Electronics > Beginners
Colpitts oscillator - role of feedback resistor
T3sl4co1l:
Yes, the Rs in network analysis are implicit as the source and sink resistances. :)
Peaking includes the total loop gain of course, so you can have something like a passive RC network and enough gain to fix it up; but the phase noise will be worse than a sharper tuned network.
A Colpitts is usually phrased in terms of a rearrangement of any other oscillator prototype, e.g., Hartley which is a strict bandpass type. It follows that the Colpitts equivalent would have a gain peak, even if it cannot have the same overall bandpass behavior because the lowpass asymptote is set by topology.
It's not necessary to stick to that format, though, so you can indeed compensate for less gain peaking with more loop gain. It's not an absolute statement.
Detailed analysis of noise response, of course, needs losses of the network, and noise of the amp. Also, nonlinearities are relevant to the conversion of thermal and 1/f noise to sidebands / phase noise. (In short, if you need performance quite that good, you'll need some good tools and/or a lot of tweaking to do it.)
Tim
Wimberleytech:
--- Quote from: aneevuser on February 03, 2019, 03:34:46 pm ---
--- Quote from: Wimberleytech on February 03, 2019, 01:42:53 pm ---
--- Quote ---...so the fact that we have a tank circuit sitting there is somewhat secondary.
--- End quote ---
:-//
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I think that's a "confused" symbol, no? If so, then my point is that one part of the condition for oscillation is that the total phase shift around the loop be 0 degrees - we are not interested in whether or not the feedback network resonates or not - that's irrelevant in the Barkhausen conditions. So the presence of resonance here is secondary to the oscillation of the circuit, AFAICS.
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The IEEE dictionary defines "tank" as follows:
tank circuit (signal-transmission system) A circuit consisting of inductance and capacitance, capable of storing electric energy over a band of frequencies continuously distributed about a single frequency at which the circuit is said to be resonant, or tuned. Note: The selectivity of the circuit is proportional to the ratio of the energy stored in the circuit to the energy dissipated. The ratio is often called the Q of the circuit.
--
The structure of a tank is inductance and capacitance. The nature of a tank is that it will resonate at some frequency.
The circuit drawn in the video is a tank circuit. And for his implementation of the oscillator is was necessary. Now, granted, you can achieve the needed phase shift without inductance. But in the video, the circuit analyzed...the LC circuit is necessary.
emece67:
.
aneevuser:
--- Quote from: emece67 on February 04, 2019, 01:31:04 am ---But, with R1 (or if you drive the CLC network with current, surely the reference to "transconductance" by the author is related to this) then C1 plays a role.
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This of course relates to my original question. If I understand correctly, it seems to me that the reference to transconductance is completely misleading - wouldn't we need to have a current source present in the circuit driving the oscillator to meaningfully talk about transconductance?
--- Quote ---Now (as before) for all f > f'0, LC2 gives 180º (V/V). For this same range of frequencies LC2 is inductive. Now you have 3 phase changes:
* 180º at the inverting amplifier
* 180º at L-C2(Ri)
* some unknown phase at R1-C1LC2Ri
To achieve the overall 0º phase you need the phase change at R1-C1LC2Ri to be 0. This can be achieved at the resonance frequency of the C1LC2Ri network, as its impedance at its resonance frequency is purely resistive and R1-C1LC2Ri is a resistive divider, thus giving 0º phase change. Thus, the frequency of oscillation is the resonance frequency of the C1LC2Ri network.
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This is a pretty nice summary. Thanks.
However, I've finally had the time to play around with this op amp based Colpitt's circuit, and whereas I've been able to get it to oscillate reliably, I've found that the frequency of oscillation has varied by up to +10% from the resonant frequency of the tank circuit (the frequency of circuit oscillation has always been greater than the resonant frequency, when it's differed). I don't have time to check these results at the moment, but if I haven't screwed up, it suggests to me that your analysis of oscillation frequency is not the whole story.
Is it usual for the oscillation frequency of a Colpitt's oscillator to differ significantly from the resonant frequency?
David Hess:
--- Quote from: aneevuser on February 05, 2019, 09:34:38 am ---This of course relates to my original question. If I understand correctly, it seems to me that the reference to transconductance is completely misleading - wouldn't we need to have a current source present in the circuit driving the oscillator to meaningfully talk about transconductance?
--- End quote ---
Transconductance outputs have a parallel load resistance like voltage outputs have a series resistance. Even if the bias is provided by a perfect current source, the collector or plate resistance is in parallel with the transconductance output. 100s of ohms to kilohms to 10s of kilohms is typical although going higher is completely feasible with additional design complexity like an output cascode which is common in precision current sources.
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