# Driving a difficult load with an Op-Amp: high capacitance, high current, high speed

I've been banging my head against this problem all day. I have a bizarre load that I need to drive over 88 feet of SMA that is a 68 ohm pullup to 15V. The cabling adds 2.64 nF of capacitance. I've sketched it below.

simulate this circuit – Schematic created using CircuitLab

I'm driving negative-going analog pulses (worst-case from 0V to 3V and back) where both the amplitude and pulse width are important, with rise times down to 20 ns. Somehow I have to achieve:

• Low overshoot (<135 mV)
• High amplitude accuracy
• Stability (this has been the most difficult!)

Because this requires sinking 18V / 68 Ω = 265 mA, I can't just use an opamp. So I tried a current-amplified opamp circuit like so:

I originally designed this without considering the capacitance of the cabling and I managed to get it perfect, but it oscillates once I add connect the 2.64 nF, as you can see below. I tried many different transistors and op amps, guessing at what parameter would affect this, but I can't get rid of the oscillation. I'm also stuck with a high-speed op-amp (BW > 50 MHz) because of the fast rise times.

Currently my only viable solution seems to be a simple voltage follower with no feedback. I'd have to calibrate out the VBE drop and temperature dependence, which makes for a terrible system.

My question is this. What causes this oscillation and what do I have to do during part selection to prevent it, or how could I damp the oscillation?

• Maybe try something like this: ti.com/lit/ds/symlink/lmh6321.pdf – John D Aug 13 '18 at 18:18
• Try removing C3 and add a capacitor (few pF) between the opamp's output and the inverting input instead. And use a better transistor, the 2N2907 doesn't have enough gain at those frequencies. – Jonathan S. Aug 13 '18 at 18:41
• @JonathanS. I have tried a faster transistor (ZXTP25100 w/ 200 MHz BW) with no luck, do you think I need even more? I also have tried a cap connected like that, no luck there either. – jalalipop Aug 13 '18 at 19:04
• @JohnD Cool part! Open-loop buffering is new to me but that looks like it could work. I was hoping to solve my problem while retaining the feedback, partially because my project is mil temp so I'd rather not depend on ICs. But I will keep that in mind! – jalalipop Aug 13 '18 at 19:05
• You MIGHT be able to put the buffer inside the feedback loop if you can figure out how to tailor the open-loop response so that it's stable when you close the loop. – John D Aug 13 '18 at 19:23