CIRCUITRY AND CIRCUIT ELEMENTS
Figure 31 shows how a two-stage amplifier increases the signal-to-noise ratio.
Figure 31 Single and Two-Stage Amplifier Circuits
The radiation detector is located some distance from the readout. A shielded coaxial
cable transmits the detector output to the amplifier. The output signal of the detector may
be as low as 0.01 volts. A total gain of 1000 is needed to increase this signal to 10 volts,
which is a usable output pulse voltage. There is always a pickup of noise in the long
cable run; this noise can amount to 0.001 volts.
If all amplification were done at the remote amplifier, the 0.01-volt pulse signal would
be 10 volts, and the 0.001 noise signal would be 1 volt. This is a signal-to-noise ratio
of 10 and could be significantly reduced by dividing the total gain between two stages
of amplification. A preamplifier located near the detector and a remote amplifier could
be used. The preamplifier virtually eliminates cable noise because of the short cable
length. If, for a total gain of 1000, the preamplifier has a gain of 100 and the amplifier
has a gain of 10, the output signal from the preamplifier is 1 volt. The signal transmitted
via the long cable run still picks up the 0.001-volt noise. The amplifier amplifies the 1.0-
volt pulse signal and the 0.001-volt noise signal by a factor of 10. The result is a 10-volt
pulse signal and a 0.01-volt noise signal. This gives a signal-to-noise ratio of 1000.