| The preamplifier is protected
from excessive input signals of either polarity by the 2N5909 junction field-effect
transistor. A nulling circuit makes it possible to set the preamplifier output voltage to
zero at a fixed low level (up to ±10e-8A) of the input current. (This level is called the
standing current and corresponds to the zero-signal level of the instrumentation.) The
opposing (offset) current is generated in the 10e9 feedback resistor to buck the standing
current. Different current ranges are reached by feeding the preamplifier output to low
and high gain amplifier chains. To reduce noise, each chain includes a 1.5 Hz corner
active filter. |