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Okay, a little background. Everyone needs a voltage reference sometimes for a regulator. Most regulators use a standard bridge rectifier at 100Hz or 120Hz output ripple. The circuit here employs a T notch filter in the current source, which modifies its frequency response to input signals in accordance to the notch of the filter. The T filter looks like a bootstrap chain.
If you decrease the Q of the filter, by damping the capacitor, with a series resistance (the 22ohm resistor), you can achieve a more flat response for a wider band around the center frequency (100Hz). In this case, we're getting -123dB output ripple rejection, from a 20dB input source, which is a very high level of noise suppression, and would be below the thermal noise threshold of the components themselves. Basically, you'll get more thermal noise at the output, than noise from the ripple.
One additional benefit of this topology is that the ripple suppression is more or less not dependent on the hFe of the transistors, so as long as the T filter is properly designed, the output would have a pretty constant ripple suppression.
Interestingly, this can also be viewed as a sort of an active notch filter without a twin T RC configuration.
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