# which type of oscillator!

Discussion in 'Wireless & RF Design' started by aj_silverthunder, Dec 8, 2009.

1. ### aj_silverthunder Thread Starter Active Member

Jun 6, 2009
116
0
GUYs tell me does this oscillator works and do think its a clapp oscillator!.
2)and the main doubt is that does that 250uF cap also add its capacitance in the oscillator to produce HF.but if it adds ,the frequency becomes will be very low,if it does not add,what is the reason behind it that it does not add its capacitance to the oscillators!...

2. ### SRH New Member

Jan 30, 2010
4
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It's a common-collector Colpitts oscillator.
- LM386 amplifies the input signal and modulates the capacitance of the dual varactor bb139.
- L1, C(15pF), Cbe(33pF) and Ce(22pF) form the rest of the tank circuit. Cbe(33pF) and Ce(22pF) determine the amount of feedback from the emiiter branch back into the tank. The center frequency of oscillation is determined by L1 and the equivalent capacitance it sees.
- The resistors that tie to the transistor are all for DC biasing, except the collector resistor 330ohm provides some signal swing at the collector, which is coupled to the output with the 15pF cap.
- The 7809 and 7805 are three pin regulators providing DC supply and the "center" tuning voltage for the varactors.

The 250uF cap will add to the tank capacitance but only in that it is in series, or mathematically in parallel, with the series branch of the varactor. I assume the LM386 has a low impedance output, so in effect the left side of the cap is at AC ground and the lower branch of the varactor is shorted out by the cap. Often we'd put a series resistor of up to 10k ohm in series with the varactor tuning node to decouple the capacitance of the low frequency circuit from the tank.

SRH

Last edited: Jan 30, 2010