Strange Colpitts oscillator topology

Question

This is the oscillator for an ultrasonic misting circuit (ye olde patent US4996502). I think that something like 99% of the Chinese HF misters (the 1.5-2.5MHz ones, not the micronozzle ones) use this topology, with slightly different values and bias schemes.

The idea would be to resonate the TD piezoceramic at its own frequency with a moderate power (10-30W, depending on the design). In my interpretation it should work in this way (correct me if I'm wrong): the whole thing is substantially a common collector Colpitts.

R1, R2, R3, R4 (often there are trimmers or pots there) gives static current for the operating point.

L3, C4 form an RF trap/DC smoothing do avoid losing power thru the supply lines.

TD and C2 are the resonant tank proper, C1 and C3 are the Colpitts capacitor divider. Nominally C1 is 1.8nF and C3 47nF but it varies in the wild. Also I think that they are different to adjust the gain, is that correct? C2 officially is only for protection against a short in TD but I reckon it participates in the oscillation. It's drawn horribly but that's the topology. The 'output' of the oscillator would be the C1-C3-L1 node.

L1 is a quite big choke (22µH to more than 220µH).

C5 and R6 gives some noise filtering and decoupling (but a lot escapes, anyway). R5 limits base current and helps stabilizing the gain (I think).

Now, for the things I have absolutely no idea: L2 is 4-5 air spire for about 100nH. Best thing I can think of is avoid spurious auto-oscillations. C6 (100pF) gives collector feedback but I have no idea of what would be its design function (in fact, it is not strictly needed, it seems to work even without it).

Now, for the question proper: this thing is different from the textbook Colpitts since the tank is anchored to the positive supply. Usually you see the 'external' terminals of TD, C4 and C1 to ground.

I interpreted this in this way: the goal of this circuit is to maximize the power on TD. So they made it the primary branch for current to flow (+ve, TD,C2,R5,Q1,L2,L1), and in fact there is a huge base current flowing (with the whole circuit at 700mA about 600mA are thru the base). Q1 is huge, too, so it works :D

The problem is that the TD transducers have many resonance pairs (they are piezoceramic discs so probably they move in many directions). And on some of them, instead of picking the nominal 1.7MHz, the oscillator locks on a 300-400kHz mode (not good, of course). The usual solution for the Colpitts would be a suitable inductor across C1 to bypass the lower modes (like for overtone crystals), but it can't be done (that would be a DC short).

I was thinking of working on C2 and C3 to steer the response, would that be a good solution? any other ideas or enlightenment about this circuit?

Answer 1

When analyzing AC circuits one important principle is that power supplies are effectively short circuits to AC. So whether it is referenced to the positive or negative supply is not important.

The circuit basically can be reduced to the standard Colpitts as below.

As @Andy Aka mentions in the comments the circuit may be more like a Clapp/Gourier oscillator depending upon the value of C2. If it is of high value then it is merely for DC isolation rather than an important factor in the oscillator.

A Clapp oscillator is basically a Colpitts with a capacitor in series with the inductor to reduce the frequency dependence on the active elements characteristics.

C1 in the original circuit is represented by C2 in the circuit below.

As Andy mentions there are certain practical aspects missing - especially DC point stabilization. Amplitude stabilization is often not needed as they will build up until limited by the power supply voltage - that is also useful in that it ensures the device reaches saturation which is often the most efficient point.

I agree, L2 is probably to avoid operation on unintended frequencies as in addition to the mechanical resonance of the transducer there will be inductive elements in the wiring that could cause oscillation at much higher frequencies.

Diagram from Wikipedia.