# which type of oscillator!

#### aj_silverthunder

Joined Jun 6, 2009
116
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!...

#### SRH

Joined Jan 30, 2010
4
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

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#### djoseph

Joined Jan 10, 2020
2
It is a colpitts oscillator. Have no experience with this circuit. Use colpitts design most of the time as its stable with many kinds of crystals/resonators.

#### Janis59

Joined Aug 21, 2017
1,523
If the tank is serial, the right name is Clapp. If the resonance is parallel, the name is Colpitts. Clapp oscillator may have a full branched capacitative divider between emitter and base or may have a hardly reduced one like in this circ. Then, using a reduced, some transistors even permit to start strong generation without of capacitor between base(gate) and emitter(source) - just it role is taken by parasythic C(be), its very handy for higher frequencies. BUT high sensitivity about used transistor type is imminent. Just may happen that one brand is working brilliantly, whilst others never and no-how.
YET, this circuit may have a firing side too, literally, smoke will persist. At the good enough the Q-factor, and I have seen a many of 100x...1000x the voltage on tank capacitor may raise up to 25 kV even if Vcc is 9...24 Volts only. Thus was the my case with this circuit when I got it to use some 20 years ago for ICP plasma ignition. Thus, if You use the very ordinary capacitor, dont be surprized why it not works, just the cap is died from the very beginning.

#### Janis59

Joined Aug 21, 2017
1,523
And one more thingy - 6k8 and 15k both biasing resistors ought not be connected to the place there are. High frequency high voltage makes them carbonize, arcing and burning in less than minute if the generation will begin once. They must give the biasing via small choke, that the RF is not let go through but DC is gong freely. I am using the 24V 10Amp with 100pF 25kV 54 MHz tank and then choke is some 30 turns of 0,3mm wire on the 5mm glass rod. Tried a 20 turns but then resistors warms up too much.
PS - explanation: in this circuit the max gate voltage must be purchased, all the mosfets with 20 or 30 V will be blown. Must find one with at least 40V allowed. And if to take in account that parasythic inductance between physical coil and gate leads, on resistors RF voltage may occur even 100V or more. Infrared thermometry of the circuit have proven this sentence.

#### Janis59

Joined Aug 21, 2017
1,523
RE: Silver thunder point (2). - I cannot to see anywhere 250 microfarads in this circuit. Where You see?
P.S. In the power source side 0,1 is hardly too small. I am using 4000 microfarads with 100 pieces 0,1 microfarads paralelly and even then it goes rather hot. But okay, we are in different sides of the supplied current value.

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