GB1155562A - Improvements in or relating to Electric Circuit Arrangements Employing Field Effect Transistors - Google Patents

Abstract

1,155,562. Measuring bridge circuits; measuring displacement electrically. GEORGE KENT Ltd. June 17, 1966 [June 18, 1965], No. 25919/65. Headings G1N and G1U. [Also in Division H3] In an arrangement for measuring displacement, displacement of core 29 in inductors 26, 27 unbalances a bridge network comprising the inductors as one branch. The second branch comprises the current paths of series-connected field-effect transistors 10, 11. The bridge network is fed with A.C. from oscillator 30 and when unbalanced produces on output which is amplified by amplifier 32 and demodulated by demodulator 33. The resulting D. C. output is fed to meter 36 and also passes through resistors 17, 18 whereby the gate electrodes of transistors 17, 18 are differentially biased, producing a differential change in impedance such as to restore the bridge to balance. The transistors obtain a fixed standing bias from battery 15. Meter 36 may be located at a remote point and connected in series with a battery 37 for energizing the apparatus, a steady potential for this purpose being picked off across a Zener diode 38 in the connecting lead. The arrangement may be made insensitive to temperature changes by connecting adjustable resistors in series with the transistors; alternatively, adjustable resistors may be connected in shunt with the transistors, further temperature sensitive resistors being connected in series with resistors (17, 18, Fig. 5, not shown). The sensitivity of the arrangement may be made adjustable by connection of the end electrodes of the transistors to a potentiometer arranged in parallel with the oscillator Fig. 6 (not shown); alternatively resistors may be arranged in series between the join of inductors 26, 27 and a centre-tapping on the oscillator and comprising a potentiometer from which the output is taken Fig. 7 (not shown). In both these arrangements temperature-variable resistors may be used to compensate for a temperature coefficient in the displacement-sensitive device. In order to prevent modulation of the gate voltage by the energizing signal across the transistors, a proportion of the voltage across the end electrodes may be applied to the gates by means of potentiometer networks Fig. 8 (not shown). In order to obtain a current output which is non-linearly related to the displacement the demodulator 33 may be followed by a non-linear network: alternatively the transistors (10, 11, Fig. 9, not shown) may have connected in parallel with their gates the gates of a further pair of field-effect transistors (57, 58), the sources of which are energized via the centre-tapped secondary (59) of a transformer, the primary (60) of which is energized from the common point (14) of the first pair of transistors, while the amplifier (32) is fed from the common point of the second pair of transistors (57, 58).