When a detector signal from the preamplifier or photomultiplier tube is of sufficient amplitude, direct coupling of that output to a timing discriminator provides the best available rise time, and minimizes the effects of noise on time resolution. When a detector signal must be amplified or shaped before deriving the time information, an amplifier specifically designed for timing should be used.

Timing amplifiers are designed to have output rise times in the low nanosecond or sub-nanosecond range. Achieving such fast rise times usually compromises linearity and temperature stability. The latter parameters are not as important as low noise and fast rise times in timing applications. The output pulse polarity is normally negative for compatibility with fast timing discriminators, which were historically designed to work directly with the negative output pulses from photomultiplier tube anodes.

Two types of fast amplifiers are available: wideband amplifiers and timing filter amplifiers. Wideband amplifiers offer no control over the rise time or the decay time of the signal. They are typically used with photomultiplier tubes [Fig. 1(e)], and silicon charged-particle detectors [Fig. 1(c)], where the fastest rise times are required for good time resolution. Wideband amplifiers rely on the preceding electronics to limit the pulse length. Timing filter amplifiers offer independent CR differentiator and RC integrator controls for adjustable pulse shaping. The timing filter amplifier is used with germanium detectors (Fig. 2), or for any other application requiring adjustment of the pulse shaping. Both types of amplifiers may be either ac- or dc-coupled. The timing filter amplifiers typically include a baseline restorer.

For timing applications with either type of amplifier, the rise time should be selected to be less than the inherent rise time of the preamplifier so that there will be no degradation of the signal rise time. Excessively fast amplifier rise times should be avoided, since they will result in more noise and no improvement in the signal rise time. If adjustment of the differentiator time constant is available, it should be set just long enough to avoid significant loss of signal amplitude.

Figure 2.  Application of the Timing Filter Amplifier.