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
The Colpitts oscillator uses a capacitive voltage divider network as its feedback
source.
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.
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?
I saw this earlier and thought someone else would answer. Have you read the Wikipedia article Colpitts oscillator ?
Oscillators used in radio applications were never my thing, so while I can see how it works I can't properly explain to anyone else. The basic idea of any LC oscillator is you have a tuned circuit consisting of an inductor and a capacitor and you link that to an amplifier, in this case a single transistor. As the transistor provides 180 degree phase shift the tuned circuit must also provide 180 degree phase shift, so there is 360 degrees around the circuit. It will then oscillate at the frequency that gives 180 degree phase shift in the LC circuit.
I hope the Wikipedia article and perhaps someone else will explain it better than I have done.
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?
The amplification is from the transistor. Yes, feedback is necessary. No, a human is not necessary, this is not quantum mechanics; no observer is needed to make it oscillate.
I'm not sure why you want to compare the circuit to a microphone amplifier. The Colpitts has an oscillator section, and an amplifier section...and a resistor network to bias the transistor. Those last two things are common to many circuits. A microphone amplifier circuit doesn't need an input oscillation to function. It is simply amplifying the input signal. Some microphone amplifier circuits have feedback too, but not to keep a "oscillation" going.
The oscillator needs a transistor to feedback the signal and keep the gain balanced to sustain oscillation. Being very general the transistor is providing amplification therefore similar to one providing gain for a microphone but it’s working with entirely different function and biasing, so this would not work well for amplifying a microphone as it is.
Feedback is vital to design any good amplifier with microphone or other uses. Stick to one type of circuit at a time.
The LC tank circuit is resonant at 2pi sqrt(LCt) your two caps are are in series. The issue with this design is that the Q must stay above 10 to maintain its frequency and putting load will change its Q and lower the resonant frequency.
PerryBebbington:
I saw this earlier and thought someone else would answer. Have you read the Wikipedia article Colpitts oscillator ?
Oscillators used in radio applications were never my thing, so while I can see how it works I can't properly explain to anyone else. The basic idea of any LC oscillator is you have a tuned circuit consisting of an inductor and a capacitor and you link that to an amplifier, in this case a single transistor. As the transistor provides 180 degree phase shift the tuned circuit must also provide 180 degree phase shift, so there is 360 degrees around the circuit. It will then oscillate at the frequency that gives 180 degree phase shift in the LC circuit.
I hope the Wikipedia article and perhaps someone else will explain it better than I have done.
The amplification is from the transistor. Yes, feedback is necessary. No, a human is not necessary, this is not quantum mechanics; no observer is needed to make it oscillate.
Thank you for the response , I didnt mean that a human is needed to make a tank circuit oscillate, I meant the oscillation of a tank circuit is similar to that of a human talking into an electrolytic microphone as both are analog signals that are being amplified
MK1888:
I'm not sure why you want to compare the circuit to a microphone amplifier. The Colpitts has an oscillator section, and an amplifier section...and a resistor network to bias the transistor. Those last two things are common to many circuits. A microphone amplifier circuit doesn't need an input oscillation to function. It is simply amplifying the input signal. Some microphone amplifier circuits have feedback too, but not to keep a "oscillation" going.
Thank you for the response, I was comparing the two in the most basic matter in such that both are analog signals who go through an amplifier and a larger version of the signal comes out of the other end.
wolframore:
The oscillator needs a transistor to feedback the signal and keep the gain balanced to sustain oscillation. Being very general the transistor is providing amplification therefore similar to one providing gain for a microphone but it’s working with entirely different function and biasing, so this would not work well for amplifying a microphone as it is.
Feedback is vital to design any good amplifier with microphone or other uses. Stick to one type of circuit at a time.
The LC tank circuit is resonant at 2pi sqrt(LCt) your two caps are are in series. The issue with this design is that the Q must stay above 10 to maintain its frequency and putting load will change its Q and lower the resonant frequency.
Crystals are much better
That's also what i thought that the transistor is to feedback the signal and for gain, the part that i am a little confused about is the 180-degree phase shift from the transistor.
I get how transistors work and i get how phase shifting works.
But can you explain in simple words why a transistor causes a 180-degree phase shift and also why the tank circuit causes a 180 phase shift because of isnt the circuit tunes so that the capacitive and inductive reactances cancel each other out? So why would there be a phase shift there also?
The transistor is in the common-emitter configuration, which has a large negative voltage gain, negative
gain means 180 degree shift.
The LC circuit is tapped in the middle and this tap is grounded, thus it oscillates in a balanced way
about ground, the two arms are neccesarily of opposite phase. In the Colpitts the capacitor is tapped.
The LC circuit has low impedance except around its resonant frequency so oscillation is only possible close to
the resonance, where it doesn't load the transistor's collector so much.
At resonance the currents round the LC tank will typically be many times larger than the rest of the circuit, perhaps
ten or more times as large.
Technically with a tank like this the susceptances cancel at resonances as they are in parallel,
sesceptance is the reciprocal of reactance.
MarkT:
The transistor is in the common-emitter configuration, which has a large negative voltage gain, negative
gain means 180 degree shift.
The LC circuit is tapped in the middle and this tap is grounded, thus it oscillates in a balanced way
about ground, the two arms are neccesarily of opposite phase. In the Colpitts the capacitor is tapped.
The LC circuit has low impedance except around its resonant frequency so oscillation is only possible close to
the resonance, where it doesn't load the transistor's collector so much.
At resonance the currents round the LC tank will typically be many times larger than the rest of the circuit, perhaps
ten or more times as large.
Technically with a tank like this the susceptances cancel at resonances as they are in parallel,
sesceptance is the reciprocal of reactance.
Thank you for the response. What you said makes sense, I do understand what a common emitter circuit is but can you explain simply how a transistor causes this shift, does it have something to do with the voltage on the base being different from the voltage on the collector?
Can you explain simply how a transistor causes this shift
As you increase the voltage (current) on the input (base) the voltage on the output (collector) falls. The output moves in the opposite direction to the input, hence the phase shift.
The NPN common emitter is basically an inverter. When the base is low/off, the collector is high (because it's tied high). When the base is high/on, the collector is pulled low.
