bjt - Increasing the compliance voltage range for a variable, two-quadrant constant current pin-driver circuit

The following is for hobbyist work and I have no commercial intentions at all. Only a handful (two?) will be built. (I use these for part testing and curve generation, though with the higher voltage compliances I may find still more uses than before.)

I've got the following pin driver circuit, which provides up to \$\pm 50\:\textrm{V}\$ output compliance voltage while providing \$\pm 10\:\textrm{mA}\$ to a load connected between the pin driver output and ground. (The larger plus and minus rails are about \$\pm 60\:\textrm{V}\$, with the opamp rails at \$\pm 15\:\textrm{V}\$.)

^{simulate this circuit – Schematic created using CircuitLab}

Slew rates at the output for the above circuit are generally no more than \$20\:\frac{\textrm{A}}{\textrm{s}}\$ or \$100\:\frac{\textrm{mV}}{\mu\textrm{s}}\$. (I drive the input at rates on the order of no faster than \$1\:\textrm{ms}\$, peak to peak, and often slower than that.)

I'd like to expand the compliance voltages to \$\pm 800\:\textrm{V}\$ and reduce the current drive capability to somewhere from \$\pm 500\:\mu\textrm{A}\$ to perhaps \$\pm 1\:\textrm{mA}\$. (The voltage slew rate then increases to \$1.6\:\frac{\textrm{V}}{\mu\textrm{s}}\$ and this may be a concern, too.)

Getting the paired high voltage supply rails of \$\pm 850\:\textrm{V}\$ isn't the problem. But I was able to pick up \$Q_1\$ through \$Q_4\$ as parts on the same dice (BCM846S, etc.) I'd like to keep the matching of \$V_{BE}\$ (and perhaps even \$\beta\$.) But now the \$V_{CEO}\$ has gone up "a lot" and the same topology isn't going to work, since I don't think there are ANY matched pairs of BJTs with that kind of \$V_{CEO}\$. In fact, I'm not sure of any discrete PNP BJT that gets close to what I'd like to see. (NPN, perhaps. But PNP?)

I can imagine setting up yet another pair of voltage rails (close to the high voltage rails, but perhaps \$40\:\textrm{V}\$ closer to ground) and using a cascoded design (using four more BJTs) in order to protect the high and low side matched mirror pairs. That added voltage supply wouldn't need to handle more than \$10\:\mu\textrm{A}\$ or thereabouts, so it may not be all that difficult to construct out of the new high voltage supply rails. But if there are other/better thoughts about the topology I'd like to hear them.

Here's what I mean:

^{simulate this circuit}

Is there a problem I missed thinking about here, or can I do better? Does anyone have a suggestion of any process by any FAB for discrete BJTs I might consider for the cascodes here?

I also know that I will also face entirely different problems related to clearances and creepage, that I didn't have to face here before. That's a different topic though, which I'll address separately and later. Right now, I'm focused on how to get the significantly higher voltage compliances I'd like to achieve.

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Just for clarity's sake, in case it isn't obvious, the circuit is a DC voltage controlled current source (VCCS) that either sinks or sources current into a grounded load. (One use has been for semiconductor curve tracing.) An input voltage of \$-10\:\textrm{V}\$ would source \$500\:\mu\textrm{A}\$ into the grounded load. An input voltage of \$+10\:\textrm{V}\$ would sink \$500\:\mu\textrm{A}\$ from the grounded load. A voltage triangle wave, oscillating smoothly between \$-10\:\textrm{V}\$ and \$+10\:\textrm{V}\$ would generate a current triangle wave into a load oscillating smoothly from \$+500\:\mu\textrm{A}\$ to \$-500\:\mu\textrm{A}\$ (whether that load was a diode or a resistor.) And the voltage compliance should support doing all of the above with a \$1.5\:\textrm{M}\Omega\$ resistor as the load. On occasion, it will be operated with a sawtooth or triangle wave as its input. I may also operate it with between \$-1\:\textrm{V}\$ and \$+1\:\textrm{V}\$ at the control input (or even with between \$-100\:\textrm{mV}\$ and \$+100\:\textrm{mV}\$ at the input.) Behavior must be monotonic, throughout. The maximum frequency I use about is \$1\:\textrm{kHz}\$, but I can sacrifice a factor of 10 on that point if necessary.

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The above circuits are also good for another purpose. If I remove (by replacing it with \$0\:\Omega\$) \$R_8\$ and use the inverting input of the opamp as a node into which I can sink or source current, and if I also then place a known precision resistor from the output to ground, then the bipolar voltage at the output will depend on the bipolar current to ground.

It's actually a rather versatile module.