One of the options for the HP3325B is the "high voltage" output, which allows up to 40Vpk-pk, at up to 1 MHz, into 500 Ohms. Usual output is 10Vpk-pk, at up to 21 MHz into 50 Ohms.
Anyway, I don't have that. I was wondering if I could perhaps build a buffer-amp that beats their spec. Ideally it'd do 40Vpk-pk at 21 MHz into 50 Ohms, without adding appreciable distortion (the HP3325 is only good for perhaps 0.01% at audio, so that's not a difficult number to reach).
I have an old design for a current-mode headphone amp that I thought might be suitable, so I spruced it up a little, thought in terms of HF rather than audio, and I think I've come reasonably close.
It's actually a challenging amp - it's gotta be DC coupled, stable with a gain of just 4, and have reasonably low offset and gobs of speed, with tolerable distortion driving 40Vpk-pk, and really good distortion at lower levels. Here's the schematic:
At audio frequency it's kinda cool, and would make a neat uber headphone amp. (0.001%, 10KHz, 40Vpk-pk into 50 Ohms). And it's got gain that just keeps going. Yeah, 12dB gain at 100 MHz: