Field effect transistor switching arrangement for amplifying only low level signals

Claim 1

Electronic amplifier switching apparatus comprising a source of high level input signals; a device connected to receive said signals and adapted to produce therefrom low level output signals; an output signal amplifier; a field effect transistor connected as a preamplifier between the device and the output signal amplifier; a switching arrangement for enabling or disabling conduction through the field effect transistor; and a control means for causing or inhibiting application of high level input signals to the device, the control means also being connected to control operation of the switching means such that whilst high level signals are being applied to the device conduction through the field effect transistor is disabled and such that in the absence of high level signals, the conduction through the field effect transistor is enabled to feed the low level signals to the output signal amplifier. 2. Electronic amplifier switching apparatus as claimed in claim 1, in which the switching means includes a first transistor having its collectoremitter circuit arranged in series combination with a first controlled electrode of the field effect transistor; a second transistor having its collector-emitter circuit arranged in series combination with a second controlled electrode of the field effect transistor; and means for commonly connecting the first and second transistors to the control means for simultaneously controlling conductivity of the first and second transistors.