In message , dated Tue, 15 Aug 2006, jasen
writes
On 2006-08-14, John Woodgate wrote:
In message , dated Mon, 14 Aug 2006, jasen
writes

You can call it two half-wave rectifiers but it's the same circuit as a
voltage doubler, and it puts a common mode ripple on the power rails

What do you mean by 'common-mode ripple'?

both go negatve or positive at the same time.

Thanks; that's what I thought.

If you draw out what happens (and look with a scope, of course), you
find that the two ripple waveforms are unsymmetrical sawtooth waveforms,
displaced in time. So there is some common-mode and some differential
mode ripple.

I would use this technique only for low-current applications, where low
ripple can be achieved with reasonable filter capacitor values, or for
feeding regulators.

to an op-amp connected accross the rails it looks like the earth is
moving,

It seems to me that the op-amp doesn't 'see' the rails until its input
or output voltage gets close to them. If it did, PSRR wouldn't be so
high.
--
OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk
2006 is YMMVI- Your mileage may vary immensely.

John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK