[sdiy] Low voltage synthesis?

Brian Willoughby brianw at audiobanshee.com
Thu Jan 13 22:08:35 CET 2022


Most op-amps lose a couple of volts near each rail; the input and output voltages can't reach all the way to the rail voltage without lots of distortion. That's one reason to use ±12V to feed op-amps that need to accurately handle ±10V signals (of course, if your circuit regulates a ±12V power source down to ±10V for the supply rails, then the op-amps are good for less than ±8V or so in terms of signal levels). Apart from maximum supply voltage ratings, op-amps generally don't care what voltages are used, but there's always the stipulation that your signal levels will be a few volts less than the rails on either side. When the rails are as low as ±2V, there'd be no room left for (undistorted) signals.

I've not seen any rail-to-rail op-amps that work with more than +5V, which can be treated as ±2.5V by using a virtual ground throughout the circuit, as well as capacitive coupling (*). Thus, it seems like there's a gap between the 5V rail-to-rail signal ranges and the typical 20V to 24V or more that normal op-amps can handle. Granted, there are R2R op-amps that work with less than 5V, e.g., 3.3V, but I'd be curious whether anyone knows of an R2R with more than 5V. In particular, I don't think you can manage ±5V - because there don't seem to be any 10V R2R chips - unless you're happy with signal levels within ±3V. At that point, you'd basically be better off with ±2.5V R2R than with standard op-amps operating at ±5V.

* You can avoid capacitive coupling of signals by operating with literal bipolar supply, but that can end up with more components than a virtual ground system (like guitar pedals usually are).

Brian Willoughby

On Jan 13, 2022, at 09:07, Mike Bryant <mbryant at futurehorizons.com> wrote:
> The other issue you run against is you need to use rail-to-rail op-amps, but most of these are specified for 5V/+-2.5V operation.   Also these opamps are often CMOS based and have what I term "input crossover distortion" as they switch from the N to P type devices conducting.  Negative feedback reduces this, but it's always slightly there.  There are a few bipolar R2R opamps without this problem - Rohm BA2510 is my favourite.





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