Basic Colpitts Oscillator Circuit.

Thread Starter

tjones91

Joined May 6, 2020
19
Hello, I do have some experience and I am passionate about learning more about electronics.
Hello, I am trying to learn about different oscillator circuits and was looking at the Colpitts oscillator here which is similar to the Hartley oscillator except the Colpitts uses capacitors instead of inductors in the tank circuit. I will have the circuit below.

I want to make sure that before I ask questions about the circuit, I want to make sure that what I know so far is correct. So tell me if this sounds right.
STATEMENTS
1) The Colpitts oscillator uses a capacitive voltage divider network as its feedback
source.
2) C1, C2, and L form a tank circuit that causes the oscillation. This oscillation goes
through C4 to the output and also at the other cycle of the wave charges and
discharges C3, which turns on an off the NPN transistor.
3) RFC is picked so that the inductor has high Reactance at the oscillation frequency
and low resistance to DC to help start the oscillation.
QUESTION
I will ask more questions about this circuit once I get some clarification but speaking very broadly is the Colpitts oscillator circuit and the Hartley in some way like a very basic Microphone into the base of a transistor circuit? Where the signal is being amplified but i believe that in the Colpitts and hartley circuits, the transistor also must
put feedback to keep the oscillation going, as with the microphone transistor circuit the source of the oscillation is the human so no feedback is necessary?
P.s I am speaking very broadly about comparing this circuit to a mic amplifier circuit. I most mean that both have analog signals that are being amplified.
Is that true? Thanksoscillator-osc13.gif
 

sparky 1

Joined Nov 3, 2018
170
The microphone to amplifier uses different principal. The resonant circuit (the tank) interacts with the transistor common emitter amplifier. The explanation is broken down the same. 1st the resonant tank senses a change in it's capacitive voltage divider. Take a moment to visualize the small potential change going on at a node between C1 and C2. The inductor is not responding to sound waves but is reacting to a magnetic and electrostatic wave so the analogies are closer to fluid however the result is a vibration of sorts. The coupling capacitor C3 keeps the DC away from the careful work you made biasing the base. I give you credit for understanding the resonant circuit as a feedback network and the amplifier. Analogies can go down in education and difficult to remove. The steam engine analogy is classic for messing with our minds. Remember the amplifiers of old used tubes so the terminology changed, voltage level changed, the parts are smaller but the Colpitts is still a Colpitts.
 
Last edited:

LvW

Joined Jun 13, 2013
1,016
Hello, I do have some experience and I am passionate about learning more about electronics.

1) The Colpitts oscillator uses a capacitive voltage divider network as its feedback
source.
2) C1, C2, and L form a tank circuit that causes the oscillation.
Well, when it is your intention to learn something, you should really UNDERSTAND the working principle of the circuit.
In this case (oscillator), it is absolutely necessary to know about the oscillation condition (formulated by H. Barkhausen).

Apart from gain requirements, this conditions requires a feedback path producing a phase shift within the whole feedback loop of 360 deg (identical to 0 deg).

Between base and collector we have phase inversion (180deg) - that means:
We need another 180 deg from the feedback network.
Question: Can you see if the "tank circuit" can produce 180 deg?
We know that - at resonance - the phase shift of the tank circuit is zero deg. Hence - this explanation does not really help.

Therefore, another explanation is much more descriptive:
Together with the output resistance ro at the collector node, the feedback elements form a third-order lowpass in ladder topology: ro-C1-L-C2
(C1 and C2 are grounded).
We know that such a circuit can produce a phase shift of -180deg at one single frequency only. This is the oscillation frequency because there is another phase shift of 180deg between base and collector.
It is to be mentioned that a detailed calculation should include the transistors input resistance at the base node.
 
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