Amplifier design

A project log for Active scope probe with no dedicated power supply

Using the probe test point to power a ultra-low capacitance, high-input-impedance amplifier, essential for accurate analog measurements!

Yann Guidon / YGDESYann Guidon / YGDES 04/14/2016 at 08:470 Comments

In this forum post one comment explains how a single-ended amplifier works, even providing this diagram:

The key part is a pair of closely matched and coupled N-FET (for temperature compensation). They can be replaced by a pair of BF256 that must be sorted/matched.

This circuit can be duplicated to become a differential amplifier, by also re-wiring the output op-amp (or using an instrumentation amplifier). The differential nature will also compensate for temperature drifts.

From this, 2 things become apparent :

I have seen other, better schematics with higher performance but the cost and complexity become overwhelming. OTOH, a handful of BF256 (VHF N-FET) doesn't cost much.

The supply can be regulated by a couple of micropower LM4041 (initially intended for the ADC of the #Discrete YASEP), drawing only 60µA, capable of up to 10V of shunt (15V abs.max.) and 10mA of sinking. I don't think a scope's probe test point can provide such a current so it's well within the bounds.

Now, it remains to be seen how much the above schematic draws in practice. Linearity, bandwidth (frequency response) etc. are also a concern.

Maybe I'll have to complete my #Quick & Dirty Frequency Generator or buy that DDS from my dear friend noodlehead

(No, I'm not shooting for GHz precision, my fastest scope reaches 200MHz only)