Patent GB-1428635-A (GB)
Title: Electrical analogue computation of arithmetical functions
IPC: G06G7/20
Filed: 1974-05-01
Abstract: 1428635 Calculating squares WESTINGHOUSE ELECTRIC CORP 1 May 1974 [14 May 1973] 19102/74 Heading G4G An analogue circuit for determining the square of the difference between two quantities comprises a P-channel and an N-channel insulatedgate, field-effect transistor (IGFET), or two such IGFETS with memory gates (MINSTORS),  connected in anti-parallel with gates connected in common i.e. as shown in Fig. 1 or Fig. 2. The saturation current of an IGFET is given by I S =K(V GS -V T ) where K is a constant, V GS  is the gate-source voltage and V T  is the threshold voltage which is fixed by fabrication. In the circuits shown, a voltage of magnitude V A  capable of causing a saturation current to flow through the IGFETS is applied across both of them, and the voltages V GS , V T  are adjusted such that N 1 /N 2 =V T /V GS  where N 1  and N 2  are the quantities concerned. The IGFETS are matched so that K is the same for each. The P-channel IGFET has a threshold potential V TP  and the N-channel IGFET, V TN ; these voltages are referred to the source and are not equal. Considering the case where V A <0, onset of current will occur when V G  (gate-ground voltage) is equal to V TP  and as V G  decreases the saturation current is given by the characteristic of the P- channel IGFET shown on the left-hand side of, e.g. Fig. 3. For increasing VG, the characteristic of the N-channel IGFET will be traced (R.H.S. of Fig. 3), but this characteristic will be displaced because V G  is measured with respect to ground. The displacement depends upon the sign of (V TP  - V TN ) and upon whether V A  is greater than, equal to or less than (V TP  - V TN ), the three conditions being shown respectively in Figs. 3, 4, 5. For V A > 0, the three characteristics are inverted (Figs. 6, 7, 8, not shown). Where (V TP  - V TN ) is negative, V TP  is set to measure the quantity N 1  and where (V TP  - V TN ) is positive, V TN  is set to measure N 1 . The invention may also be performed using IGFETS having memorygates, known as MINSTORS. The circuit of Fig. 18 is basically similar to those of Figs. 1 and 2 and uses N-channel and P-channel MINSTORS respectively M n  and M p . To set the memory gate to a chosen threshold voltage, a polarizing voltage is applied by setting 1SW, 2SW and 3SW to position 2 whereby the sources and drains are all connected in common to a chosen negative potential and the gates to a positive polarizing potential which is applied for an appropriate length of time by a timing mechanism. To erase the threshold voltage, the switches are set to position 1 which reverses the voltages. To perform a calculation, the switches are set to position 3, whereby a voltage V G  representing the quantity N 2  may be impressed on the gates and a voltage V A  applied across the MINSTORS. The saturation current is measured by the meter ME. The switch 4SW enables V A  to be varied. The MINSTORS are matched, so that the same polarizing voltage produces the same threshold voltage in each MINSTOR. Where unmatched MINSTORS are used, it is necessary to use different polarizing voltages and/or time intervals in order to obtain equal threshold voltages.