OK, I need to charge a capacitor with a stable constant current. The desired slope is about a volt per nanosecond.
So I did this:
Things like this tend to oscillate, so I used a fairly slow, high-beta transistor, BCX71K. The ferrite in the collector is supposed to isolate the ramp cap from the transistor capacitance and make the ramp linear.
Well, the ramp looked not much better than an R-C curve, and the BCX71 oscillates at 80 MHz. Increasing R1 from 50 to 150 ohms kills the oscillation and makes the curvature worse.
So we went to the opposite extreme, a BFT92, a 5 GHz PNP. Typ beta is
50! The ramp is now visually linear, and the oscillation frequency went up some. Tried a ferrite instead of R1, and it *really* oscillates. A 100 ohm base resistor seems to work.