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High speed voltage to current

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Jun 29, 2023
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I have a signal oscillating at tens or hundreds of MHz (up to 200 MHz expected), with 2.5 V mean and approx 200 mV peak to peak. I'd like to produce a current proportional to this signal at approx 40-50 mA mean to drive a laser diode.

I'm thinking about using this kind of floating load voltage to current circuit:


Is this kind of circuit suitable for up to 200Mhz signal?

If yes, does the AD8000 opamp would be suitable?

If not, which kind of (simple) circuit should I use (I'm running single supply)?

Thank you very much for your help.

Basic sim seems to work. Note I did not bias part to yield a
+ only input V. Note I did not have a laser diode model so just used
a small signal diode for sim purposes.

The challenges will be stability at this frequency. I would look at the
manual for the dev board AD sells, paying specific attention top layout
and their specific recommendations for bypass caps.


I did a bode plot but do not trust it as my circuit does not have all the parasitics in
it and actual components used. But at 100 Mhz still a lot of G left and phase flattens out
at 80 degrees, so externals phase contribution could bite you in the rear end. Amp has

PSRR at these freq can also cause you a lot of grief :


Regards, Dana.
Last edited:

"RF over fiber" is a real thing and you can probably find a lot of info
on how people do modulation for linear signals.

While a transconductance amp would fit the theoretical bill, it's liable
to be over-slow.

Laser modulation must be single ended (can't go darker than dark)
so you might find a resistor network that does the job (perhaps a
bias legRbase to +5 for lasing current at half scale, and a CR-Rbase
divider port that gives the right current-for-voltage scaling (this
presumes the 2V input is well lower-Z than the rest of it).

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