That's not too bad. And yeah, it's bootstrapped, so you can put it wherever you want, plate or cathode.
Some downsides: the 100k bias resistor must supply a minimum base current, which means saturation will necessarily be rather high. In this example, I found around 5V (more on this later). That's fine for a plate load. For a cathode load, it doesn't matter, because you wouldn't use this circuit: you'd use a circuit with more range.
Negative resistance is an obvious problem around reactances. Since it can be adjusted, this isn't necessarily a big problem, but it's something to be aware of.
Your analysis of ~4.5Mohm ignores some fundamental features of this circuit. For instance, it can do quite a bit better than this! The proof is in your negative resistance oscillator: if impedance can go negative, it must go to infinity somewhere inbetween. Unfortunately, such a point is likely only a balance point, with arbitrarily large resistance (positive/negative) on either side. The trick is to adjust that point so the curvature is as gradual as possible.
I have produced some DC sweeps which may be of interest. First: plain DC sweep, from 0-100V. This is using an identical circuit as shown above, using both 2N3904s for convienience (the model does not specify breakdown voltage, so the sweep can go beyond 60V quite easily). I cannot guarantee that my 2N3904 models are identical to yours.
The second graph shows a zoom on the saturation region, with two points highlighted. The red cursor is approximately the infinite impedance point, showing a peak of 10.8941mA. The current drops off on both sides away from this point. The blue cursor indicates a point towards the edge of the sweep, at 90V, indicating a change of -0.1479mA, or an average incremental resistance (ugh, what an abomination of terminology) of -281kohms. (The actual incremental resistance at the blue cursor will be smaller than this.) That's not very good, and a cascode will outperform it easily.
It gets worse. The third plot is a temperature sweep. At room temperature, everything is fine, but as temperature drops (the lowest is -50C, at the end of the military temperature range), saturation voltage rises considerably (to about 14V), and the saturation current is 21.9% higher. As temperature rises, saturation voltage falls, but current falls precipitously, by a whopping 36.9% at 150C!
The saturation voltage is affected by hFE, which "freezes out" at low temperatures, so it is reasonable to anticipate this effect. Likewise, Vbe drops at high temperatures, reducing the "threshold" voltage which appears on the current sense resistor.
A better current source might use a ratio of Vbe's at different currents to set the reference voltage (i.e., a bandgap reference), or an IC incorporating this method (e.g., TL431). Additionally, extra gain could be added to the system, to linearize it, flattening the curve.
Current sources can also be chained to a limited extent, eliminating the need to compensate the bias resistor, at the expense of a much larger "bias CCS bias resistor", which could be compensated in the same manner while having far less impact on total current.
Other options include below-ground sensing, so that the total circuit current is measured, thus including all base currents, and using a suitable reference and feedback loop to stabilize it.
Tim
