You can successfully use an ordinary low-cost line transformer as an isolation transformer in ac circuits that require floating sources. However, at frequencies below 20 to 30 Hz, high distortion and excessive phase shifting occur. The simple circuit in Fig 1 restores the phase and frequency response down into the millihertz region. Based on phase-response data, the low-end frequency response extends below 100 mHz.
The circuit samples the current at the end of the primary winding and applies frequency-dependent positive feedback. As the frequency goes below 50 to 60 Hz, the permeability of the iron core severely degrades, as does the inductance. This change causes a proportional increase in current at sense resistor RS. The circuit ac couples this error voltage and injects it into the noninverting input of the op-amp driver.
The setting of RS depends on the loop gain of the stage, losses in the primary winding, and the source load of the secondary. You can excite the circuit with a low-frequency triangular or square wave and adjust RS for minimum distortion at the output. The transformer in Fig 1 is wired in series to obtain high initial inductance, and a nonconcentric secondary provides for minimum capacitance between windings (approximately 25 pF). Isolation is rated to 2000V. Any op amp with fairly low 1/F noise, such as the LF356 or even lower noise OP-07, is suitable for this circuit. (DI #1577)