Second stage overload protection for amplifiers

Claim 2

In an amplifier circuit including a gain stage connected to a supplemental amplification stage, an output stage having an output transistor and an output terminal, said output transistor having a base and an emitter, said amplifier circuit having feedback means coupled between said output stage and said gain stage, said supplemental amplification stage having an input transistor having a collector, an overload protection circuit comprising: switchable resistive coupling circuit means connected between said collector of said input transistor and said base of said output transistor for switching overload currents in said switchable resistive coupling circuit means to cause a voltage to develop across said switchable resistive coupling circuit means under overload conditions at said output terminal, said voltage being sufficiently large in magnitude to saturate said input transistor.

Claim 3

The overload protection circuit is recited in claim 1 wherein said switchable resistive coupling circuit means includes a first resistor connected between said collector and said base, and said output stage includes a second resistor connected between said output terminal and said emitter of said output transistor and said switchable resistive coupling circuit means includes first and second diodes connected in series, the anode of said first diodE being connected to said base of said output transistor and the cathode of said second diode being connected to said output terminal.

Claim 4

The overload protection circuit as recited in claim 1 wherein said input transistor and said output transistor are NPN transistors.

Claim 5

The overload protection circuit as recited in claim 1 wherein said switchable resistive coupling circuit means include a transistor having a second collector, a second emitter, and a second base, said second collector being connected to said collector of said input transistor and said second emitter being connected to said base of said output transistor.

Claim 6

In an amplifier circuit having a gain stage coupled to a supplemental amplification stage, said supplemental amplification stage being coupled to an output stage having an output transistor and an output terminal, said amplifier circuit having feedback means connected between said output stage and said gain stage, said supplemental amplification stage having an input transistor coupled to an output of said gain stage, an overload protection circuit, the improvement comprising: means coupled between said input transistor and said output stage for saturation of said input transistor, said means including a resistive circuit connected between said output stage and an output of said supplemental amplification stage, said resistive circuit comprising a coupling transistor for providing high resistance under output overload conditions and a low saturation resistance under normal operating conditions and a constant current source circuit for saturating said coupling transistor under normal operating conditions, said coupling transistor having a collector electrode connected to a collector electrode of said input transistor and an emitter electrode connected to a base electrode of an output transistor coupled to said output terminal and a base electrode connected to said constant current source and also connected to circuit means responsive to said output terminal for saturating said input transistor when excessive current is drawn from said output terminal, thereby protecting said supplemental amplification stage from damage under overload conditions by limiting the current therein.

Claim 7

In an amplifier circuit having a gain stage coupled to a supplemental amplification stage, said supplemental amplification stage being coupled to an output stage having an output transistor and an output terminal, said amplifier circuit having feedback means connected between said output stage and said gain stage, said supplemental amplification stage having an input transistor coupled to an output of said gain stage, an overload protection circuit, the improvement comprising: means coupled between said input transistor and said output stage for saturation of said input transistor, said means including a resistive circuit connected between said output stage and an output of said supplemental amplification stage, said resistive circuit comprising a coupling transistor for providing high resistance under output overload conditions and a low saturation resistance under normal operating conditions and a constant current source circuit for saturating said coupling transistor under normal operating conditions, said coupling transistor having a collector electrode connected to a collector electrode of said input transistor and an emitter electrode connected to a base electrode of an output transistor coupled to said output terminal and a base electrode connected to said constant current source and also connected to circuit means responsive to said output terminal for saturating said input transistor when excessive current is drawn from said output terminal, thereby protecting said supplemental amplification stage from damage under overload conditions by limiting the current therein.

Claim 8

An amplifier circuit having a gain stage connected to a supplemental amplification stage and an output stage, said output stage being connected to said supplemental amplification stage and having an output terminal, and a second stage overload protection circuit coupled between said gain stage and said output stage, wherein: said supplemental amplification stage includes a PNP input transistor, a first diode, a second diode, and a first constant current source circuit in series connection between a power supply conductor and ground, said diodes being connected to allow current to flow from said power supply conductor to ground; said output stage includes an NPN transistor, two emitter resistors, and a PNP transistor in series connection between said power supply and ground, said NPN transistor being connected between said power supply conductor and one of said emitter resistors, said PNP transistor being coupled between ground and the other of said emitter resistors, each of said emitter resistors being coupled between one of said NPN transistor and said PNP transistor and said output terminal and; said second stage overload protection circuit includes, in combination, a PNP coupling transistor having a collector connected to the collector of said PNP input transistor, an emitter connected to the base of said NPN transistor, and a base connected to a second constant current source circuit, and third, fourth, and fifth diodes connected in series between the base of said NPN coupling transistor and said output terminal, and a sixth diode connected between said output terminal and an output of said supplemental amplification stage, for saturating said PNP input transistor when said output terminal is shorted to ground.

Claim 9

An amplifier circuit having a gain stage connected to a supplemental amplification stage and an output stage, said output stage being connected to said supplemental amplification stage and having an output terminal, and a second stage overload protection circuit coupled between said gain stage and said output stage, wherein: said supplemental amplification stage includes a PNP input transistor, a first diode, a second diode, and a first constant current source circuit in series connection between a power supply conductor and ground, said diodes being connected to allow current to flow from said power supply conductor to ground; said output stage includes an NPN transistor, two emitter resistors, and a PNP transistor in series connection between said power supply and ground, said NPN transistor being connected between said power supply conductor and one of said emitter resistors, said PNP transistor being coupled between ground and the other of said emitter resistors, each of said emitter resistors being coupled between one of said NPN transistor and said PNP transistor and said output terminal and; said second stage overload protection circuit includes, in combination, a PNP coupling transistor having a collector connected to the collector of said PNP input transistor, an emitter connected to the base of said NPN transistor, and a base connected to a second constant current source circuit, and third, fourth, and fifth diodes connected in series between the base of said NPN coupling transistor and said output terminal, and a sixth diode connected between said output terminal and an output of said supplemental amplification stage, for saturating said PNP input transistor when said output terminal is shorted to ground.

Claim 10

An amplifier circuit having a gain stage connected to a supplemental amplification stage and an output stage, said output stage being connected to said supplemental amplification stage and having an output terminal, and a second stage overload protection circuit coupled between said gain stage and said output stage, wherein: said supplemental amplification stage includes an NPN input transistor, a first diode, a second diode, and a first constant current source circuit in series connection between a power supply conductor and ground, said diodes being connected to allow current to flow from said power supply conductor to ground; said output stage includes an NPN transistor, two emitter resistors, and a PNP transistor in series connection between said power supply and ground, said NPN transistor being connected between said power supply conductor and one of said emitter resistors, said PNP transistor being coupled between ground and the other of said emitter resistors, each of said emitter resistors being coupled between one of said NPN transistor and said PNP transistor and said output terminal and; said second stage overload protection circuit includes, in combination, a PNP coupling transistor having a collector connected to the collector of said NPN input transistor, an emitter connected to the base of said PNP transistor, and a base connected to a second constant current source circuit, and third, fourth, and fifth diodes connected in series between the base of said NPN coupling transistor and said output terminal, and a sixth diode connected between said output terminal and an output of said supplemental amplification stage, for saturating said NPN input transistor when said output terminal is shorted to ground.

Claim 11

An amplifier circuit having a gain stage connected to a supplemental amplification stage and an output stage, said output stage being connected to said supplemental amplification stage and having an output terminal, and a second stage overload protection circuit coupled between said gain stage and said output stage, wherein: said supplemental amplification stage includes an NPN input transistor, a first diode, a second diode, and a first constant current source circuit in series connection between a power supply conductor and ground, said diodes being connected to allow current to flow from said power supply conductor to ground; said output stage includes an NPN transistor, two emitter resistors, and a PNP transistor in series connection between said power supply and ground, said NPN transistor being connected between said power supply conductor and one of said emitter resistors, said PNP transistor being coupled between ground and the other of said emitter resistors, each of said emitter resistors being coupled between one of said NPN transistor and said PNP transistor and said output terminal and; said second stage overload protection circuit includes, in combination, a PNP coupling transistor having a collector connected to the collector of said NPN input transistor, an emitter connected to the base of said PNP transistor, and a base connected to a second constant current source circuit, and third, fourth, and fifth diodes connected in series between the base of said NPN coupling transistor and said output terminal, and a sixth diode connected between said output terminal and an output of said supplemental amplification stage, for saturating said NPN input transistor when said output terminal is shorted to ground.