A current mirror circuit to bias beam intercepting pick up electrodes

Beam properties are often determined by measuring the current induced on intercepting electrodes. To collect the full signal current, it is desirable to bias the electrodes. The system discussed in this note makes use of a current mirror circuit to allow the use of a shared bias supply while dispensing with the need for isolation amplifiers. Low power, high voltage bipolar transistors, matched for best performance, form the mirror. The circuit has a wide dynamic range, which can exceed 104. Though it is unipolar, versions can be made for either positive or negative polarity. Spark protection is provided by including a current limiting circuit. A current mirror system has been constructed and used in a TRIUMF H- ion source. The device provides eight measurement channels and supplies 400 V at up to 1 mA