Tuesday, 17 January 2017

A fast 12dB gain current-mode buffer for my HP3325B function generator

I'm sprucing up my HP3325B, after it stopped working (shorted out tantalum).

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 the hard end of the spec it manages 0.08% THD pushing a 1 MHz 40Vpk-pk sine wave into 50 Ohms. Relax any of that and it gets a lot better. For example at 1Vpk-pk, 1MHz, 50 Ohms it does 0.002%. At the "High Voltage option" max spec (500 Ohm load), it manages 0.05%, not a lot different from the 50 Ohm case, due mainly to the heavy feedback current.

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:

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