The circuit provided below diagrams a high voltage booster
and has 10 mA output current. It is called as a positive-output-only
circuit that will drive 350V into a 30k load and is almost immune to
load shorts and reverse voltages.
Though it shown a 350V limit, this output booster circuit
can be extended the output capacity into several kilovolts. The
parallel diodes at the summing junction prevent high voltage from
destroying the amplifier during circuit start-up and slew rate limiting.
See more explanation about 300V-10mA (High Voltage) Output Booster Circuit Diagram in detailed in this Op Amp Booster application notes (source: national.com). See another Amplifier/Conditioner article you may interest in this blog.