positive feedback capacitor and the emitter resistor are all in series from VCC (or +ve) to ground....not output level.
The choke doesn't do any good in series with the emitter. A 3nF cap across the choke is a dead short at your high frequency.mstechca said:so I should have a capacitor in parallel with the choke?
Then you would have a useless shorted choke.then I would have 2 tank circuits. I'm trying to avoid extra controls.
You didn't hear me. An electolytic cap is an inductor at your high frequency. It might have just enough capacitance between its wires to allow your circuit to work. A 1000pF ceramic disc cap with very short wires is far better as an RF supply bypass. The cap should be across the supply terminals of your pcb, not at the battery. Each inch of wire adds inductance in series with it.As soon as the cap in parallel with the battery was removed, the transmitter did not work. So I always try to have the cap in.
The bigger is the cap, the higher is its inductance. Inductance is the opposite of the capacitance that your circuit needs.I even went to 1000uF (not pF) for the supply bypass cap and I didn't notice much of a difference.
I see what you mean. The high impedance of an inductor in series with the emitter allows the total feedback signal to go into the emitter, instead of dividing some into an emitter resistor. Also, the low resistance of an inductor allows low DC voltage drop so the transistor gets the full supply voltage allowing more output power.Ron H said:The RF gain is not reduced by adding an inductor in the emitter circuit. In fact, gain should be increased, since this is a common-base circuit as far as the oscillator is concerned.
I agree. If you use an inductor, you still need a resistor in series.audioguru said:I think that DC emitter degeneration would still be required for bias stability.