Patent GB-1125897-A (GB)
Title: Frequency selective electronic apparatus
IPC: H03F13/00
Filed: 1966-06-14
Abstract: 1,125,897. Semi-conductor devices. WESTINGHOUSE ELECTRIC CORP. 14 June, 1966 [18 June, 1965], No. 26480/66. Heading H1K. A frequency selective device comprises a substrate mounting for an extended vibratory member and a field effect transistor, which co-operates electrostatically with the vibrating member to produce electrical responses corresponding to the vibrations. Fig. 1 shows a ceramic or semi-conductor substrate 10 with a cantilever vibrating member 12 overlying the channel portion of an insulated gate field effect transistor 23. The member 12 may consist of a nickel, phosphor bronze or goldplated tungsten wire or of a deposited metal layer and may be vibrated by electrostatic action produced by applying an A.C. signal of appropriate frequency to plate electrode 16 underlying the extended portion of member 12. An additional contact which is engaged when 12 vibrates, may be provided on the substrate to operate a switch. The member 12 may also be arranged to overlie part of the drain electrode of the transistor, the electrostatic reaction between drain and vibrator then effecting positive feedback so that an oscillator can be provided; alternatively the elements may be positioned for balanced neutrality so that feedback is avoided. Vibrator 12 may be D.C. biased and voltage gain may be achieved by operating at pinch-off conditions. Resonant frequencies of 100 to one million c.p.s. are practicable. Details relating to the production of a suitable device by photoresist and deposition processes are described; the photo-resist may be put on by spinning. Chromium, gold and a further layer of gold may be provided to form the vibrating member which is bonded at one end to the substrate; the extended vibrating portion is provided by removing part of an underlying support and resist layer after the gold has been deposited electrolytically to leave the cantilever portion 1 mil. wide, ¢ mil. thick and 38 mils. long with a resonant frequency of 3000 c.p.s. The body may be of silicon covered with silicon oxide and the source and drain electrodes may consist of aluminium and silver. A plurality of vibrating members with different resonant frequencies may be provided and two or more members may be loosely mechanically coupled to give a band-pass effect. Fig. 11 (not shown), describes a modulating arrangement in which signals from source (115) modulate the carrier provided by feeding signals from source (15) at the resonant frequency of the vibrating member (12) so that a modulated amplified output apepars at high-pass filter (42). In another embodiment (Fig. 12, not shown), member 12 is momentarily disconnected from a D.C. bias source so that it oscillates which, when the bias is restored, results in a pulse of damped oscillations; a plurality of such devices may be used signalling pulses of different frequencies in a push-button telephone. Fig. 13 (not shown) shows a frequency controlled oscillator with a bipolar transistor amplifier (63) in the feedback path, which after wave-shaping and frequency dividing may be used to drive a watch mechanism. In a further embodiment (Fig. 14) the impedance of the device, which varies as resonance is approached, is used as a variable reactance to control tuning functions. The member 12 may alternatively be a plate, diaphragm or rod mounted at both ends.