Stabilized Preamplifiers

Generally, ac signals are symmetric about a zero-volt baseline. If a dc amplifier is used to process such a signal, it may be necessary to use a dc offset to put the ac signal into the operating range of the dc amplifier. This is also true of an ac signal which possesses a dc component. 

Both mechanical and electronic choppers are used. The mechanical units frequently switch at the power-line frequency (60 Hz) and through the arrangement of the switch contacts produce a chopped signal at the second harmonic (120 Hz). Mechanical choppers consist usually of a set of fixed-switch contacts, an armature (moving reed with contacts), and an electromagnet assembly which actuates the armature. An operating frequency of 60 Hz, although convenient, is not desirable because of possible hum pickup 

Chopper amplifiers do have several disadvantages. Mechanical choppers have a short lifetime and are expensive. In general, this system is satisfactory for dc signals and slowly varying ac signals. Unless quite sophisticated circuitry is used, chopper amplifiers are not satisfactory for processing repetitive 

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Alternating-Current, Direct-Current, and Chopper-Stabilized Preamplifiers... 

Alternating-, Direct-Current and Chopper-Stabilized Preamplifiers 159... 

The eommonly used MCPs and PMTs deliver single-electron pulses of 20 to 50 mV when operated at maximum gain. Although these pulses can be detected by the input diseriminators of most TCSPC modules, a preamplifier is recommended for several reasons. The most obvious one is that a good preamplifier, if it is con-neeted elose to the detector output, improves the noise immunity of the system. Moreover, with the amplifier the CFD ean be operated at a higher discriminator threshold, which improves the timing and the threshold stability. 

The gain of the preamplifier allows the PMT to be operated at a correspondingly lower gain. The reduction of the average output eurrent at a given eount rate improves the timing stability and inereases the lifetime of MCP PMTs. 

Fig. 2.22. Langmuir-Taylor detector, (a) Side and top elevation. The detector s filament is made of rhenium [252]. (b) Schematic of the electronic measuring circuit. The filament is heated by a stabilized current Is and set to a positive potential by a second highly stabilized voltage f/s. The preamplified anode current I a is detected by a lock-in technique or by a picoammeter...

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