Differential amplifier and current sensing circuit including such an amplifier

A current sensing circuit for sensing an output current of a power semiconductor arrangement, which arrangement has major and minor current carrying sections, each section comprising at least one basic semiconductor element, said major and minor current carrying sections having a common first electrode and corresponding major and minor second electrodes, the current sensing circuit comprising:

• means for comparing the voltages at the major and minor second electrodes;
• means for controlling a current drawn from the minor second electrode in response to an output of the comparing means so as to cause the voltage at the minor second electrode to end to substantial equality with that at the major second electrode;
• and means for measuring the current drawn from the minor second electrode, wherein the comparing means comprises a differential amplifier comprising first and second matched field-effect transistors (FETs) having their source electrodes connected together and to a current source, and their drain electrodes connected respectively to an input and an output of a current mirror circuit, the junction of the output of the current mirror circuit and the drain electrode of the second FET forming a tap for output current of the differential amplifier, characterized in that said first and second FETs are depletion-mode FETs, and in that said current source comprises parallel-connected third and fourth depletion-mode FETs matched to the first and second FETs, each of the third and fourth FETs having its gate and source electrodes connected together and to a voltage supply, and its drain electrode connected to the source electrodes of the first and second FETs, means being provided for coupling the gate electrodes of the first and second FETs respectively to the major and minor second electrodes of the power semiconductor arrangement.

An integrated circuit comprising a power semiconductor arrangement having major and minor current carrying sections, each section comprising at least one basic semiconductor element, said major and minor current carrying sections having a common first electrode and corresponding major and minor second electrodes and a current sensing circuit comprising means for comparing the voltages at the major and minor second electrodes;

• means for controlling a current drawn from the minor second electrode in response to an output of the comparing means so as to cause the voltage at the minor second electrode to tend to substantial equality with that at the major second electrode;
• and means for measuring the current drawn from the minor second electrode, wherein the comparing means comprises a differential amplifier comprising first and second matched field-effect transistors (FETs) having their source electrodes connected together and to a current source, their gate electrodes coupled respectively to said major and minor second electrodes, and their drain electrodes connected respectively to an input and an output of a current mirror circuit, the junction of the output of the current mirror circuit and the drain electrode of the second FET forming a tap for output current of the differential amplifier, characterized in that said first and second FETs are depletion-mode FETs, and in that said current source comprises parallel-connected third and fourth depletion-mode FETs matched to the first and second FETs, each of the third and fourth FETs having its gate and source electrodes connected together and to a voltage supply, and its drain electrode connected to the source electrodes of the first and second FETs.