Low-voltage AC-powered oscillator

Interesting question. The main problem is going to be that even a symmetric FET is still polarized, i.e. an NFET is different from a PFET. If you turn a common-source stage upside down, you get a source follower, so your volt age gain goes away. Another way of putting this is that in an NFET the drai n current increases when the gate goes more positive. Thus at a given frequ ency inverting the supply turns positive feedback into negative, so there's not enough gain for it to oscillate there.

There are common-drain oscillators (Colpitts for instance) as well as commo n source ones, and all the reactances are frequency dependent.

If you don't mind the frequency being different on the two half-cycles, it' s probably possible, I'd say.

Cheers

Phil Hobbs

Since the supply is ac there are times when the active element can't oscill ate, so you're stuck with an oscillator that runs part of the time driving a tank that keeps resonating. Given that you must accept that why not just oscillate on +ve half cycles only. Spectral purity is maximised with high q - can you run a crystal at

How about common gate? Seems to be symmetric enough.

The question is out of pure curiosity, so different frequencies are fine.

Best reagrds, Piotr

Which is exactly what I want. :-) I was thinking about a weirdo isolated zero crossing detector. Having 2xLND150 to cut the current down to 1.5mA the problem is now to notify the other side that the current is flowing. The AC-opto approach is too boring and a magamp too bulky. :-)

Please share your ideas then. The assumption: the circuit must be self-powered (i.e. no external power supplies are allowed, but it can steal up to 1.5mA from the input).

Best reagrds, Piotr

Doesn't fix the polarization of the FET. It won't amplify if you come in via the drain.

Cheers

Phil Hobbs

fairly easy... pass your oscillation through a small C

I expect your no-oscillate region will be too long to make a good enough zero cross detector, but some interpolation on the other side might get it passable for some things.

FWIW... Put the 0.2v through a transformer and run a more conventional oscillator Use L,C to create an out of phase supply line too, then give each supply line its own oscillator feeding the same tank so oscillation can be continuous Mechanical rectification to get a bit more voltage. Mechanically gated smpsu to get more volts out

Only the 1st has the faintest chance of meeting your requirements.

NT

Inductive coupling is far easier. If the potential difference is high, the C will be nowhere close to any reasonable definition of "small". An X2 cap would be bulky, but several inches of TIW wire is cheap and solve the issue easily.

Why? Do you mean the startup time?

Do not underestimate the capabilities of modern JFETs. :-) It is pretty easy to make a blocking oscillator for 200mV (google => Joule thief) and then couple two of them to oscillate in the separate half-cycles. I'm just curious whether one device would suffice, given its symmetry.

BTW, you don't need mechanical switching for boosting small voltages. Just use regular low RDS_on MOSFETs and power the controller from the switcher's output in the "perpetuum mobile" fashion ;-) and add a JFET-based Joule thief to kickstart the whole caboodle. BTDT.

Best regards, Piotr

I'm afraid I don't understand what you mean, Phil. The only polarization in a JFET I know of is the polarization of the gate diode, which is a P region somehow embedded within the N silicon bar which makes up D and S. Drain is the end with higher potential and source with the lower, but it flips as the AC waveform changes. Is it correct?

Do you mean the problems with *external* gate polarization circuitry, which would inject current through the gate if powered incorrectly?

Best regards, Piotr

Phil only thinks discrete devices which (usually) have back gate/substrate tied to source. In my I/C world it's trivial to have both gates independent, so a _symmetrical_ grounded-gate situation is quite possible.

(I am currently working with a major ASIC foundry to get a good JFET structure added to their repertoire.) ...Jim Thompson

A symmetrical JFET is symmetrical with respect to interchanging gate and drain, but not with respect to inverting the supply polarity--otherwise there would be no difference between NFETs and PFETs. A common-gate amplifier can have gain if you come in via the source, but not if you come in via the drain. Inverting the power supply turns the one into the other.

Cheers

Phil Hobbs

The voltage gradient along the channel is what makes the drain different from the source, even in a perfectly symmetrical FET. (These exist in discretes, e.g. the BF862.)

Inverting the supply in a common-gate amplifier moves the pinch from one end of the channel to the other, effectively interchanging drain and source. It'll amplify one way, but not the other.

Cheers

Phil Hobbs

This is not what you asked, but the LTC3108 uses a MOSFET and operates down to 20mV:

So it can be done and is available commercially.

Wrong >:-} ...Jim Thompson

Show your work.

Cheers

Phil Hobbs

Which is drain and which is source is determined by potential direction... in this (PJFET) case drain is defined by the source/drain terminal that is negative relative to the other terminal. ...Jim Thompson

Presently under analysis for practicality...

...Jim Thompson

Right. So if you invert the power supply, the drain and source effectively get swapped. Seems like we're in violent agreement.

Cheers

Phil Hobbs

Yep. I should read more carefully :-( ...Jim Thompson

Instead of a JFET, I think one could do some interesting things with a dual-gate depletion-mode MOSFET like so:

For example, build an oscillator that starts out blocking, then creates its own boosted voltages to a) bias it up on the second gate into the linear region of operation, and b) develop enough negative voltage to half-wave rectify the input.