An effective "ripple eater" design

 This post is regarding something that the Theremino approach towards high-resolution XRF spectroscopy emphasizes.  That is the noise+ripple voltage on the PMT (photomultiplier tube) high voltage supply.  They indicate that it should be in the microvolt range because the PMT gain is very dependent on its supply voltage.  This can be a significant problem because the PMT voltage can be in the 1,000 volt range, so we're looking at something on the order of 10^-6/10^3 or a ratio of 1:10^9.  This is a very demanding requirement.  Their approach is to use a passive multiple-order RC filter chain, but this also introduces a large phase shift vs. frequency ( in addition to gain vs. frequency) that is difficult to control in terms of circuit stability.  So its response to a pulse-style load isn't all that great, either.  Thus a lower-order filter scheme looked more attractive in terms of better response to impulse-style loads (as in, the signal generated by a PMT+scintillator when presented with a single x-ray photon).

I've found a few descriptions of a 1-transistor circuit called a "ripple eater" but the circuit wasn't all that intuitive to me.  I also suspected it wasn't the best-possible approach so I tried a couple of different schemes to see what was possible.  But the nice thing was that it offered the possibility of greatly reducing the HV supply's ripple+noise while reducing the order of the filter ... which also would improve the transient response of the HV supply.  So I tried a few different approaches.  The first was a relatively simple "capacitance multiplier" circuit -- basically, an emitter follower driven by an RC low-pass filter.  The idea behind this is that the transistor's base current is low so we can use a very high-value R in the base-input RC low-pass filter.  One very significant limitation with any kind of HV low-pass filter is the fact that high-value and high-voltage capacitors can get VERY expensive, so I limited my options to a maximum of 47 nano-farads.  My SPICE simulation showed that about the best "ripple eater" attenuation I could get with this simple circuit was in the neighborhood of 30dB.  Not too bad, in fact about a reduction of 1000:1.

But we can do much better than that if we use a high-gain, low-noise operational amplifier in a special kind of summing-node application.

More on that soon.