Abstract: A turn off circuit for an inductivity loaded power switching transistor utilizes a transistor current path in the base circuit and two series connected diode junctions in the emitter circuit to reverse bias the base-emitter junction of the power switching transistor without the need for an active reverse bias voltage source.

Abstract: A method and apparatus for applying a reverse bias voltage to the base-emitter junction of a transistor, for turning off the transistor from a normal conduction state, or for connecting a relatively low impedance current path between the base and emitter electrodes for turning off the transistor from a fault conduction state.

Abstract: A transistor having a selectively saturated emitter collector path is protected.The emitter collector voltage level is sensed as being greater than a predetermined level to indicate saturation as is the presence and absence of drive current to the base. In response to the transistor not being activated to saturation while the drive current is applied to the base, a fault indication is derived. In response to the fault being indicated, the transistor is prevented from conducting by decoupling a source of operating voltage and current from the emitter collector path and/or decoupling the drive current from the base.

Abstract: A method and circuit for protecting the output stage of an amplifier from thermal overload. A normally-open, temperature sensitive switch is thermally coupled to the transistors in the output stage and electrically connected across a portion of the feedback resistance element of the amplifier, in one circuit embodiment. Whenever the temperature of the transistors reaches a predetermined threshold level, the switch closes and acts as a low resistance path across that portion of the feedback resistance element, thereby reducing the gain of the amplifier to a safe level which is zero in the preferred embodiment. Thereafter, when the temperature decreases below the predetermined threshold level, the switch automatically opens to restore normal operation of the amplifier.

Abstract: A normally-closed temperature sensitive switch which is heat coupled to the output transistors of an amplifier, is connected in series with a zener diode and the operating coil of a relay. When the voltage applied to the series circuit, which is the same voltage as applied to the amplifier, is insufficient to cause breakdown of the zener diode, the relay remains inactivated, and a pair of relay contacts across a feedback resistor of the output transistors remains closed. This keeps the output transistor off so that transients (which are the response of the amplifier to the rapid change in operating voltage when the amplifier is first turned on or when it is turned off) are substantially reduced. When the voltage reaches a given level at or close to the nominal operating voltage level, the relay is activated and the output transistors are turned on. When the temperature reaches a value such that the switch opens, the relay is inactivated and the output transistors are thereby turned off.

Abstract: It is known in the art to place a low impedance path across the input circuit of an output amplifier when the power dissipation of the amplifier exceeds a given value. The present circuit includes, in addition, a gate controlled rectifier, the main conduction path of which is connected across the output terminals of the amplifier, and which is turned on in response to a flow of current through this low impedance path. This additional circuit protects the amplifier from voltage spikes produced, for example, by an inductive load, such as a speaker driven by the amplifier.

Abstract: The amplitude is detected of both the input and output signals of the circuit (such as an audio amplifier) it is desired to protect. When, for example, a short circuit occurs across the load being driven, there is a sudden change in the ratio of these amplitudes and in response to this change, the signal to the output driver stage of the circuit is shunted to ground.