rectifier

The trick is that you have to use them backwards because of the body diode.

Best regards, Spehro Pefhany

As in "FullWaveRectifier.pdf"

on the S.E.D/Schematics page of my website.

...Jim Thompson

When the MOSFET is conducting the source will be positive wrt the drain on an n-channel device. The body diode is thus in parallel with the channel, and hopefully not conducting significantly. The opposite polarity when it's blocking.

Best regards, Spehro Pefhany

No, it's not that easy. The MOSFET won't really start to conduct until the gate voltage is several volts, so it can't really be a 2-terminal device and still be better than a silicon diode. You have to drive the gate from some other circuitry. What you'd get if you did what you suggest is just a slow and expensive diode with normal voltage drop.

For measuring applications often we use a "precision rectifier". There are a few ways of doing it, most of which involve an op-amp and a diode or two. Rather than drone on (in the Larkinsian sense), I'll suggest something you might not find-- If you just need to indicate AC mV on a moving-coil panel meter, you can do something like this:

Or maybe this where the extra diodes shunt current away from overloads and damp the movement.

Best regards, Spehro Pefhany

That's very clever!

What do you mean by backwards?

Check out synchronous rectification.

Graham

MOSFETs are used instead of rectifying diodes because they have less resistance, IIRC.

lg, Bernhard

Graham, what you showed me is just plainly fantastic! It's just what I need for a certain application (AC voltage measurement).

Spehro, maybe you can educate me a little bit. I am hoping that I could use a mosfet with its gate not energized (connected to drain), as a diode with low voltage drop, acting, rather, like a resistor. It would conduct from drain to source.

Is it correct that I can do something like that?

If so, what mosfet would you suggest for low voltage, low current applications?

thanks

Pleased to have been of assistance. :-)

I'm intruiged now !

Graham

Ah - you mean you want a precision rectifier for measurement purposes where there's no Vf from the diodes ?

Spehro has posted one example but it can be made to provide a ground referenced output too.

Graham

Graham

no, I didn't want anything really... I was just wondering if there were such things ;) I figured it would be "generalization" of sorts and might be useful... just wondering if there were such circuits that were similar but with other components replaceing the diodes. I think the wheatstone bridge is of similar design but with resistors or whatever?

AD

Slow diodes can take even hundreds of microseconds to turn off though popular fast turn off/on diodes like the 1N914 (used in computers) go over their expected dc level upto even 0.5V for 10 to 20ns before the diode turns on. Using the emitter base junction of BJTs like the

You'll find diode circuits that need speed in switchers , fast pulse detectors and some voltage clamps.

In the field of audio I've heard of transistors used as diodes exploiting the 2nd harmonics generated by BJTs to give better/different sound outputs compared to diodes, but as you know this may just be another audio myth!

I prefer the base-collector diode.

My experiments with 2N4401 show the B-E junction has recovery time comparable to the storage time, which makes reasonable amounts of sense (IANA semiconductor physicist).

Tim

-- Deep Fryer: a very philosophical monk. Website:

yes, but in that case the breakdown voltage is usually limited to about 5 V; and the CB junction is usually not a fast. it is tradeoffs.

We have one product whose operation involves deliberately forward biasing a power diode to +48 volts for a short time.

John