Low voltage rectification

You could step up the voltage with a transformer and rectify it.

How do you define a passive device. Since a diode is a considered a passive device is a transistor a passive device too.

An opamp based active rectifier circuit would fit the bill perfectly.

Otherwide gain the signal up with transistors (common emitter,etc) and rectifiy that, then attenuate to the original levels.

On a sunny day (Fri, 20 Jul 2007 10:48:48 -0000) it happened James wrote in :

If it is not to power something, then use active rectifier:

opamp Uin ----------- + diode out ----a k----------------- peak rectified ---- - | | | -------------------------| | | [ ] R === C | --- | | /// ///

The voltage on C will equal the posivie peak voltage of Uin.

I would say you need to abandon the idea of doing this "on the cheap" with a passive setup.

Even though you seem to class diodes as active components, I don't know of any that will conduct this low - let alone what you are going to do with the signal once you have removed the diode "voltage". Chances are anything that will is gonna be mil-spec and/or hideously expensive/unobtainable. Germanium stuff can come down as low as 180mv but this is still way over what you are after.

google "precision rectifier" there's plenty out there and depending on the opamp, they can work in this formation (i.e as a rectifier) down to a few micro-volts (some even lower)

the first thing that came up for me here was

which gives a nice precision full wave rectifier circuit which you could construct for

The "backwards diode" conducts at very low voltages. Nobody makes them any more because active rectifiers are commonly used today.

You can bias a normal silicon diode part way on and rectify small waveforms that way. The result is not very accurate.

An ALD110800 with its gate hooked to the drain is fairly nonlinear near zero volts so it would also rectify.

further browsing turned up this very concise little page on precision rectifiers with single opamps which further reduces your cost/complexity.

hant tried any of these - experienced eyes in this group will no doubt be able to comment on particular solutions to greater improve everyone's knowledge... gentlmen; the floor is yours.

"no forward tunnel current" is not exactly correct. A back diode is a tunnel diode with a low Ip.

Several people still make back diodes. They are photolithographically fabbed on germanium, unlike the classic/horrible tunnel diode fab. Back diodes are still the lowest noise unbiased RF detectors.

(I think the Ip's are uA, not mA)

John

The first question is what is this for? Is it RF detector or something else? Is it a measurement circuit? How accurate?

Vladimir Vassilevsky

DSP and Mixed Signal Design Consultant

There are some diodes that have significant rectification in this voltage range. These include some very low voltage Schottky types.

Here are some data sheets for low forward drop Schottkys, as examples:

With lower resistance values and high GBW OpAmps I've used the following up into the MHz range...

...Jim Thompson

Use a synchronous rectifier.

Yes, the opamp based rectifier makes a perfect sense. But he asked for the passive circuit.

Vladimir Vassilevsky

DSP and Mixed Signal Design Consultant

I know. But, as most posters here, he probably doesn't know what he really wants, or couldn't find a canned solution in AoE ;-)

...Jim Thompson

look at this mine first upper schematics (slide C); it is a 70Khz IF amp for measuring signal strenght. (for TDA7000 IC)

Last half of the schematic- passive part at the end: The right last 2 resistors provide current biasing two germanium point glass demodulator type diodes to the knee of conducting signal .. So, you can measure from few mV & up to 2V RMS signals..

-- Regards , SPAJKY ® mail addr. @ my site @

The Esaki (tunnel) diode is the closest passive device able to do that.

That isn't very passive and a backwards diode is closer.

precision rectifier via an op-amp and a couple of components.

Or fewer with a comparitor such as LM339:

Vcc ! ------!+\\ OC ! >------+-/\\/\\---Vcc --!-/ ! ! ! ! ! Vee ! ! ! ------------+ ! --- --- ! GND