In an IR sensor having plural readout channels, the combination comprising
- (1) an evacuated cryogenic enclosure having an aperture for admission of IR image radiation through a predetermined aperture, and a cold radiation shield within said enclosure and spaced from the walls thereof,
- (2) a thermally and electrically conductive cold finger extending into said enclosure for conductively cooling and mechanically supporting said cold radiation shield,
- (3) a focal plane array installed within said cold radiation shield, mechanically supported by, electrically grounded to, and conductively cooled by said cold finger, including
- (a) an M.times.N plurality of IR detector elements for intercepting said IR image radiation within said aperture,
- (b) a multiple channel electrical readout circuit for sensing an electrical signal representing the IR image at each of (N.times.M) IR detector elements and grouping said sensed signals from sets of N IR detector elements into M separately scanned readout channels to facilitate a higher scanning rate and/or higher signal to noise ratio, and
- (c) M preamplifiers, one for each set of N IR detector elements, each preamplifier including four field effect transistors (T1-T4), each transistor having a gate and as principal electrodes, a source and drain;
- three field effect transistors (T1-T3) forming a cascoded amplifier having their principal electrodes connected in series between a ground node (VSS) and a drain node (VDD), both nodes being common to said M preamplifiers,
- the first transistor (T1), with source connected to said ground node (VSS), having the gate thereof connected to the set of N IR detector elements associated with that preamplifier,
- the second transistor (T2), with source connected to the drain of the first transistor, having the gate thereof connected to a cascode gate node common to said M preamplifiers,
- the third transistor (T3), with the source connected to the drain of the second transistor having the gate thereof connected to a load gate node common to said M preamplifiers, and
- the fourth transistor (T4) forming a source follower, the source of which is connected via a current setting resistor to said ground node (VSS), the drain of which is connected to said drain node (VDD), and the gate of which is connected to the drain of the second transistor of the same preamplifier, and
- (4) electrical conductors of minimum number and cross sectional area entering said cryogenic enclosure for operating said focal plane array with minimum heat loss, comprising,
- (a) a first conductor for supplying the load current of said M preamplifiers, connected between said drain node and a first terminal for a first external source (VDD') of dc drain potential, and having substantial electrical resistance (RDD) to minimize thermal conductance,
- (b) said cold finger, which returns the load current of said M preamplifiers, being electrically connected between said ground node and an external ground terminal for said first external source, and having negligible electrical resistance,
- (c) a second conductor of minimum thermal conductance for providing a gate potential with negligible current drain for effecting cascode operation of said first and second transistors, connected between said cascode gate node of said M preamplifiers and a second terminal for a second external source of dc potentials (VGC), and
- (d) a third conductor of minimum thermal conductance for providing a gate potential with negligible current drain for operating said third transistors as loads, connected between said load gate node of said M preamplifiers and a third terminal for a third external source of dc potentials (VGL), and
- (e) a low ac impedance connected between said third terminal and said cold finger for eliminating crosstalk between said preamplifiers due to a signal induced voltage perturbations on said first conductor.