Active filter for electromagnetic interference (EMI) reduction using a single connection point and a negative impedance converter

A single-connection-point active filter comprising:

• a bi-directional connection point to a noisy line having a noisy current;
• a connection capacitor connected from the bi-directional connection point to a first node;
• a gain control circuit having the first node as an input, the gain control circuit having a second input connected to a second node, and an output connected to a third node;
• a power amplifier circuit having the third node as an input, the power amplifier generating a sensing current on a fourth node in response to the third node;
• a transfer impedance connected between the fourth node and the first node, the transfer impedance converting the sensing current to an output voltage;
• wherein a voltage of the first node causes the transfer impedance to inject a compensation voltage back through the connection capacitor and into the noisy line at the bi-directional connection point;
• wherein the compensation voltage compensates the noisy line for changes in the noisy current.

An active noise filter comprising:

• a connection capacitor coupled between a connection point on a current-carrying line and a first node;
• an op amp having the first node applied to a non-inverting input, the op amp generating an op amp output on a third node in response to the first node;
• a first gain-setting impedance device connected to a second node applied to an inverting input of the op amp;
• a second gain-setting impedance device connected between a ground and the second node;
• a power amplifier generating an output current to a fourth node in response to the op amp output, wherein a peak value of the output current is greater than a peak value of a current generated by the op amp;
• a transfer impedance connected between the fourth node and the first node, for converting the output current into a transfer voltage;
• wherein the transfer voltage on the transfer impedance induces an injected voltage across the connection capacitor to compensate the current-carrying line,
• whereby Electro-Magnetic Interference (EMI) generated by the current-carrying line is reduced by the active noise filter.