I have a control voltage at one point in a circuit which I need to duplicate at various other points (3 or 4, if I remember correctly). I'd like to use one potentiometer to control these voltages, so I need to "duplicate" the control voltage at the points I need it in. The problem is that I need to raise and lower voltages in the circuit by the control values, so I can't just use a buffer amp--it needs to be a floating source. Essentially I'm looking to build something like: 
The output resistance isn't very important, as in all cases the next input stage is an op amp, and \$V_x\$ is no more than 3-5 volts. How would I go about building something like this? I know I could convert the voltage to AC, use two transformers, and then filter the output, but I'd like to keep this DC if possible.
Answer
A difference amplifier, with a buffered virtual ground and unity gain is close to your VCVS.
Difference amps are often drawn with the output relative to ground (replace U2 below with ground). But the output can be relative to any low impedance reference point. Below I've added U2 to create a virtual ground, so that the output is Vx relative to U2's noninverting input.
simulate this circuit – Schematic created using CircuitLab
It's not perfect:
- the input impedance is finite (could buffer though)
- all the inputs have to stay within the PS rails and op amps' CM input ranges
- input and output need to share a ground (it's floating but not isolated)
- will need compensation if it oscillates.
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