Garry Thorp wrote:
With the 723, you can make the reference noise as low as you want, by
heavy RC filtering. This applies whether you use its own reference or a
better external reference.
The 723 also seems to work quite happily with a feedback capacitor from
the output to the inverting input, reducing the AC gain to unity. The
output noise will then just be the buffer amplifier's input noise
voltage, ~5-6nV/rtHz. This is about 20dB better than most modern
'low-noise' LDOs will do. (I can't remember what the amplifier's noise
was like at very low frequencies - it was many years ago at a
different company, and I don't have the results any more.)
Garry
Using a capacitor from the output to the inverting input of the error
amplifier to reduce the high frequency noise gain of the regulator is
often used in high performance series regulators.
Another option/enhancement, when using a 723 with external pass
transistors is to preregulate the 723 input.
Even a simple resistor + zener + emitter follower can be effective in
improving the line rejection.
The extra supply voltage required is easily generated using a simple
diode voltage doubler driven from the transformer secondary.
Another possibility is to use a tracking preregulator such as an LM317
or similar regulator.
In this case the tracking pre-regulator maintains a constant voltage
drop between the LM723 supply and output pins.
With some elaboration (perhaps using a higher current regulator) this
can be extended to the case where an external pass transistor is used.
However this increases the minimum regulator input to output voltage.
Bruce
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