Is this a colpitts oscillator or something else? - Page 1
Forum section: electronics / beginners

author: Infraviolet
author_rank: Super Contributor
timestamp: June 17, 2025, 06:55:51 pm
content: Falstad simulationIn regards to the circuit on the left using an NPN transistor, is this a colpitts oscillator or some other type?On the right I've assembled an op amp colpitts design, which gives an output at the correct f=1/(2*pi*sqrt(L*C_para) ) with C_para being 1/( (1/C_1) + (1/C_2) ) (= 0.5C_1 if C_1==C_2).The single transistor oscillator has its frequency affected by the size of the current limiting resistors, so does not match the colpitts formula precisely. Is it still of a colpitts type though?Just wondering whether the single transistor type can ever be suitable for use in a fairly crude inductance meter, once one has taken account of how the resistances (kept constant) affect the frequency. And is it reliably self-starting? And so long as the capacitors are of an appropriate (C0G, NP0) type should it be pretty consistent in frequency, the need for capacitors with minimal variation caused by their dielectric being the same for the op amp colpitts too. I used 6.8nF caps in this example because that's about the highest capacitance for which C0G/NP0 dielectric seems common.A single transistor, whilst usually more bothersome for "analogue" circuits than just using an op amp, would have the advantage that it could run at high enough speeds to measure smaller inductances for which an op-amp might not have enough slew rate to keep up*. And it could deliver bursts of higher current if an unknown inductor's DC resistance was very low, whereas an op-amp doing so might be damaged.Is the single transistor version a valid design, or is there some major flaw to it?Thanks*One cannot always go to a larger capacitance when using smaller inductors so as to reduce the frequency...With physical circuits in the past I have found that when the capacitance/inductance ratio gets above a certain level the oscillator will not run, I understand it relates to how non-ideal real components have some resistance and this limits the Q factor they can reach.

author: Tation
author_rank: Regular Contributor
timestamp: June 18, 2025, 10:42:46 am
content: It is a Colpitts oscillator, although a somewhat rude one. Single transistor Colpitts oscillators are useable well into the GHz range (I have no experience above UHF, though).There are many things to consider when designing an LC oscillator:to ensure self start ⇒ ensure the Barkhausen criteria is meet with enough margin (depends on small signal gain and ratio C1/C2)to ensure low distortion ⇒ ensure previous margin is not too high (how does large signal gain decay with amplitude? Sometimes an amplitude limiting network is added, even as a separate 2nd, active, stage, with the 1st stage working always in the small signal regime)to ensure there are no parasitic oscillations ⇒ ensure there are no undesired couplings between output and input (proper DC design)to ensure frequency stability ⇒ ensure parasitic devices have small effects (sometimes a Clapp, instead of Colpitts, can be safer here. Have you considered the tempco of the coil? And of the parasitic devices of the transistor?). Also ensure resonator has high Q (usually limited by the coil, but avoid the DC network to interfere here. A high resonator Q aids in many other ways)to ensure load variations do not affect frequency/start-up ⇒ ensure load is always small (inserting a 2nd transistor as a buffer?)needs to be tunable? If so, a Clapp instead of Colpitts may be more appropriateSo, if in a hurry, use any oscillator circuit known to work at your target frequency range and take care of the PCB design. Many years since the last time I read the ARRL handbook, but it used to contain valuable descriptions of circuits. Sorry, but I do not have anymoderncomprehensive reference about RF circuit design.Hope this helps, regards.

author: Tation
author_rank: Regular Contributor
timestamp: June 18, 2025, 11:59:33 am
content: Vackář oscillator at 30-240 MHzVackář oscillator at 7-14 MHzJust in case: basic LC oscillators are of, mainly, 3 types:Colpitts: the resonator has 2 capacitors and one inductorHartley: the resonator has 2 inductors and one capacitorClapp: as the Colpitts, but there's a 3rd capacitor (smaller than the others) in series with the inductorNow, you can put the signal ground in any of the 3 nodes of the resonator, thus you can have all three above in common emitter, common collector or common base configurations. Yours is a common emitter Colpitts, adding resistors does not change it into another type. Vackář's above are common emitter Colpitts with the addition of a capacitive divider before the base.For crystal oscillators:Colpitts: it is a common collector Colpitts where the inductor is replaced by a crystalPierce: it is a common emitter Colpitts where the inductor is replaced by a crystal

author: MariuszD
author_rank: Regular Contributor
timestamp: June 18, 2025, 06:57:52 pm
content: I have been using the Peltz oscillator to measure inductance.https://www.edn.com/simple-5-component-oscillator-works-below-0-8v/This oscillator works correctly over a very wide range of inductance and capacitance.To ensure that the measurement of inductance based on frequency is accurate, the waveform on the LC circuit should not be distorted. In the Peltz circuit, the resistor can be replaced with a potentiometer and adjusted to achieve small distortion.Vackář oscillator is also good choice.