A self-adjusting constant-current source follower for CMOS preamplifiers of semiconductor detector signals

A class A-B CMOS output stage, conceived for low-noise integrated preamplifiers of X- and gamma-ray detector signals, is proposed. It is based on a negative-feedback loop which stabilizes the working current of the output transistor. As a result we obtained both a low output impedance and a large voltage swing. We realized the output stage in a 5V 0.8um CMOS technology. The obtained output voltage swing on a 100 ohm load resistor is -2.5V against a negative power supply of -3V. We inserted the output stage into a charge-sensitive preamplifier for germanium detectors to prove its functionality, obtaining the same large negative voltage swing and a signal rise time as fast as 12.8ns on a terminated 50-ohm cable (equivalent to a load resistor of 100 ohm), as well as a negligible additional noise. The detector capacitance for these measurements was 15pF.