Transient and thermal protection

A method for thermally protecting an amplifier, and for reducing the transients normally produced in the amplifier when the operating voltage of the amplifier rapidly changes, said amplifier including an output stage, and a feedback connection which includes impedance means whose value of impedance affects the gain of said amplifier, comprising the steps of:

• sensing the amplitude of said operating voltage;
• sensing the temperature of at least the output stage of said amplifier;
• and
• reducing substantially the effective value of the impedance in said feedback connection whenever either said operating voltage is lower than a given value or said temperature is greater than a given value.

A circuit for providing thermal overload protection for the transistors in the output stage of an amplifier, and for substantially reducing transients in the amplifier when the operating voltage of the amplifier changes rapidly, said amplifier including a feedback element coupled between two different signal terminals, for controlling the gain of said amplifier, and a pair of terminals across which an operating voltage may be applied, comprising:

• means coupled between said terminals for sensing the amplitude of an operating voltage applied across said pair of terminals;
• means heat coupled to and at the same ambient temperature as said output stage for sensing the temperature of the transistors thereof;
• and
• means responsive to said temperature and amplitude sensing means for connecting a relatively low resistance across said feedback element, whenever the sensed temperature exceeds a predetermined temperature, or the sensed amplitude is lower than a predetermined percentage of the nominal amplitude of said operating voltage.

In combination:

• an amplifier having a noninverting input terminal coupled to a point of reference potential, an inverting input terminal to which a signal may be applied, an output terminal, and a feedback resistance connected between said output and inverting input terminals for controlling the gain of said amplifier;
• a relay comprising a coil and normally-closed contacts controlled by said coil connected across said feedback resistance;
• a pair of terminals across which an operating voltage for said relay and said amplifier may be applied;
• and
• a series circuit connected between said terminals comprising said coil, a normally-open voltage responsive switch which conducts only when the voltage across it exceeds a given value, and a normally-closed, temperature sensitive switch which is heat coupled to said amplifier and which opens when the temperature of said amplifier exceeds a given value.