# How can you build a floating two-port VCVS?

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.

• How much money do you want to spend? You can use standard isolation amplifiers such as the Analog Devices AD210 or any of a myriad of similar devices. The downside is that they can be expensive. – Dwayne Reid Jun 20 '15 at 4:03
• More information would be helpful. How much voltage between the various ports? – Dwayne Reid Jun 20 '15 at 4:04
• No more than 3-5 volts. I'll update the question. – interplex Jun 20 '15 at 17:33
• Could you describe the reason why you think you need such a thing? Although they exist in simulations it is extremely rare to need one in a practical circuit. Usually the circuit can be rearranged so that a voltage source referenced to ground can be used instead. – Kevin White Jun 20 '15 at 17:59
• At one point in the circuit, for example, I need only voltage above a user defined limit (Vu) to pass into the next part of a circuit. I'm currently doing this by adding a voltage drop of Vu before a voltage rise of .7 volts before a diode. The problem is Vu isn't exactly user-defined, there's some other processing in its exact value, so I need to take the output of that process and use it twice, once for the positive part of the signal and once for the negative. – interplex Jun 20 '15 at 18:18

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.
• Thanks for the idea. I figured out how to use voltages referenced to ground though, and I believe I can just use a summing amplifier to do the level shifting. – interplex Jul 8 '15 at 4:53
• Yes wouldn't recommend substituting the above, there are probably more elegant approaches. Summing amp to add an offset is a good candidate. I'd like to see your design with ideal vcvses, and try to redo with current mirrors! – tariksbl Jul 8 '15 at 16:01