Public address/radio switching system
AssigneeMotorola, Inc.
InventorSamir Thakorbhai Desai
Life of this patent
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Abstract
Public address add-on circuitry to a conventional radio includes a microphone coupled to a preamplifier which draws significant idle current, and is optimally operable when coupled to a proper load resistance. Permanently affixed and capacitively coupled to the audio input stage of the radio is a series element containing a semiconductor junction device and a resistor selected to properly load the microphone preamplifier. When switching from radio operation to public address the quiescent current of the microphone amplifier reduces the resistance of the junction from a high to a low value thereby allowing the preselected resistor to load the microphone. Semiconductor action further disables the tuner portion of the receiver.
Claims14
In combination with a radio receiver having a tuner and an audio amplifier coupled thereto
- a signal source means including an electrical gain stage, the gain stage biased to conduct a quiescent current, the signal source means optimally operable with a fixed load,
- a switching means operable in a first or a second state, the switching means operable in the second state to couple the source means to the audio amplifier and operable in the first state to decouple the source means from the audio amplifier,
- a load means including a semiconductor device series connected to a load element selected to optimally operate the signal source means, the device having a pair of electrodes and an internal resistance therebetween, the resistance relatively low in response to a common current flowing from the first to the second electrode, the resistance otherwise relatively high,
- means coupling the source means quiescent current to the load means device in response to the switching means assuming a second state.
A public address and radio receiver system comprising:
- a radio receiver having a tuner and an audio amplifier,
- a microphone means including a transducing element coupled to an electrical gain stage, the gain stage biased to conduct a quiescent current,
- a switching means operable in an active or an inactive state,
- a semiconductor device having a gate terminal, a first terminal and a second terminal, the first and second terminals having a resistance therebetween, the resistance relatively low corresponding to gate current, the resistance otherwise high,
- the switching means coupling the microphone means to the audio amplifier in the activated state,
- the switching means further coupling the quiescent current to the gate terminal in the activated state, and
- means disabling the tuner responsive to a low device resistance.
Dependencies are inferred from the claim text.