A simple derivation of field-effect transistor characteristics
In the conventional treatment of the field effect 
transistor, the first step is the specification 
of an impurity profile that describes the 
nature of the gate-channel contact. Solutions 
for the static and small-signal characteristics 
are then valid only for the particular 
impurity profile chosen, and must be repeated 
from the beginning for different 
structures. 
 
The purpose of this communication is to 
present a simple, though approximate, 
development of the characteristics of an 
FET without specifying the detailed nature 
of the structure. The charge-control approach is used, and it is shown that in the pinch-off region the relation between the drain current and the gate-source voltage is 
approximately square law. The results are 
applicable to all gate-channel structures, 
including the insulated gate types.
An Active FET Receiver Front End Mixer
In many recent receiver designs, the front-end mixer determines the receiver's sensitivity and its susceptibility to distortion from large input signals. Two balanced FET mixers are developed for such an application; a common source configuration and a common gate configuration. Each exhibits a range of 110 dB between the available S+N input power necessary for 10 dD sensitivity and the available input power of a two-tone signal which causes intertnodulation products down 40 dB from the desired signals* An expression for the conversion transconductance of a junction FET used as a mixer is first derived from the device transfer characteristic equation* The advantages of a balanced mixer con figuration are then discussed* A noise equivalent circuit which describes the sources of noise in each of the proposed mixers is developed* This leads to the calculation of noise factor and hence, sensitivity of the common source mixer as a function of R , the generator resistance* The dependence of distortion on R for the common source mi'xer is then found and plotted* A comparison of this plot and the sensitivity vs. R plot shows that the maximum dynamic range results from a choice s of R equal to 200 ohms. The dynamic range of the common gate mixer is investigated and found to be equal to that of the common source mixer. The sensitivity, however, is found to be 6 dB worse.
rPET detectors design and data processing
Small animal PET systems based on rotating planar detectors posses some interesting advantages for high sensitivity, high resolution imaging. We have designed the rPET detectors based on MLS crystals assembled on a 30times30 matrix optically coupled to a flat-panel PS-PMT. Weighted position readout circuits pre-process the 64 signals from the 8times8 anodes matrix, which are digitized using a charge-integrating converter. The amplification electronics, including the trigger output for coincidence detection, and the high voltage supply are integrated in a three PCBs stack that forms the base attached to the back of the PMT. The whole assembly is enclosed in a light tight, lead (Pb) shielded aluminum box. The detectors are mounted on a rotating gantry with more than 180 degrees rotation span. The digitized events are screened and histogramed, and a modified center of gravity algorithm removes from the position calculation those signals with poor signal to noise ratio. Apparent mean crystal size on the 511 keV field-flood images is 0.6 mm, mean peak-to-valley ratio is better than 8, and intrinsic resolution is 1.5 mm at the central row, with the energy window wide open. Sensitivity (CPS) for a pair of these detectors set in coincidence at 160 mm distance is 1%