Rectifying an unbalanced signal

[fryingpan]

I've never done this. How are you supposed to do it? The easy way is to use a phase splitter. The best way? (No opamps!).

[ElectricDruid]

In what sense do you mean "unbalanced"? Do you mean "not a differential signal", or "with different positive and negative peaks", or something else?

And why no op-amps? They might be the best way, which is what you wanted.

Thanks!

[fryingpan]

In what sense do you mean "unbalanced"? Do you mean "not a differential signal", or "with different positive and negative peaks", or something else?

And why no op-amps? They might be the best way, which is what you wanted.

Thanks!

Because there's this stupid desire of mine to design something without using opamps. I know how you rectify a balanced signal (easy, you just connect the bridge rectifier to the two different taps of a transformer). But if you want to rectify a signal (say, for controlling a VCA or whatever in a compressor)...

[ElectricDruid]

Ok, I think I get it. It's a "normal" single-ended signal.

The trouble with any sort of diode-rectifier is that nothing happens until the level of the signal goes over the forward voltage of the diodes. That's crossover distortion, effectively.

You can minimise the effect by using diodes with the lowest forward voltage you can find. John Hollis' compressor does this with germanium diodes in a diode bridge, IIRC. Another way is to use a phase splitter to give you in-phase and out-of-phase versions of the signal, and then use simple halfwave rectifiers to deal with each bit in turn. This is the approach used in the Dynacomp/Ross.

Another way to overcome the forward voltage of the diodes is to put them in the feedback loop of some device or other, to make a circuit called the "precision rectifier". This uses the device gain to eliminate the forward voltage problem and gives accurate results down to a few mVs. The only trouble is that the device in question might be an op-amp...

As usual, there's a good page on ESP about this:
https://sound-au.com/appnotes/an001.htm

There's probably ten other ways that I've forgotten, but the commonest are probably the "two half-wave rectifiers" technique borrowed from the Dynacomp (although you don't have to implement it the same way), or some sort of diode bridge. Precision rectifiers are less common but do turn up.

[R.G.]

My standard warning about using the word "best" applies. "Best" has no real meaning until it's attached to the list of qualities you will measure to determine what "best" means. In this particular niche, it clearly means "uses no opamps", but there are other issues, too. Cheapest? Smallest? Fewest parts? Best accuracy (and measured as accurate ...how?) Widest/narrowest/most specific frequency response? Widest input size range? Smallest input voltage range coupled with a given accuracy?

But you already knew all that. 

I'd use a differential amplifier to make the opposing phases. It's still a phase splitter, but more accurate and wider range than a single common emitter stage with equal emitter and collector resistors. Add on a few current mirrors to get the currents reflected to a common level and direction, and you have your full wave rectification. But then, you've replicated a lot of the circuitry inside an OTA, but with poorer accuracy and matching.

The things you're fighting are the DC offset on either/both phases and the forward voltage of any diodes. The 0.5v and up make it a non-starter for signals of anything lower than a couple of volts. Germanium is better, but still non-zero. The classical pedals doing envelopes generally use a slight on-bias on a silicon junction to get rid of most of the forward voltage, but it's still tricky.

An approach that I've never seen anyone use in small signal stuff is to use a synchronous rectifier. You generate opposing phases, then use some kind of comparat... er, saturating differential amplifier  to enable switching between two signals with MOSFETs or JFETs to literally pick the half of the signal that is positive right ... NOW... JFETs do this, OK, but can only get down to maybe 50-200 ohms when on. MOSFETs can saturate to lower ohms when used as switches, and can be easier to drive. Effectively, you're using a circuit to turn on a much lower-drop FET instead of a diode. A MOSFET full wave bridge driven from a wildly amplified input signal to tell the bridge which way to switch could be done discretely, I think. The MOSFET "on" voltage can be down in the millivolts if you're careful.

[PRR]

Rectifying an unbalanced signal

You don't say "full wave" (or "both sides"). A lot of audio can be done single-side rectifier.

> There's probably ten other ways

In the old days, any good BOOK would have 10 ways to do anything. At least one unique to that author.
This is not the book I was thinking of, it's been fluffed-up with some goofy things, but still worth downloading.
https://www.technicalaudio.com/pdf/Burr-Brown/Burr-Brown_Applications_Handbook_1994_ocr.pdf
also:
https://www.worldradiohistory.com/BOOKSHELF-ARH/Technology/Technology-General/Handbook-of-Operational-Amplifier-Applications-Burr-Brown-1963.pdf

[PRR]

Audio rectification is such a solved problem that you really should say "what-for?" Meter? AGC? Anti-clip? Pitch doubling? Important: instantaneous or peak-catching?

When relentlessly optimized, a discrete rectifier may be far simpler than chip-heads might think. Here is a NEVE program meter which meets the original BBC PPM specs fine. Bottom circuit:

The current at Q3 collector is fairly accurately the peak-held absolute value of the input. (The voltage is LOGged by the diode chain.)