Resistive feedback RF preamplifiers

Under normal circumstances, with no source on the detector, the preamplifier test point voltage when operated at the recommended bias voltage was —1.6 V. Over time, the detector started to exhibit poor detector resolution to which no obvious solution was found (4/2 in Table 12.1). 

It would help if, in times of misery, one had available a printed copy of a test point voltage versus bias curve measured when the detector was working satisfactorily for comparison. This is, perhaps, another item to place in the detector log book when commissioning a new detector (see Chapter 11, Section 11.2). In this particular case, the assumption was made that the enhanced leakage current was due to condensation of gas traces onto the detector within its vacuum enclosure. A thermal cycle was undertaken (see below. Section 12.3) and good resolution was restored. Presumably, the thermal cycle successfully removed the surface contamination from the germanium crystal. 

If the test point voltage reads 22-23 V, i.e. the supply voltage, then it is likely that the FET has been destroyed any high value unaffected by bias voltage points to a defective FET. 

If the resolution of a detector has degraded with no apparent reason and all the usual checks and adjnstments, such as pole-zero cancellation, fail to cure the problem it may be that a thermal cycle is necessary.  

Unidentified resolution problems may be a consequence of contamination of the detector itself, causing surface leakage currents. This contamination arises because of desorption of gases from the absorber within the vacuum 

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The mechanisms of the resistive feedback (RF) preamplifiers and transistor reset preamplifiers (TRPs) were discussed in Chapter 4, Section 4.3. Some of their properties are compared in Table 14.2, and some of the differences between them are explored in the following sections. 

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