Method and circuit for preventing forward bias of a parasitic diode in an integrated circuit

A method of reducing a base current of a bipolar transistor to zero as the bipolar transistor collector voltage approaches ground so that a collector-substrate diode in an integrated circuit with the bipolar transistor does not become forward biased, the diode substrate begin biased to ground, the method comprising the steps of:

• (a) providing the bipolar transistor base current from a current source;
• (b) decreasing the current from the current source that is available to the bipolar base as a collector current of a first transistor increases responsive to a decrease in the collector voltage of bipolar transistor;
• (c) providing second and third transistors having a common base and a common collector, the second transistor having an emitter connected to ground, and the third transistor having an emitter connected to the bipolar transistor collector voltage;
• (d) increasing the collector current of the third transistor relative to the collector current of the second transistor as the bipolar transistor collector voltage decreases to ground;
• and
• (e) increasing the collector current of the first transistor as the collector current of the third transistor increases,
• whereby the base current of the bipolar transistor is zero when the bipolar transistor collector voltage is at ground.

A circuit for preventing forward bias of a collector-substrate diode comprising in an integrated circuit;

• a bipolar transistor having forward gain .beta. and a substrate biased to ground, said bipolar transistor having a collector-substrate diode therein which will become forward biased when said bipolar transistor base current times .beta. exceeds said bipolar transistor collector current and when said bipolar transistor emitter voltage is less than ground;
• means for sensing a difference between said bipolar transistor collector voltage and ground;
• and
• means for varying said bipolar transistor base current responsive to the sensed difference so that the base current is zero when the collector voltage is equal to ground and so that the collector current will not be less than .beta. times the base current when the emitter voltage is less than ground, whereby said diode will not become forward biased,
• said means for varying comprising means for providing the base current for the bipolar transistor, and a first transistor with a collector current that increases responsive to a decrease in the collector voltage of the bipolar transistor, whereby the current that is available to the base of the bipolar transistor decreases as the collector current of said first transistor increases,
• said means for sensing comprising,
• second and third transistors having a common base and a common collector, said second transistor having an emitter connected to ground, and said third transistor having an emitter connected to the bipolar transistor collector, and
• whereby the collector current of said third transistor increases relative to the collector current of said second transistor as the bipolar transistor collector voltage decreases to ground, and
• wherein said means for varying further comprises means for increasing the collector current of said first transistor as the collector current of said third transistor increases.

A circuit for reducing a base current of a bipolar transistor to zero as the bipolar transistor collector voltage approaches ground so that a collector-substrate diode in an integrated circuit with the bipolar transistor does not become forward biased, the diode substrate being biased to ground, the circuit comprising:

• a current source for providing the bipolar transistor base current;
• a first transistor having a collector connected for changing the bipolar transistor base current and a base connected so that said first transistor collector current increases responsive to a decrease in the collector voltage of the bipolar transistor;
• second and third transistors having a common base and a common collector, said second transistor having an emitter connected to ground, and said third transistor having an emitter connected to the bipolar transistor collector;
• whereby the collector current of said third transistor increases relative to the collector current of said second transistor as the bipolar transistor collector voltage decreases to ground;
• and
• means operably connected to the base and emitter of the first transistor for increasing the collector current of said first transistor as the collector current of said third transistor increases,
• whereby the base current of the bipolar transistor reduces to zero as the bipolar transistor collector voltage approaches ground.

A circuit for preventing forward bias of a collector-substrate diode in an integrated circuit with a bipolar transistor having forward gain .beta. and in which the substrate is biased to ground, wherein the diode will become forward biased when the bipolar transistor base current times .beta. exceeds the bipolar transistor collector current and when the bipolar transistor emitter voltage is less than ground, the circuit comprising;

• means for sensing a difference between the bipolar transistor collector voltage and ground, said means for sensing comprising,
• a pair of transistors having a common base and a common collector, a first one of said pair of transistors having an emitter connected to ground, and the other one of said pair of transistors having an emitter connected to the bipolar transistor collector, and
• whereby the collector current of said other one of said pair of transistors increases relative to the collector current of said one of said pair of transistors as the bipolar transistor collector voltage decreases to ground;
• and
• means for varying the bipolar transistor base current responsive to the sensed difference whereby the base current is zero when the collector voltage is equal to ground and so that the collector current will not be less than .beta. times the base current when the emitter voltage is less than ground, whereby the diode will not become forward biased.