DRIVEN-SHIELD AMPLIFIER FOR THE RECORDING OF BROAD-BAND, NANO-AMP SCALE, CURRENT TRANSIENTS FROM SOURCES LOCATED INSIDE A VACUUM CHAMBER

A fast, high-sensitivity amplifier has been developed and successfully used to record transient photoconductivity current of high-impedance, hydrogenated amorphous silicon thin film samples mounted inside a cry ostat. The key feature of this amplifier is adjustable, positive feedb ack to the guard conductor of the input coaxial line. The amplifier ci rcuit is located entirely outside the cryostat. It is characterized by a useful bandwidth from oc to 5 MHz, current to voltage gain in exces s of 8 x 10(6) V A(-1), good linearity and low noise. Averaging over s everal hundred waveforms with the help of digital storage oscilloscope improves signal-to-noise ratios dramatically and allows transient pho tocurrents of the order of 1 nA to be accurately followed in a microse cond time scale. The amplifier was also successfully used to record th e ion current of a Faraday cup in time-of-flight mass spectroscopy.