Sans Choke Smoothing

[Gingertube]

See this from Tentlabs:
Electronic Choke

It has a link on the board
:
Link Out: it works as a "Gyrator" or simulated inductor.
Link in: It works as a Capaciance Multiplier.



Cheers,
Ian

[R.G.]

I get it now...I think.

Yes, you do get it.

Ok so the Gate needs to be 5-10v higher than the Source for the FET to conduct. In the circuit I posted, the Source is only 3v lower than the Gate. I guess there is a RANGE and not so much a solid figure for Vgs.

Yes. The fogginess is that there is a minimum "cut in" Vgs voltage, which is different per type of MOSFET, and mildly different within the same type of MOSFET.

MOSFETs work by having the voltage on the gate literally pull current carriers up out of the substrate into the channel. Until there's enough voltage to pull enough to get started, nothing at all happens. Once you get just a trickle of current going, you need to pull in more charge carriers to let more current go through. This is the "transconductance" of the MOSFET from the datasheet, and is usually in the range of 1 ampere per volt for the power MOSFETs. That amounts to about 1ma/mV, so the amount the gate has to move to let through, say, 100ma, is down in the 100mV range, and the source really doesn't have to drop much to make tube currents flow. But that tiny drop rides on the threshold/cut-in voltage.

The threshold is usually somewhere from 2-4V for "logic level" MOSFETs, and often 5-8V for the big hulks, although the thresholds get smaller and the transconductances get bigger as MOSFETs get better.

Mostly for tube circuits we can call the gate-source drop equal to the threshold, and ignore the tiny changes to support changing currents.

Another way to look at this circuit is as a "capacitance multiplier". There is a BFC filtering the gate voltage. That capacitance is multiplied by an effective amount of the "gain" of the MOSFET. Since a MOSFET has a huge current gain, the capacitance is effectively multiplied. The MOSFET source looks like a huge capacitor as long as the MOSFET has enough supply voltage to let current through.

On gyrators: they work but have some issues with audio noise. May not be a problem in power supply circuits because of the decoupling caps.

[lowell]

Great info. thanks!

I'll report back once it's built. Will take me a few weeks to a couple months to get around to it. Can't wait to try it though.

[R.G.]

Remember to worry about how much heat has to get out of the MOSFET, and how that will happen.

[eschertron]

See this from Tentlabs:
Electronic Choke

It has a link on the board
:
Link Out: it works as a "Gyrator" or simulated inductor.
Link in: It works as a Capaciance Multiplier.

[ATTACH=CONFIG]42366[/ATTACH]

Cheers,
Ian

And they spec a 330uF/450v cap on the output?!