# Differential to single-ended converter

Originally I wanted to setup a simple curve trace circuit to measure the characteristics of a BJT I salvaged. This circuit operates by measuring the voltage and current through the transistor for different applied voltages on the base and collector. For this, I found a simple tutorial which uses the same USB oscilloscope that I have (https://www.instructables.com/id/Semiconductor-Curve-Tracer-With-the-Analog-Discove/). The hitch arose when trying to implement the circuit on the breadboard; I found that their circuit measures the differential voltage across the collector resistor to determine the current. This wouldn't have been a problem for me until I (rather permanently) connected a BNC probe adapter to my USB oscope, which ties the negative inputs of all BNC probes together. So as a simple remedy I figured I could just build a differential to single-ended converter to avoid the issue of ground loops in my probes.

For even more background, I did a quick google search on the terms 'differential to single-ended converter' and it gave me this article as a top result (https://www.analog.com/en/analog-dialogue/raqs/raq-issue-145.html). In my haste to get to my primary project, I threw this circuit together on a breadboard.

My results were decently accurate for differential signals, but I found the common-mode signals weren't completely attenuated.

Great, so I guess this has something to do with the non-idealities of the opamps. Then I got to thinking back on what I know about circuits, and I realized that an OPAMP is essentially a differential to single-ended converter itself. I quickly found a 'differential amplifier' circuit that would essentially achieve the same thing with only one opamp (https://www.electronicshub.org/differential-amplifier/).

So my question (finally) is, what advantages does the dual opamp circuit offer versus the single opamp solution? Furthermore, would either one be better for simple (near-DC) current measurement? Thanks in advance